chm 2005 lab manaul summer 2010 - peter

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CHM 2005 • Organic Chemistry Laboratory I Manhattanville College • Department of Chemistry • Summer 2010

Instructor:

Professor Sapan L. Parikh

Lab: Office:

Tuesdays, Wednesdays and Thursdays 1:00 – 5:00 PM Brownson 3A, 117 & 122

Telephone: (914) 323 – 5332 Email: [email protected]

Course Website: http://faculty.mville.edu/parikhs/

Text:

1) “Custom Manual for Manhattanville Organic Chemistry Laboratory”

2) “Techniques in Organic Chemistry” Mohrig, Hammond and Schatz,

2nd edition 2007 (on reserve at library).

Course Objective:

CHM 2005 is designed as an introduction to research techniques in organic chemistry using the concepts covered during lecture. Through chemical experimentation, the lab is designed as an introduction to research techniques. The chemical experiments will familiarize students with various synthetic techniques, purification methods, and characterization methods used by research chemists.

At completion of these courses, the student should be able to: Isolate, identify and characterize starting material and product Record data and observation Interpret chemical and instrumental data of organic compounds Verbalize chemical concepts Construct technical experimental reports

Attendance Policy: Attendance is very important for each student’s academic performance in this

course. Accordingly, there are no make-ups for missed labs. A student will receive an F grade if he or she is absent from two or more labs. A single absence is acceptable if a student can supply a written health or other valid excuse. Such a student is still responsible for all material covered in the missed lab

Grading: The laboratory grade is composed of pre-labs, lab reports and lab quiz.

9 pre-labs 10 pts 90 pts 9 lab reports 50 pts 450 pts 1 lab quiz 160 pts 200 pts

700 pts

A 100 – 90 B 90 – 80 C 80 – 70 D 70 – 60 F < 60

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Pre-Lab Procedures: Every student must be in lab at the staring time listed above and complete the experiment within the time allotted. In preparation for the laboratory, study the theory and familiarize yourself with the techniques and procedures required for that experiment. A pre-lab lecture (the first 10 - 15 minutes of the lab) will be held to cover any experimental changes and demonstrate laboratory techniques. Prior to arriving and conducting the scheduled experiment, a thorough outline of the experiment must be typed (2 copies). This will entail all the experimental procedures required for that day (i.e. recipe for that day). One copy will be submitted to the instructor prior to start of lab. Students who fail to prepare for the lab will not be allowed to conduct the experiment and receive a zero for the experiment. During the lab, you will record any changes in procedures, measurements, data, results and observations. After completing the experiment, you will type a lab report and answer discussion questions assigned by your instructor. Note: All changes to the procedures should be modified in your lab report.

Lab Quiz:

One cumulative lab quiz will be given at the end of each summer session. The quiz will refer to the theory and practical applications of the experiments performed during the semester.

Lab Reports: The purpose of the laboratory report is to give a complete and concise description of the experiment preformed. All lab reports must typed and submitted to your instructor exactly one week from completion of experiment (see schedule at end of syllabus). Hand written reports will not be accepted under any conditions. Reports handed in late will be marked late and 5 points will be deducted per each day the report is late. Reports more than 2 days will not be accepted. Unlike papers in other disciplines, chemistry lab reports are technical documents and should be written in a very specific manner. Lab reports are not based on feelings or opinions; they are based on factual observations and results. Do not use the terms “I” or “we” in a lab report. Formal laboratory reports should be typed (5-8 pages, double-spaced with figures, appendices, illustrations, etc.) according to the guidelines below.

A complete lab report will include the following: (1) Cover sheet/Header (2 pt): Name, date, Title of Experiment, CHM 2005 (Download from course website or emailed). At the top of every page should appear a header with name, title of experiment and date, and a footer with page number on bottom right of the page (2) Abstract (10 pts): This is a brief, but thorough, statement describing the experiment conducted including evidence of product characterization and identification. The statement should be 2-4 sentences long, written in the past tense, passive voice. Include any physical (i.e. MP, BP, RI, , etc…)

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and/or spectroscopic (i.e. IR, NMR, GC-MS, etc.) evidence to supporting you findings. example: The melting point of Benzoic Acid and Mandelic Acid were determined to be X C and Y C, respectively. Introduction of impurities to Benzoic Acid resulted in a depression and broadening of the melting point. The eutectic temperature was determined to be Z C for Benzoic Acid and Mandelic Acid. Note: There are no raw numbers with regards to grams/mL isolated or procedures mentioned in the abstract. (3) Experimental (5 pts): This is a concise account of the materials and methods used in the experiment. Since you will have already prepared a detailed procedure section, you can make electronic changes to your procedures and insert directly into your lab report. (4) Chemical Table (8 pts): Table of reagents and product with data on the compounds’ names, physical constants like structure; molecular formula, mp, bp (literature and experimental values), density, volume, weight and moles theoretical and percent yield (where applicable). (5) Reaction and Mechanism (0 pts): Write out overall reaction and a detailed mechanism for the experiment done (if applicable). (6) Results and Discussion (10 pts): This is the core section of your lab report. Before you begin writing it, you should gather together any and all data and/or observations you made during the experiment. Numerical data are best represented in tables (or graphs when applicable) which should be labeled and put in the last part of your lab report (see below). Your results should state only what you found in your experiment, not what you expected to find or what you were supposed to have observed. Be sure to write in the past tense, passive voice, no “I” or “we”. example: The recrystallized benzoic acid appeared as a powdery, white solid weighing 0.67 g (54% yield) and with a melting range of 98-101 °C. Discuss your data and observations as they relate to the reactions and the experiment in a logical manner. You must talk about your results in a way that describes how your results support your lab objective. Exclude those points that are not relevant to your experiment. This is the part of the lab where you can explain or rationalize errant data or describe possible sources of error and how they may have affected the outcome of the experiment. Even if your experiment was a complete disaster you can still write an excellent lab report, as long as you understand what went wrong and can explain it; the discussion section is where you can do that. In the end you should relate back to the introduction section and come to a definitive conclusion.

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(7) Figures, Tables, Data, Spectra (5 pts): The last part of your report contains the hard data - tables, graphs and spectra. All of these should be labeled and have a title, and you should include appropriate units. Spectra must have the name and structure with characterization of significant findings. (8) Post-lab question (10 pts): The instructor will assign additional discussion questions from your lab manual.

Laboratory Safety: Lack of understanding or ignorance of proper guidelines and methods can make any experiment dangerous; therefore students must pay careful attention to safety. Eye protection: Safety glasses/goggles must be worn at all times in the lab. The purpose of eye protection is to guard you from accidents caused by you or your neighbor. Contact lenses are prohibited; you must substitute contacts with prescription glasses. Clothing: Open-toed shoes and excessively loose or flowing clothes are forbidden in lab. Long hair must be tied back. Students are strongly encouraged to wear socks, long pants, and other clothing that will help to protect the skin from hazardous chemicals. Spills and splashes do occur; therefore lab coats can be worn to protect you and your clothing. Gloves should be worn when using chemicals. Even if you use gloves, you should always wash your hands after each lab. Chemicals: Chemicals and equipment used in lab can be dangerous if handled improperly, it is very important that you familiarized yourself with the experiment before beginning it. Always assume that chemicals are dangerous unless you know otherwise. Vapors of volatile liquids and gases can be particularly dangerous; they can be toxic, corrosive, and/or flammable. Therefore, smoking, eating, drinking, and applying cosmetics in the lab are prohibited. Open flames are forbidden. Accidents/Injuries: In case of an emergency, you should know the location and operation of the proper equipment.

Eye Wash/Safety Shower Station Fire Blanket Nearest Fire Alarm Fire Extinguishers Nearest Emergency telephone First Aid Kit

Exits from Lab and Building Report all personal injuries to the instructor who will assess the wound and summon professional help if necessary. Students who fail to adhere to safety rules will not be allowed in lab. Behavior deemed by the instructor to be unsafe

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to yourself or to others will result in your dismissal from that experiment.

Waste Disposal: Disposal of chemical waste is a serious matter, and responsibility for safe disposal in the laboratory begins with you. Improper waste disposal can lead to hazardous conditions. Always think before you discard chemical waste!

Here are some general guidelines: Neutral or slightly basic or acidic aqueous solutions of non-toxic

chemicals may be flushed down the drain with excess water. Strongly acidic or basic solutions must be disposed of in bottles marked

"Acidic Waste" or "Basic Waste", located in the fume hoods. It is very important that these types of solutions not be poured into bottles labeled "Organic Waste". Organic solvents must be disposed of in the correctly labeled waste

bottle. Halogenated wastes (F, Cl, Br, I) will be kept separate from non-halogenated wastes.

Containers will also be available for solid chemical waste. Note that

these containers are only for the chemical waste itself and not for filter paper, paper towels, Kimwipes, etc.

A broken glassware container is located near the door of the lab room. Do not place paper waste in the broken glassware bin.

Some specific guidelines are also included with each laboratory.

Lab Cleanliness:

It is essential that the laboratory as a whole be kept clean and tidy. Try to be reasonably neat and clean. All equipment and spaces (personal and general) must be cleaned and left in good condition at the end of each lab. Equipment that is broken should be reported to the instructor so that it can be fixed or replaced. Students will be assigned a clean-up task (common area of the lab including balances, hoods and sinks) every week.

The reagents for each experiment will be placed either on a common bench top or in the fume hood. They should be closed and returned immediately after you have obtained what you need for the experiment. Do not keep them open or at your personal workspace.

Disabilities:

Students requesting special accommodations should contact the ADA Coordinator Jean Baldassare in Founder's Hall Room at (914) 323-7127.

Academic Dishonesty:

Academic dishonesty will not be tolerated. All work to be graded in this course must be done independently unless specifically designated otherwise by the lab professor. While discussions of the experiments and results with others are

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encouraged to enhance your understanding of the concepts involved, you are expected to write independent lab reports. Anyone caught copying lab reports will receive an automatic zero.

Lab Schedule – Summer Session I – 2010

Date Experiment (post-lab questions in parentheses) Due DateW 6/02 Check In and Lab Rules/Safety --

Th 6/03 TECH 701 - Measuring the Melting Points of Compounds & Mixtures (1 – 3, 5) Th 6/10

Th 6/03 TECH 703 - Purifying Acetanilide by Recrystallization (1 – 3) Th 6/10

T 6/08 Extraction - Solvent Extraction of Fluorene, Fluorenone and Crystal Violet T 6/15

Th 6/10 TECH 704 - Separating Cyclohexane and Toluene by Distillation (1 & 2) Th 6/17

T 6/15 Handout - Isolation of (R)-(+)-Limonene from Orange Peels (1) T 6/22

W 6/16 TECH 707 - Separating a Mixture of Thin Layer Chromatography (1 – 3) W 6/23

T 6/22 Spinach - Column Chromatography: Isolation of Pigments From Spinach T 6/29

W 6/23 REAC 712 - Dehydrating Cyclohexanol (1 – 7) W 6/30

Th 6/24 SYNT 719 - Brominating Alkenes (1 – 4) Th 7/1

T 6/29 Lab Quiz and Checkout

Practicing Safety in theOrganic ChemistryLaboratoryprepared by Michael W. Rapp, University of Central Arkansas

PURPOSE Review the basic rules of laboratory safety. Recognize the commonhazards in an organic chemistry laboratory. Learn the properresponses to incidents that may occur in the laboratory.

SAFETY RULES FORTHE ORGANIC CHEMISTRY

LABORATORY

Follow all rules. A Safety Contract is included within this module.You must hand in a completed contract to indicate your willingness tofollow the standard rules of laboratory safety before you will beallowed to work in the laboratory.1. Wear safety goggles while in the chemistry laboratory. Usesplashproof goggles rated as ANSI Z87.1. Goggles are to be worn overprescription glasses. Supply your own goggles because sharinggoggles can lead to eye infection from another wearer. Use of contactlenses under the goggles is discouraged because contact lenses mayincrease the damage done if an irritant gets in your eye. If you mustwear contact lenses under your goggles to avoid unreasonably lim-ited vision, indicate that need on your Safety Contract.

2. Wear proper clothing to provide protection from reagent spills.Long pants are required and long-sleeved shirts are preferred. Alaboratory coat that extends below the knee is recommended. Shoesmust be closed-toe and made of nonporous material. Do not wearloose-fitting clothing because it may catch on objects and cause spills.Avoid loosely woven or fuzzy fabrics because they increase thechances of fire hazard to the wearer. Tie back hair that is longer thanshoulder length.

3. Use good housekeeping practices to ensure a safe workplace. Callto the attention of the laboratory instructor any conditions that seemunsafe. Avoid cluttering the work area, especially the work areasshared by many students. Place personal items, such as coats andbackpacks, in separate storage areas rather than in the laboratorywork space. Return items promptly to their proper locations. Disas-semble and clean glassware directly after use because residues inglassware may become resistant to cleaning if not washed promptly.Allow hot glassware to cool to room temperature before washing.

TECH

700m o d u l a r · l a b o r a t o r y · p r o g r a m · i n · c h e m i s t r ypublisher: H. A. Neidig organic editor: Joe Jeffers

Copyright © 1997 by Chemical Education Resources, Inc., P.O. Box 357, 220 S. Railroad, Palmyra, Pennsylvania 17078No part of this laboratory program may be reproduced or transmitted in any form or by any means, electronic or mechanical, includingphotocopying, recording, or any information storage and retrieval system, without permission in writing from the publisher. Printed in theUnited States of America 00 99 — 15 14 13 12 11 10 9 8 7 6 5 4 3 2 1

4. Do only authorized experiments, and work only when the labora-tory instructor or another qualified person is present. Do not enter thelaboratory until the laboratory instructor is present. Unauthorizedexperimenting will waste time and may expose you and others tounreasonable risk. Authorized experimental procedures take intoaccount the special hazards of the materials used. Do not treat labora-tory reagents and equipment as playthings. Do not remove anyreagents from the laboratory. Injuries from laboratory incidents mostoften occur from violations of the precautions given in this paragraph.

5. Treat all laboratory reagents as if they are poisonous andcorrosive, unless told otherwise. Immediately wash spills off your skinwith plenty of water. Then notify your laboratory instructor. Thisresponse is especially important for many organic compoundsbecause their fat solubility enhances their ease of absorption throughthe skin. Wash your hands thoroughly with soap or detergent beforeleaving the laboratory. Special hazards of laboratory reagents will beindicated by appropriate labels on the reagent bottles.

Containers from chemical supply companies may use the NationalFire Protection Association’s diamond or some similar indicator ofpotential hazard, as shown in Figure 1. A number from 0 (low) to 4(high) in each category indicates the degree of hazard.

6. Dispense reagents carefully and dispose of laboratory reagents asdirected. Do not use reagents from unidentified containers. Doublecheck each label before dispensing a reagent. To prevent contamina-tion, do not return any reagent to its original container. Place anyexcess reagent in the recovery container provided by your laboratoryinstructor. Dispose of reagents as directed by the laboratory instructorand the written procedure. Promptly notify the laboratory instructor ofany spill. Clean up a spill only if directed to do so by your laboratoryinstructor. Spills should be cleaned up immediately to prevent contact ofthe chemicals with persons who are not aware of the spill. When weigh-ing reagents, dispense them into containers so that reagents do not spillonto the balance.

7. Do not eat, drink, use tobacco, or apply cosmetics in the labora-tory. Violation of this rule can introduce poisons into your system.Especially avoid any contamination to your mouth or eyes. Neverbring food or drinks into the laboratory.

8. Immediately report all incidents to the laboratory instructor. Anincident is any situation in the laboratory that might endanger thosepersons present. Your laboratory instructor must give prompt attention

2 TECH 700/Practicing Safety in the Organic Chemistry Laboratory

© 1997 by Chemical Education Resources

Figur e 1 National Fire Protection As-sociation label

Common Hazards 3

when injuries occur. Even minor incidents may require attention. Thelaboratory instructor may also be able to use the information youprovide to help other students avoid a similar difficulty.

9. Be familiar with the location and use of all safety equipment in thelaboratory. All laboratories should have an eyewash station, a safetyshower, a fume hood, and more than one fire extinguisher. Anticipatethe steps to be taken in the event of an incident. Prompt response toan incident can make the difference between a minor event andpermanent injury. The laboratory instructor may direct you to assist inresponding. However, do not place yourself or others at risk in order torespond to an incident in the laboratory. A subsequent sectiondescribes proper responses to incidents in the laboratory.

10. Become familiar with each laboratory assignment before comingto the laboratory. Pay particular attention to cautions given in theprocedure and by the laboratory instructor. Use of some equipmentpresents special hazards. For example, vacuum operations include thepossibility of implosions, and ultraviolet light is damaging to the eyes ifviewed directly. Each laboratory experiment will give special cautionsfor any hazardous reagents used in that experiment. Your laboratoryinstructor will provide other information and reminders as needed.

By law, chemical supply companies must provide a Material SafetyData Sheet (MSDS) with each reagent they sell. The MSDS is atechnical bulletin that gives detailed information on the properties ofa laboratory reagent. Some information from an MSDS for 1-propanolis shown in Figure 2 on the next page. Your laboratory instructor mayhave you consult the reagent MSDS as part of your laboratory work.

COMMON HAZARDS INTHE ORGANIC CHEMISTRY

LABORATORY

Anticipate common hazards encountered in the laboratory. Experi-ence has shown that ignoring the following common concerns will leadto mishaps.

1. Never pipet by mouth. Many organic chemicals are toxic.

2. Do not use chipped or cracked glassware, which may cause cutsor may crack and spill its contents unexpectedly.

3. Obtain approval from your laboratory instructor before starting adistillation. Make certain the apparatus has an opening. Do not heat aclosed apparatus because abrupt release of the increased pressuremay propel reagents or pieces of glass at persons nearby. Use a freshboiling chip each time a liquid is boiled to avoid bumping, the sud-den eruptive release of vapor. Such a release can burn personsnearby. Do not heat any distillation pot to dryness because the resi-due that remains may be heat sensitive, and overheating could causeit to detonate. Also, glassware that is superheated could crack. Beforeheating a flask, clamp the neck of the flask to support it in an elevatedposition to allow withdrawal of the heat source and rapid cooling, ifneeded.

4. Lubricate and clamp ground glass joints so they will not freeze orspring open in use. Use lubricant sparingly.

5. Do not point the open end of a container at anyone. Abruptformation of bubbles, such as from boiling, could propel the contentsinto a person’s face.


6. Place heated glass and other hot objects on an appropriate sur-face, such as a wire gauze or ceramic pad, until they have cooled. Hotglass or metal may look like cool glass or metal, so cautiously touchobjects that have been heated before handling them. Place a notenearby any hot objects remaining at the end of a laboratory period, sostudents in a subsequent laboratory period will not be endangered.

7. Use a fume hood when working with reagents whose vaporsare harmful. Except for small quantities heated by steam or ahot-water bath, heating of highly flammable organic substances inopen containers must be done in a hood. In using a fume hood, posi-tion any apparatus well within the hood space, keeping your headoutside the hood. The flow of air through the hood must be adequateand unobstructed. The hood sash should be lowered, except whenmaking manipulations within the hood. Place within the hood onlythose items necessary for the operation being performed. Keep theexhaust fan on as long as any reagents remain within the hood.

8. When testing odors of reagents, gently waft vapors from the con-tainer toward your nose. Do not directly sniff the contents of a container.


Figure 2 Selected information from MSDS for 1-propanol

MATERIAL SAFETY DATA SHEETANY SCIENTIFIC COMPANY EMERGENCY #: (8OO)-555-XXXX

YOURTOWN, USASection 1 (Identity): 1-PROPANOL (and synonyms given) Mol. Formula C3H8O Mol. Wt. 60.1

CAS # 71-23-8 NFPA Ratings (scale 0–4): Health = 1, Fire = 3, Reactivity = 0

Section 2 (Hazardous Ingredients): 1-PROPANOL (100%)Exposure Limits: 200 ppm (492 mg/m3) OSHA TWA

250 ppm (614 mg/m3) OSHA STEL

Section 3 (Physical & Chemical Characteristics): Description: Colorless liquid with mild alcohol odorBoiling Point: 207 F (97 C) Melting Point –195 F (–125 C) Vapor Pressure (20 C): 15 mm HgOdor Threshold: 30 ppm

Section 4 (Physical Hazards): Dangerous fire hazard when exposed to heat or flame. Vapors are heavierthan air and may travel a considerable distance to a source of ignition. Flash Point: 74 F (23 C) (CC)Upper Explosive Limit 13.7% Fire Fighting Media: Dry chemical, carbon dioxide, water spray, or alcohol-resistant foam. Transportation Data: US DOT Hazard Class 3 — flammable liq.

Section 5 (Reactivity): Stable under normal temperatures and pressures. Incompatibles: alkali & alkalineearth metals. Attacks coatings, plastics, and rubber.

Section 6 (Health Hazards — Inhalation, Skin Contact, Eye Contact, and Ingestion):INHALATION: Irritant/Narcotic. 4000 ppm is immediately dangerous to life or health.Acute Exposure: Inhalation of vapors may cause moderate irritation of the upper respiratory tract with

coughing and shortness of breath. High concentrations may cause CNS depression, with dizziness,headache, and vomiting.

Chronic Exposure: Reproductive effects have been reported.First Aid: Remove from exposure area. Perform artificial respiration if necessary. Get medical attention immed.

Section 7 (Precautions for Safe Handling and Use — Storage, Disposal, Spill & Leak Procedures):May be ignited by electrostatic sparks, so should be stored in grounded container, as specified in NFPA77-1983. Disposal must be in accordance with 40 CFR 262 (EPA Hazardous Waste Number D001). For smallspills, take up with sand or other noncombustible absorbent and place into containers for later disposal.

Section 8 (Control Measures — Ventilation, Firefighting, Clothing, Gloves, Eye Protection): Wearappropriate protective clothing and equipment to prevent prolonged skin contact.


Responses to Incidents 5

9. Do not use open flames (Bunsen burners) in the presence offlammable materials, especially organic solvents such as acetone,diethyl ether, or petroleum ether. Use of a flameless heat sourcediminishes the danger of a fire, but such heat sources remain hot forquite some time after they are turned off. Overheated sand baths, hotplates, or heating wells can ignite fumes from volatile organic solvents.

10. Wear gloves when dispensing irritating reagents. This precautionis especially important for organic reagents, which can penetrate theskin readily. Your laboratory instructor will designate gloves that areappropriate for the reagents to be used. Latex surgical gloves are notappropriate because they allow passage of many organic reagents.Gloves should be inflated to check for breaks by whipping themthrough the air. Do not check gloves by inflating them by mouth.

11. Take special care when working with strong acids or strong bases.Contact with these materials can cause severe chemical burns. Preparedilute acids by slowly adding the concentrated acid to a larger volumeof water, with stirring. The water dissipates the evolved heat and pre-vents localized boiling that could spew the contents from the container.

12. If you must insert glass tubing into stoppers, follow the directionsgiven by your laboratory instructor.

RESPONSES TO INCIDENTSIN THE ORGANIC

CHEMISTRY LABORATORY

Become familiar with actions to be taken in the event of incidents inthe laboratory. Provide appropriate assistance to others in emergen-cies. The items that follow describe the actions that should be taken incertain situations.1. Report all incidents to the laboratory instructor, who is responsiblefor actions to be taken in response to incidents and for reports to bemade to other authorities. As defined in Safety Rule 8, an incident isany situation in the laboratory that might endanger those persons pres-ent. An improper response may change a trivial difficulty into a muchmore hazardous situation. Sometimes an irritation or personal injury isnot manifested immediately. A student who experiences an irritationlater in the day, and who has a reasonable suspicion that contact withlaboratory reagents could have caused the problem, should contact thelaboratory instructor or a health care professional for advice.

The safety of persons in the laboratory has absolute priority over allother considerations. While you will not have the responsibility fordirecting others in the laboratory, you should be aware that the typicalsequence of actions to take in the event of an incident in the laboratoryis ALERT, CONFINE, and EVACUATE. If you are the first to notice ahazard in the laboratory, you should alert your laboratory instructorand others nearby. After you and others are clear of danger, yourlaboratory instructor will confine the hazard. If the hazard persists, thelaboratory instructor may give instructions to evacuate the area.

Severe injuries may result from unreasonable responses to unex-pected situations. For example, a person who spills a corrosive reagenton himself or herself might hope no one else notices, waiting until leav-ing the laboratory to wash off the spill. In the meantime, the burn fromthe reagent may have progressed from a superficial irritation to onethat requires medical attention. Or a person who has lifted a test tubeat the time the contents ignite might throw the tube through the air


onto another person, catching that person’s clothes on fire. Considerthe consequences of your actions.

2. Dispose of broken glass as directed by the laboratory instructor.Use a hand brush and dust pan to collect the pieces. Do not attempt togather sharp glass by hand. Place broken glass in specially designatedreceptacles in order to avoid placing other persons at risk. Placevery small, sharp objects—for example, syringe needles and pieces ofcapillary tube—in specially designated receptacles.

If a mercury thermometer is broken, step back from the work areaand notify the laboratory instructor, who will use special techniques tocollect the spilled mercury. The special hazard with mercury is not fromcontact with the skin, but from prolonged exposure to the vapor. Acut bya broken thermometer should get the same attention as other cuts.

3. For either minor cuts or burns, wash the affected area using soapor detergent. Tissue damage from a superficial burn will be minimal ifthe affected area is cooled quickly, so you should flush the affectedarea with cold water. Then notify the laboratory instructor. Whenwork is resumed, protect any break in the skin by wearing a glove, inorder to prevent introduction of laboratory reagents.

4. In the event of a reagent spill, notify the laboratory instructor. Appro-priate steps to be taken in response to a reagent spill will vary, dependingon the amount and identity of the reagent. Concerns for hazards otherthan the reagent itself, such as danger of shorting electrical equipment,may even take precedence. Spills of organic solvents may be a fire haz-ard. In such an event, remove all ignition sources, including any equip-ment that could produce a spark—for example, switches being turned onand off. Hot plates and sand baths at a high temperature do not cool rap-idly on turning off, so move these heat sources away from the spill. If aspill creates a large amount of fumes, evacuate the laboratory. Stop anyexperiments, if doing so doesn’t place anyone at risk.

Deal promptly with reagent spills on a person. Wash the affectedarea with large volumes of water. Rapid response is necessary becausemany organic solvents are fat soluble and can be absorbed through theskin. Use the sink or safety shower as needed, depending on the size ofthe spill. Remove clothing and wash skin with soap or detergent tocomplete the removal of the reagent. Do not remove goggles beforewashing any reagent spill from the face, to lessen the likelihood ofgetting the reagent in the eyes.

A person whose eyes have had reagents splashed into them requiresassistance from others. A person’s automatic response to an irritation tothe eyes is closing of the lids and rubbing, actions that will only increasethe irritation. Other persons should assist the person to the eyewashfountain and operate the water flow, while the person holds openhis/her eyelids. The flow of water must get to the entire eye surface,continuing for twenty minutes. Cold water may be intolerable for suchan uninterrupted period, so periodic washing may have to be done. Irri-gation of the eye will not be adequate if contact lenses that are presentare not removed. Further treatment of irritation to the eye from areagent spill must be done only by a health care professional.

5. Many common solvents used in the organic chemistry laboratory arehighly flammable, and a small fire may occur in the laboratory. Do not



Responses to Incidents 7

react without thinking. The immediate response to a fire in the laboratoryis to take those actions that remove individuals from the hazard. For exam-ple, stepping back from a small fire and cautioning neighbors of the haz-ard would be a reasonable response. Move flammable materials away, andturn equipment off or remove equipment from the vicinity of the fire. Shutoff the gas spigot or heating element. Place a watch glass or beaker over asmall container to smother the burning material. Some small fires, such asalcohol fires, may be allowed to burn out.

If a fire spreads to a larger area of the bench, the laboratory instructoror other authorized persons should operate the fire extinguisher. Shoulda fire reach a stage where it cannot be easily controlled, the laboratoryinstructor will direct you to evacuate the laboratory and the building.

The most distressing incidents in laboratories are those where anindividual is on fire. Using small quantities of flammable substancesand following safe practices in the laboratory ensure that such an eventis unlikely to happen. Proper response can make the differencebetween loss of some clothing or, in the extreme case, loss of life. If aperson’s lungs are seared from inhaling flames, there will be littlechance of recovery.

The safety shower or water from the sink may be sufficient toextinguish a fire on a person. In a severe situation, the proper responseto fire on an individual’s clothing is to STOP, DROP, and ROLL. That is,if you have fire on your body, stop where you are, drop to the floor, androll to smother the flames. Staying upright will allow the flames to riseto the face. Nearby persons can use a laboratory coat to beat out theflames. When the flames are extinguished, remove any smolderingfabric. If the person has been burned, place the person under the safetyshower. Other persons nearby can assist as needed, such as extinguish-ing fire on the bench, shutting off equipment, and cleaning up. Mostother persons in the laboratory should simply move away.

6. Ingestion or inhalation of a reagent will likely require the assis-tance of a health care professional. In such an event, immediatelynotify the laboratory instructor, who will gather the informationneeded to report the incident to the poison control center. Space isprovided at the end of this module for you to record the phone num-ber of the poison control center in your area.

Avoid the inhalation of unsafe levels of irritating or toxic vapors byfollowing the directions for using laboratory reagents and by using thereagents in a fume hood. While you must not depend upon your sensesto alert you to inadvisable conditions, notify your laboratory instruc-tor promptly if your eyes begin to sting or if you develop a headachethat may be caused by fumes in the laboratory. Especially avoid breath-ing the vapors from chlorinated solvents and aromatic compounds.

7. Immediately notify the laboratory instructor if you or a neighborfeels faint. A person who has become unconscious from inhalation offumes must be removed from the source of the fumes. Other thanchecking the person’s airway and treatment for shock (elevatinglimbs, keeping warm), further treatment should only be made by ahealth care professional.

This module is to serve only as a starting point for good safety practices and does notpurport to specify minimum legal standards or to attest to the accuracy or sufficiency ofthe information contained herein, and Chemical Education Resources, Incorporated,assumes no responsibility in connection therewith.


SAFETY INFORMATION

Complete this form and keep it for possible use.

1. Emergency health providers (telephone numbers to call)

Campus Health Services:

on campus ______________________________ off campus __________________________________

Emergency Medical Assistance:

on campus ______________________________ off campus __________________________________

State Poison Control Center: _________________________

Campus Police:

on campus _______________________________ off campus __________________________________

City Police or Fire Department:

on campus _______________________________ off campus __________________________________

2. Contacting the laboratory instructor

Name: __________________________________________ Office: _______________________________

Office phone: on campus _________________________ off campus ____________________________

Home phone: ___________________________________ E-mail address: ________________________

3. Reporting incidents (for reference)

The following information will be needed when communicating with health professionals and/orrecording incidents.

Nature of the incident (description – including fire, substances involved, number of individualsinvolved and their physical conditions):

Individuals involved (identification – name, gender, age):

Location of the incident, including who will meet any emergency vehicle, and where:

Person reporting the incident (name, phone number being used to report the incident. Note: Do notallow this phone to be tied up for calls unrelated to control of the incident.):



Safety Contract 9

Safety Contract

Complete this form and give to your laboratory instructor.

I have carefully read the organic chemical laboratory safety module. I have given myanswers to the accompanying safety quiz and given that completed quiz to the labora-tory instructor as an indication of my familiarity with the module. Whenever I am in anarea where laboratory reagents are being used, I agree to abide by the following rules:

1. Wear safety goggles.

2. Wear proper clothing.

3. Use good housekeeping practices.

4. Do only authorized experiments, and work only when the laboratoryinstructor or another qualified person is present.

5. Treat laboratory reagents as if they are poisonous and corrosive.

6. Dispense reagents carefully. Dispose of laboratory reagents as directed.

7. Do not eat, drink, use tobacco, or apply cosmetics in the laboratory.

8. Report all incidents to the laboratory instructor.

9. Be familiar with the location and use of all safety equipment.

10. Become familiar with each laboratory assignment before coming to thelaboratory.

11. Anticipate the common hazards that may be encountered in laboratory.

12. Become familiar with actions to be taken in the event of incidents inthe laboratory.

student signature date

laboratory instructor date

In the space below, give any health information, such as pregnancy or other circum-stance, that might help the laboratory instructor provide a safer environment for you,or that could aid the laboratory instructor in responding to an incident involvingyou in the laboratory.

1. I do/do not (circle one) expect to wear contact lenses during laboratorywork. [Note: Goggles must still be worn when contact lenses are worn.]

2. List any known allergies to medication or other chemicals.


Safety Quiz 11

name section date

Safety Quiz

1. On a separate sheet of paper, sketch the layout of the laboratory. (a) Note thelocation of each important safety feature (fire extinguisher, fume hood, eyewash, safety shower, and exits). (b) Draw a line from your work location,showing the path you would take to evacuate the laboratory. (c) Indicate thenearest location where you can activate the fire alarm.

2. Describe the steps to be taken in the event 10 mL of ethanol in a 50-mLbeaker ignites in the laboratory.

3. Identify two important reasons for notifying the laboratory instructor ofany incidents that occur in laboratory.

4. Why is unauthorized experimenting by a student in the laboratory notallowed?


5. Describe the steps to be taken in the laboratory if a large bottle ofacetone (noncorrosive, nontoxic, highly volatile, water-soluble, flamma-ble solvent) is broken and spilled.

6. According to the information in the MSDS (Figure 2), which hazardouscategory (health, fire, or reactivity) is of greatest concern for 1-propanol?

7. What is the first action to be taken in the event a person spills somereagent on himself or herself? What is the second action to be taken?

8. Identify three precautions to be taken before beginning the distillationof an organic liquid.


ISBN 0-87540-700-5


Using Exponential Notation andSignificant Figures

prepared by Norman E. Griswold , Nebraska Wesleyan University

Purpose of the Experiment

Review exponential notation and use it to solve problems with and without acalculator. Review rules for determining significant figures and use them toround off calculations.

Background Information

I. Exponential Notation

During your study of chemistry, you will encounternumbers ranging from the incredibly large to the ex-tremely small. For example, a 100-mL sample of watercontains more than 3 septillion molecules of water, or3,000,000,000,000,000,000,000,000 molecules. Eachwater molecule has a mass of approximately 30 septil-lionths of a gram, or 0.000 000 000 000 000 000 000 03grams. Representing very large or very small numbersthis way is awkward and time consuming. Conse-quently, we usually use exponential notation, some-times called scientific notation , to express suchnumbers.

A. Expressing Numbers Using ExponentialNotation

Exponential notation expresses numbers asthe product of two factors. The first factor, the digitterm , is a number between 1 and 10. The digit term is

multiplied by the second factor, called the exponentialterm , which has the form 10x, 10 raised to a specificwhole number power called the exponent .

For example, using exponential notation we rep-resent 126 as 1.26 × 102, which we read as “one pointtwo six times ten to the second”.As shown in Figure 1,the digit term in this expression is 1.26. This term in-cludes all the significant figures of the number beingrepresented. (We will review the rules for determiningsignificant figures in Part II of this module.)

1.26 × 102 exponent

digit term exponential term

Figure 1 Exponential notation

The exponential term in this example is 102. Apositive exponent represents the number of times thedigit term must be multiplied by 10 to give the num-ber represented. For example, 1.26 × 102 means1.26 × 10 × 10 = 126. Note that there are three figuresin the digit term and three figures in the number beingrepresented.

M I S C

490m o d u l a r · l a b o r a t o r y · p r o g r a m · i n · c h e m i s t r yprogram editor: Conrad L. Stanitski

Copyright © 1997 by Chemical Education Resources, Inc., P.O. Box 357, 220 S. Railroad, Palmyra, Pennsylvania 17078No part of this laboratory program may be reproduced or transmitted in any form or by any means, electronic or mechanical, including photo-copying, recording, or any information storage and retrieval system, without permission in writing from the publisher. Printed in the UnitedStates of America 00 99 98 97 — 15 14 13 12 11 10 9 8 7 6 5 4 3 2 1

Some additional examples of numbers ex-pressed in exponential notation are:

273.15 = 2.7315 × 102

0.08206 = 8.206 × 10–2

0.001 = 1 × 10–3

These examples show that, when expressed using ex-ponential notation, numbers greater than 10 have pos-itive exponents and numbers less than 1 have negativeexponents.

A negative exponent represents the number oftimes the digit term must be divided by 10 to give thenumber being represented.Thus, 2.46 × 10–3 means

2.46 × × × =110

110

110

0.00246

Another way to interpret the exponent is to saythat the exponent is equal to the number of places wemust move the decimal point in a number to convertthe number into the digit term. If the decimal pointmust be moved to the left , the exponent is positive.For example, the number 126 can be expressed as1.26 × 102. The decimal point (following the 6 in 126)must be moved two places to the left to give the digitterm, 1.26, so the exponent is a positive 2.

If the decimal point must be moved to the right ,the exponent is negative. As another example,0.00246 can be expressed as 2.46 × 10–3. The expo-nential term is 10–3, because the decimal point in0.00246 must be moved three places to the right togive the digit term, 2.46. We could also express0.00246 as 24.6 × 10–4, 246 × 10–5, or even as 0.246× 10–2.However, scientists usually keep the digit termbetween 1 and 10. For this example then, 2.46 × 10–3

is preferred, although the other expressions areacceptable.

B. Exponential Notation Using a Calculator

An electronic calculator is an important aid forperforming chemical calculations. Your calculatormay be slightly different from the one used for the fol-lowing examples. If so, use your calculator’s instruc-tion book when performing these tasks.

To use exponential notation with your calculator,it must have an exponent key, usually labeled

(or or on some models).

1. Entering Exponential Numbers on a Calcula-tor To enter 1.26 × 102 on a calculator with an

key, press the following keys in the order shown.

To enter 2.46 × 10–3 on a calculator with an key,press the following keys in order.

The key may be labeled for “change sign”.Some calculators can be set so that the answers

are automatically expressed in exponential notationon the display. If your calculator has the appropriatekeys, select the exponential notation mode by press-ing , then . Other calculators require differ-ent keystrokes to select the exponential notation mode.

The following example shows the different an-swers obtained using the normal mode and the expo-nential notation mode.

normal mode: (3.2 × 10–3) × (5 × 10–4) = 0.0000016

exponential notation mode: (3.2 × 10–3) × (5 × 10–4)= 1.6 × 10–6

2. Adding, Subtracting, Multiplying, and Di-viding Exponential Expressions In order to use acalculator to add, subtract, multiply, or divide expo-nential expressions, we use the keys , , , or

, which represent these operations, just as wewould when manipulating numbers in normal nota-tion. The only difference is that you must first selectexponential notation mode. For example, use the fol-lowing sequence of keystrokes to calculate (3.2 ×10–3) × (5 × 10–4).

= 1.6 × 10–6

3. Determining Square Roots and Cube Roots ofExponential Expressions To obtain square rootsof exponential numbers, remember that A A= 1 2/

and use the or key. For calculators with akey, use the following sequence of keystrokes to

find the square root of 2.7 × 1010.

= 1.6 × 105

The and keystrokes are used because ½ = 0.5.To obtain cube roots of exponential numbers, re-

member that A3 = A1/3 = A0.333, and use the key.For example, to take the cube root of 2.7 × 1010, usethe following sequence of keystrokes.

= 2.97 × 103

4. Taking Logarithms and Antilogs of ExponentialNumbers A logarithm is an exponent: It is the powerto which 10 must be raised in order to produce a givennumber. For example, 1.5 × 104 = 104.18. The givennumber is 1.5 × 104, and its logarithm is 4.18.The log-arithm of 1.5 × 104, written as log 1.5 × 104, can be de-termined by the following sequence of keystrokes.

= 4.18

2 MISC 490/Using Exponential Notation and Significant Figures

EXP EEX

EXP

1 2 6 EXP 2•

+/– CHS

2nd SCI

+ ×–÷

3 • 2 4EXP 3+/– × 5 EXP =+/–

EE

2

•

7 EXP 1 0 yx • 5 =

32 • 7 EXP 1 0 yx • 3 3 =

1 • 5 EXP 4 log

yxyx

yx

5

•

2 • 4 6 EXP 3

EXP

+/–

x

The reverse of obtaining (“taking”) a logarithm istaking the antilog. To take the antilog of 4.18, selectthe exponential notation mode on your calculator.Then use the following keystrokes to calculate antilog104.18.

= 1.5 × 104

If your calculator is not set for exponential notationmode, the answer will appear as 15135.612.

C. Using Exponential Numbers without aCalculator

1. Adding and Subtracting Exponential Num-bers To add or subtract exponential numbers with-out a calculator, the numbers must have the same ex-ponents.Consider the following example: (1.27 × 103)+ (4 × 101).One way to express these numbers so thatthey have identical exponents is to rewrite 4 × 101 as0.04 × 103.Moving the decimal point two places to theleft increases the exponent by two. The example thenbecomes (1.27 × 103) + (0.04 × 103). Next, add thedigit terms, the sum of which becomes the digit termof the answer. The final answer is the answer digitterm multiplied by the common exponential term, asshown.

1.27 × 103 1.27 × 103

+ 4 × 101 + 0.04 × 103

1.31 × 103

The following is the result if we rewrite 1.27 × 103,rather than 4 × 101, before adding.

1.27 × 103 127 × 101

+ 4 × 101 + 4 × 101

131 × 101 = 1.31 × 103

The answer is the same: It does not matter which num-ber we change before addition. However, the firstmethod directly gives the result in preferred exponen-tial form (only one digit to the left of the decimal point).

The rules for subtraction of exponential numbersare the same as for addition, except that the digitterms are subtracted rather than added. Here are twoexamples:

1.0 × 102 10 × 101

– 4 × 101 – 4 × 101

6 × 101

3.2 × 10–3 3.2 × 10–3

– 5 × 10–4 – 0.5 × 10–3

2.7 × 10–3

2. Multiplying Exponential Numbers Whenmultiplying or dividing exponential numbers without acalculator, it is not necessary for the numbers to haveidentical exponents. To multiply exponential num-bers, first multiply the digit terms. Then add the expo-nents to obtain the exponential term of the answer. Ageneral expression for this procedure is:

(A × 10n) (B × 10m) = (A × B) × 10n+m

Here are some specific examples:

(2 × 104)(4 × 102) = (2 × 4) × 104+2 = 8 × 106

(2 × 104)(4 × 10–2) = (2 × 4) × 104+(–2) = 8 × 102

(2 × 10–4)(4 × 10–2) = (2 × 4) × 10(–4)+(–2) = 8 × 10–6

Sometimes multiplying exponential numbers re-sults in an answer in which the exponential term is100. In such cases, remember that 100 = 1 exactly; theexponential term can be dropped from the answer.For example:

(2 × 104)(4 × 10–4) = (2 × 4) × 104+(–4) = 8 × 100 = 8

3. Dividing Exponential Numbers To divideexponential numbers, first divide the digit terms.Thensubtract the exponent of the denominator fromthe exponent of the numerator to obtain the exponentof the answer. A general expression of this procedureis:

A

B

AB

n

mn m×

×= × −10

1010

Some specific examples are:

6 10

3 10

63

10 2 104

24 2 2×

×= × = ×−

6 10

3 10

63

10 2 104

24 2 6×

×= × = ×

−− −( )

6 10

3 10

63

10 2 104

24 2 2×

×= × = ×

−

−− − − −( ) ( )

MISC 490/Using Exponential Notation and Significant Figures 3

1 0 yx • 1 84 =

Problem Set 1

(Use the spaces provided for the answers and additional paper if necessary.)

1. Some numbers of interest to chemists are givenbelow. Express each number in proper exponentialnotation.

(a) 96,485 C (the Faraday constant)

(b) 299,792,458 m/s (speed of light in avacuum)

(c) 0.0000000128 cm (radius of a metallic cop-per atom)

(d) 0.000001315 m (wavelength of an iodinelaser)

2. Solve the following problems and express youranswer in proper exponential notation. Try doing(a)–(d) without a calculator, first. Then do them with acalculator.

(a) (3.8 × 10–4) + (4.000 × 10–2) =

(b) (2.40 × 106) – (4 × 104) =

(c) (2.10 × 108)(3.00 × 10–14) =

(d)7 69 10

2 00 10

6

2

.

.

×

×=

−

(e)2 73 10

5 46 101 00 10

6

43.

.( . )

×

×

× =

−−

(f)2 40 10

1 20 103 48 10

6

48.

.( . )

×

×

+ × =

−−


name section date

II. Significant Figures

One of the first concepts taught in chemistry is den-sity , the mass of a substance divided by its volume.Suppose that, to help understand density, you areasked to determine the density of a metal sample asaccurately as possible. Using an analytical balance,you determine the mass of the assigned metal sampleas 14.3216 g. If its volume is 2.00 mL, what should youreport as the density of the metal?

When more than 100 students were asked thisquestion, they gave the following answers: 7.1608(most common), 7.160, 7.161, 7.16, 7.1, 7.2, and“about 7.” Are all these answers correct? If not, whichis correct? Would these answers have differed if themass had been reported as 14 g? How can you reportexperimental results in a way that indicates the exact-ness of the measurements involved? All these ques-tions can be resolved by using some simple rules todetermine the proper number of figures to use whenreporting a result obtained from measurements. Theproper number of figures to include are called signifi-cant figures or significant numbers .

The basic rule for determining significant figuresis: only those figures that are reasonably reliableare significant . The following sections describe howto determine which figures in a measurement arereasonably reliable and, therefore, are significantfigures.

A. Kinds of Experimental Values

Experimental values in chemistry consist of twobroad groups: exact numbers and inexact num-bers .The first group includes numbers that arise fromcounting or from certain definitions. For example, ifwe count the students in a chemistry class, we knowthe exact number of people in the class. Similarly,some numerical relationships are exact by definition.Such numbers can be thought of as having an infinitenumber of significant figures. Some examplesinclude:

1.000 L = 1000 mL1.000 cm = 1.00 × 107 nm

1.00 g = 1.00 × 10–3 kg

By definition, 1.000 liter is exactly equal to 1000 millili-ters. These examples are all conversions within agiven system of units, in this case, the metric system.

The second group, inexact numbers, consists ofnumbers resulting from measurements and approxi-mate conversion factors. The exactness of a mea-surement depends upon the measuring device. For

example, Figure 2 shows arrows positioned at identi-cal locations on three scales that differ only in thenumber of measuring marks. In Figure 2(a), the esti-mated position of the arrow is 6 or 7. A more exact po-sition cannot be obtained using the scale in Figure2(a). Figure 2(b) shows that the arrow is slightlycloser to 7 than to 6. Using the scale in Figure 2(b), wecan estimate that the arrow is at 6.5 or 6.6. The scalein Figure 2(c) makes it clear that the arrow is closer to6.6 than to 6.5. Using the scale in Figure 2(c), a rea-sonable estimate for the arrow position is about 6.58or 6.59. As you can see, the exactness of a measure-ment depends on the measuring device.

Figure 2 Examples of measurement usingscales of varying precision

Certain conversion factors are also inexact. Thissituation occurs when converting from one system ofunits to another system, such as converting from theEnglish system to the metric system. For example, bydefinition, the conversion of the mass unit called theEnglish pound to the metric kilogram is:

1.00 lb = 0.45359237 kg

However, a more common (but less exact) conversionfactor found in many tables is:

1 lb = 0.4536 kg

The number of significant figures in 0.45359237 and0.4536 is different. Rules for determining the correctnumber of significant figures to use when reporting ameasurement or calculation are given in the followingsection.

B. Determining the Number of SignificantFigures

A reasonably reliable measurement contains atleast one figure that is known with certainty, plus one


estimated figure to the right of the last known figure.In Figure 2(b), for example, a reasonably reliable esti-mate of the arrow position is 6.6, although the arrowcould be at either 6.5 or 6.7. In this case, the ones fig-ure is known with certainty, and the tenths figure isestimated. Therefore, based on Figure 2(b), the num-ber 6.6 contains two significant figures. If we were toreport 6.62 as the arrow position for Figure 2(b), thesecond estimated figure, 2, would not be significant.In a reasonably reliable estimate, only one estimatedfigure can be included among the significant figuresreported.

Similarly, for reported measurements or results,we assume that only the last numeral is estimated.Based on this assumption, it is not hard to determinethe number of significant figures in reported values.For example,

1.75 has 3 significant figures



The following two rules apply to correctly reportedvalues.

1. All nonzero numerals are counted as signifi-cant figures.

2. The position of the decimal point has no effecton the number of significant figures, as long as thenumber contains no zeros.

For numbers containing zeros, common sense isvery useful for determining the number of significantfigures. For example, 2.016 clearly contains four sig-nificant figures: The zero is in the middle of the num-ber, so it must be included. On the other hand, withsmall numbers like 0.08206 and large numbers like135,000, there can be some confusion about whetherzeros at the beginning or the end of a number shouldbe counted. The following rules apply to counting ze-ros as significant figures.

3. Zeros to the left of all nonzero numerals arenot significant.

This means that you start counting significant fig-ures at the nonzero numeral farthest to the left in thenumber, and count to the right. The following exam-ples illustrate this rule:

0.0821 contains 3 significant figures (startcounting at the 8 and count to the right)

0.002 has one significant figure (startcounting at the 2)

4. Zeros surrounded by nonzero numerals aresignificant.

The following examples illustrate this rule:



0.08206 has 4 significant figures (startcounting at the 8)

Note again that the position of the decimal point doesnot affect the number of significant figures.

5. Zeros to the right of all nonzero numerals,called trailing zeros ,may or may not be significant.

(a) If a decimal point appears in the number,all trailing zeros to the right of the decimal pointare significant. For example:

0.00640 has 3 significant figures (start counting atthe 6; the last zero is to the right of all the nonzero

digits and to the right of the decimal point;it is therefore significant)

75.0 has 3 significant figures (same reasoning)

1000.0 has 5 significant figures (all zeros aresignificant, because the last one is to the

right of the decimal point)

(b) If trailing zeros are all to the left of thedecimal point, then we must know more about thenumber to determine whether any of these zerosare significant. Sometimes a reasonable guess isnecessary. The following examples clarify this rule.

The number 1000 may contain from one to foursignificant figures. For example, if you lift an objectwith your hand and guess that it weighs about 1000 g,this is obviously not an exact measurement. In thiscase, the measurement and the number has only onesignificant figure, the 1. If you weigh the same objecton a balance that determines mass to the nearest 10g, then you can be reasonably certain of the first threefigures in the measurement. In this case, 1000 has 3significant figures. If you weigh the object on a bal-ance that determines mass to the nearest gram, thenall four figures in 1000 are significant.

As another example, the number 135,000 ozprobably represents an approximate measurement,so it likely has only 3 significant figures. The zerosprobably are there only to show the position of thedecimal point. In cases like this, we cannot be surewhether any of the zeros are significant until we knowsomething about the method of the measurement.

If we know the number of significant figures in135,000 oz, we can indicate this clearly by using


exponential notation to report the measurement num-ber. This is because the digit term in exponential no-tation contains only significant figures. For example,if we know that a measurement of 135,000 oz hasonly one significant figure, we can show this clearlyby expressing the number as 1 × 105 oz. If a muchmore exact measurement is made so that 135,000 ozhas four significant figures, then we can express themeasurement as 1.350 × 105. Thus, an importantuse of exponential notation is to clearly indicatethe number of significant figures in a reportedmeasurement.

C. Rounding Off Numbers

The next few sections explain how significant fig-ures are used in calculations. The general rule aboutrounding is that a calculated result can only be asreliable as the least precisely known measure-ment in the calculation . This rule makes it neces-sary to round off some numbers, that is, to drop cer-tain digits.

Conventions for rounding off numbers focus onthe digit farthest to the right of those that will be kept,the retained digit , and the next digit to the right, thedropped digit . Thus, if we round off 1.743 to 1.7, theretained digit is 7, and the dropped digit is 4. The fol-lowing examples show numbers being rounded off tothree significant figures.

1. If the dropped digit is less than 5, the retaineddigit remains unchanged.

For example:

1.634 rounds off to 1.63 (4 is less than 5,so the 3 remains unchanged)

1.6729 rounds off to 1.67 (2 is less than 5,so the 7 remains unchanged)

2. If the dropped digit i s a 5 followed by zeros orno digits, the retained digit remains unchanged if itis an even number and is increased by one if it isodd.

For example:

1.635 rounds off to 1.64 (5 with no followingdigits is dropped, 3 is odd, so the 3

is increased by 1 to 4)

1.625 rounds off to 1.62 (5 with no followingdigits is dropped, 2 is even, so the 2

remains unchanged)

1.07500 rounds off to 1.08 (5 followed by zerosis dropped, 7 is odd, so the 7 is increased to 8)

3. If the dropped digit is greater than 5 or is a 5followed by nonzero digits, the retained digit is in-creased by 1.

For example:

1.637 rounds off to 1.64 (7 is greater than 5,so 3 is increased to 4)

1.647 rounds off to 1.65 (7 is greater than 5,so 4 is increased to 5)

1.48533 rounds off to 1.49 (5 is followed bynonzero digits, so 8 is increased to 9)

D. Rounding Off Calculated Results

In Part C, we noted that a calculated result is onlyas reliable as the least precisely known measurementin the calculation. We use this rule to determine howmany digits to drop when rounding off a calculated re-sult. The type of calculation determines how the ruleis applied.

1. Rounding Off in Addition and SubtractionIn addition and subtraction, the least precisely

known factor will be the one with the smallest numberof decimal places. Therefore, the calculated resultmust have no more decimal places than the least pre-cisely known number being added or subtracted.

For example, suppose a solution contains 99.6 gof A, 31.62 g of B, and 9.765 g of C. What should bereported as the total mass of the solution? We solvethis problem as follows:

mass of A: 99.6 g → 99.6 gmass of B: 31.62 g → 31.6 gmass of C: 9.765 g → 9.8 g

total mass: 141.0 g

In other words, we round off all factors until there areno blank spaces in the right-hand column. When usinga calculator to do the above addition, we either roundoff before adding (which requires fewer keystrokes) orwe round off the result. In this case, if we use a calcula-tor to add the original numbers, the result is 140.985.We then round off this number to 141.0, whichmatches the result we obtain when we round off thenumbers before adding.

Rounding off in subtraction is done in the sameway as in addition. For example, suppose that abeaker containing a solution weighs 72.654 g, whilethe empty beaker has a mass of 59.6 g. What is themass of the solution?

mass of beaker and solution: 72.654 g → 72.7 gmass of beaker: 59.6 g → 59.6 g

mass of solution: 13.1 g


Again, if we use a calculator, we must either round offthe result or round off the factors first, as shown.

2. Rounding Off in Multiplication and DivisionIn multiplication and division, the result can be no

more reliable than the least precisely known factor.The least precisely known factor in a multiplication ordivision problem calculation is simply the factor withthe fewest significant figures, regardless of the posi-tion of the decimal point. The calculated result mustbe rounded off so that it contains no more significantfigures than does the least precisely known factor.

For example, if we use a calculator to multiply3.142 times 2.2 we get 6.9124. However, we shouldnot report 6.9124 as our result, because the factor 2.2contains only two significant figures. Therefore, thereported result can have only two significant figures,so 6.9124 must be rounded off to 6.9.

We can do some rounding off before multiplyingor dividing. This will decrease the number of key-strokes needed. First, find the factor with the fewestsignificant figures. Round off all other factors so theyhave one more significant figure than the least pre-cise factor. The calculated result will be the same as ifyou used the original factors and then rounded off theresult at the end.

For example, consider the density calculationdiscussed at the beginning of this part of theProcedure:

d =← ←

14 3216

2 00

6

3

.

.

( )

(

g

mL

significant figures

significant figures)

Using a calculator, we get the result 7.1608 g/mL,which must be rounded off to 7.16 g/mL (3 significantfigures). The reported results, 7.16 g/mL, has thesame number of significant figures as does the leastprecisely known factor, 2.00 mL.

To save keystrokes, we can round off the factors be-fore dividing. In this case, we can round off 14.3216 g

to four significant figures (14.32 g), one more than thethree significant figures in 2.00 mL. Then we can divideas follows:

14 32

2 00716

.

..

g

mLg / mL=

Both methods yield the same result.

E. Significant Figures in Logarithms

Several areas of chemistry use logarithms, whichhave two parts, the characteristic and the mantissa.The characteristic consists of the digits to the left ofthe decimal point.The mantissa consists of the digitsto the right of the decimal point. For example, log2578 = 3.4113. In the logarithm 3.4113, the charac-teristic is 3 and the mantissa is 4113.

One basic rule governs the number of significantfigures that should be reported in a logarithm: themantissa of a logarithm should have the samenumber of significant figures as does the originalnumber. Some examples are:

log 2 = 0.3 (1 significant figure in 2)

log 2.0 = 0.30 (2 significant figures in 2.0)

log 2.00 = 0.301 (3 significant figures in 2.00)

log 2.0 × 104 = 4.30 (2 significant figures in 2.0)

log 2.00 × 10–5 = –4.699 (3 significant figures in 2.00)

The rule also applies when determining antilogs.Some examples:

antilog 0.48 = 3.0 (2 significant figuresin the mantissa)

antilog 0.477 = 3.00 (3 significant figuresin the mantissa)

antilog 3.4771 = 3.000 × 103 (4 significant figuresin the mantissa)


Problem Set 2

(Use the spaces provided for the answers and additional paper if necessary.)

1. How many significant figures are contained ineach of the following numbers?

(a) 0.9463 _______________

(b) 0.08206 _______________

(c) 6.0225 × 1023 __________________

(d) 1.0 × 10–12 __________________

(e) 1010 _______________

2. Round off each of the following numbers to foursignificant figures.

(a) 273.15 _______________

(b) 12.652 _______________

(c) 19.9743 _______________

(d) 4.32156 _______________

(e) 0.019807 _______________

3. Complete the following calculations, and expresseach result using the proper number of significantfigures.

(a) 4.196 + 0.0725 + 14.3 =

(b) 74.321 – 4.2 =

(c) (8.2156 × 102) × (3.12) =

(d)6 042

7

. =

(e)0 98 0 230

0 08206 298

. .

.

××

=


name section date

ISBN 0-87540-490-1

Measuring the Melting Pointsof Compounds and Mixturesprepared by Joseph W. LeFevre, SUNY, Oswego

Measure the melting points of pure benzoic acid and pure mandelicacid. Determine the eutectic composition and the eutectic temperature ofbenzoic acid–mandelic acid mixtures. Identify an unknown compoundusing mixture melting points.

None

The melting point of a compound is the temperature at which the solidis in equilibrium with its liquid. A solid compound changes to a liquidwhen the molecules acquire enough energy to overcome the forces hold-ing them together in an orderly crystalline lattice. For most organic com-pounds, these intermolecular forces are relatively weak.

The melting point range is defined as the span of temperature fromthe point at which the crystals first begin to liquefy to the point at whichthe entire sample is liquid. Most pure organic compounds melt over anarrow temperature range of 1–2 °C.

The presence of a soluble impurity almost always causes a decreasein the melting point expected for the pure compound and a broadeningof the melting point range. In order to understand the effects of impuri-ties on melting point behavior, consider the melting point–mass percentcomposition diagram for two different fictitious organic compounds, Xand Y, shown in Figure 1. The vertical axis represents temperature andthe horizontal axis represents varying mass percent compositions of Xand Y.

Both compounds have sharp melting points. Compound X melts at150 °C, as shown on the left vertical axis, and Y melts at 148 °C, as shownon the right vertical axis. As compound X is added to pure Y, the melt-ing point of the mixture decreases along curve CB until a minimum tem-perature of 130 °C is reached. Point B corresponds to 40 mass percent Xand 60 mass percent Y and is called the eutectic composition for com-pounds X and Y. Here, both solid X and solid Y are in equilibrium withthe liquid. The eutectic temperature of 130 °C is the lowest possiblemelting point for a mixture of X and Y. At temperatures below 130 °C,mixtures of X and Y exist together only in solid form.

C E R · M o d u l a r · L a b o r a t o r y · P r o g r a m · i n · C h e m i s t r y

editor: Joe Jeffers

Copyright © 1997 by Chemical Education Resources, Inc., P.O. Box 357, 220 S. Railroad, Palmyra, Pennsylvania 17078No part of this laboratory program may be reproduced or transmitted in any form or by any means, electronic or mechanical, including photocopy-ing, recording, or any information storage and retrieval system, without permission in writing from the publisher. Printed in the United States ofAmerica. 00 99 98 97 — 15 14 13 12 11 10 9 8 7 6 5 4 3 2 1

TECH

701

PURPOSE OF THEEXPERIMENT

BACKGROUND REQUIRED

BACKGROUNDINFORMATION

Figure 1 Melting point–mass percentcomposition diagram for a two-component mixture

150

146

130

1000

8020

6040

4060

2080

010

tem

pera

ture

, °C

A

C

B

solid + solidX Y

liquid +solid X liquid +

solid Y

liquid + liquidX Y

mass %mass %

XY

Consider a 100-microgram (µg) mixture composed of 20 µg of Xand 80 µg of Y. In this mixture, X acts as an impurity in Y. As the mix-ture is heated, the temperature rises to the eutectic temperature of130 °C. At this temperature, X and Y begin to melt together at point B,the eutectic composition of 40 mass percent X and 60 mass percent Y.The temperature remains constant at 130 °C until all 20 µg of X melts.At the eutectic temperature, X and Y will melt in the ratio of 40 parts Xto 60 parts Y. If 20 µg of X melts, then 30 µg of Y (20 µg X × 60/40 ratio= 30 µg Y) also melts. At this point, the remaining 50 µg of solid Y (80µg – 30 µg = 50 µg) is in equilibrium with a molten mixture of the eutec-tic composition.

As more heat is applied to the mixture, the temperature begins torise, and the remaining Y begins to melt. Y continues to melt as the tem-perature increases, shown by curve BC.

Finally, at 142 °C, point C, where the liquid composition is 20 masspercent X and 80 mass percent Y, all of Y is melted. At temperatureshigher than 142 °C, liquid X and liquid Y exist together with a compo-sition of 20 mass percent X and 80 mass percent Y. Thus, the meltingpoint at which the entire mixture liquefies is 142 °C, six degrees lowerthan the melting point of pure Y. Also, the melting point range130–142 °C is quite broad.

In the previous example, X acts as an impurity in Y. Compound Ycan also act as an impurity in X, as indicated in Figure 1 earlier in thisexperiment. For example, in a mixture composed of 80 µg of X and 20µg of Y, the mixture begins to melt at the eutectic temperature of 130 °C.As before, at this temperature, the eutectic composition is 40 mass per-cent X and 60 mass percent Y. The temperature remains at 130 °C untilall 20 µg of Y melts. At the eutectic temperature, X and Y will melt inthe ratio of 40 parts X to 60 parts Y. Thus, if 20 µg of Y melts, 13 µg ofX (20 µg Y × 40/60 ratio = 13 µg X) also melts.

The remaining 67 µg of X (80 µg – 13 µg = 67 µg) melts over therange of 130–146 °C, shown by curve BA. At 146 °C, the last traces of Xmelt. This melting range is larger than the range over which 20 masspercent X and 80 mass percent Y melts.

If a mixture has exactly the eutectic composition of 40 mass percentX and 60 mass percent Y, the mixture shows a sharp melting point at130 °C. Observing this melting point could lead to the false conclusionthat the mixture is a pure compound. Addition of either pure X or pureY to the mixture causes an increase in the melting point, as indicated bycurve BA or BC, respectively. Observing this melting point increaseindicates that the original sample is not pure.

The initial melting that occurs at the eutectic temperature is some-times very difficult to observe. This difficulty is especially true if only asmall amount of an impurity is present, because the quantity of liquidproduced at the eutectic temperature is very small. However, the tem-perature at which the last trace of solid melts can be accurately meas-ured. Hence, a sample with a small amount of impurity will have anobserved melting point much higher than the eutectic temperature, butlower than that of the pure compound.

Because the melting point of a compound is a physical constant, themelting point can be helpful in determining the identity of an unknowncompound. A good correlation between the experimentally measuredmelting point of an unknown compound and the accepted melting point

2 TECH 701/Measuring the Melting Points of Compounds and Mixtures

1997 by Chemical Education Resources

of a known compound suggests that the compounds may be the same.However, many different compounds have the same melting point.

A mixture melting point is useful in confirming the identity of anunknown compound. A small portion of a known compound, whosemelting point is known from the chemical literature, is mixed with theunknown compound. If the melting point of the mixture is the same asthat of the known compound, then the known and the unknown com-pounds are most likely identical. A decrease in the melting point of themixture and a broadening of the melting point range indicates that thecompounds are different. A flowchart for using a mixture melting pointto identify an unknown compound is shown in Figure 2.

Melting points can also be used to assess compound purity. A melt-ing point range of 5 °C or more indicates that a compound is impure.Purification of the compound causes the melting point range to narrowand the melting point to increase. Repeated purification may be neces-sary before the melting point range narrows to 1–2 °C and reaches itsmaximum value, indicating that the compound is pure.

In practice, measuring the melting point of a crystalline compoundinvolves several steps. First, a finely powdered compound is packedinto a melting point capillary tube to a depth of 1–2 mm. Then the cap-illary tube containing the sample compound is inserted into one of sev-eral devices used to measure melting points.

Background Information 3


Figure 2 Flowchart for mixture melt-ing point determination of an unknown

Measuring Melting Points

Figure 3(a) shows the Thiele tube apparatus, filled to the base of theneck with silicone oil or mineral oil. The capillary tube is attached to athermometer so that the sample is located next to the middle of thethermometer bulb. The thermometer is inserted into the oil and thenthe side arm of the Thiele tube is heated with a Bunsen burner flame.

The Thomas–Hoover Uni-Melt device, shown in Figure 3(b), con-tains silicone oil that is stirred and heated electrically. Silicone oil can beheated to temperatures up to 250 °C. With this device, up to seven sam-ples can be analyzed at one time.

The Mel-Temp apparatus, shown in Figure 3(c), consists of an alu-minum block that is heated electrically. The aluminum block can beheated easily to temperatures up to 400 °C, and can tolerate tempera-tures up to 500 °C for brief time periods. A thermometer and up to threesamples can be inserted into the block at one time. A light and magni-fier permit easy viewing of the sample(s).

If the melting point of the compound is unknown, it is convenientto first measure the approximate melting point of the compound, calledthe orientation melting point. The sample is heated at a rate of10–15 °C per minute until it melts. Then the melting point apparatus iscooled to approximately 15 °C below the orientation melting point. Anew sample is heated, increasing the temperature at a much slower rateof 1–2 °C per minute, to accurately measure the melting point. A slowheating rate is necessary because heating a sample too rapidly maycause the thermometer reading to differ from the actual temperature ofthe heat source. The result would be an observed temperature readingthat differs from the actual melting point temperature.

If the melting point of the sample is known, the sample can bequickly heated to within 10–15 °C of its melting point. Then the heatingrate can be slowed to increase 1–2 °C per minute until the sample melts.



Figure 3 Different types of meltingpoint apparatus: (a) Thiele tube; (b)Thomas–Hoover; (c) Mel-Temp

Errors in observed melting points often occur due to a poor heattransfer rate from the heat source to the compound. One cause of a poorheat transfer rate is the placement of too much sample into the capillarytube. Finely ground particles of the compound are also necessary forgood heat transfer. If the particles are too coarse, they do not pack well,causing air pockets that slow heat transfer.

Sometimes slight changes, such as shrinking and sagging, occur inthe crystalline structure of the sample before melting occurs. Also,traces of solvent may be present due to insufficient drying and mayappear as droplets on the outside surface of the sample. This phenom-enon is called sweating and should not be mistaken for melting. Theinitial melting point temperature always corresponds to the firstappearance of liquid within the bulk of the sample itself.

Some compounds decompose at or near their melting points.This decomposition is usually characterized by a darkening in thecolor of the compound as it melts. If the decomposition and meltingoccur over a narrow temperature range of 1–2 °C, the melting pointis used for identification and as an indication of sample purity. Themelting point of such a compound is listed in the literature accom-panied by d or decomp. If the sample melts over a large temperaturerange with decomposition, the data cannot be used for identificationpurposes.

Some compounds pass directly from solid to vapor without goingthrough the liquid phase, a behavior called sublimation. When subli-mation occurs, the sample at the bottom of the capillary tube vaporizesand recrystallizes higher up in the capillary tube. A sealed capillarytube is used to take the melting point of a compound that sublimes ator below its melting point. The literature reports the melting point forthese compounds accompanied by s, sub, or subl.

In this experiment you will measure the melting points of benzoicacid, mandelic acid, and mixtures of these two compounds. Both com-pounds melt near 122 °C. You will use these data to construct a meltingpoint–mass percent composition diagram. From this diagram, you willestimate the eutectic temperature and eutectic composition for benzoicacid and mandelic acid. Finally, using the mixture melting pointmethod, you will identify an unknown compound.

Measuring the Melting Points of Compounds and Mixtures

Equipment

graph paper metric ruler (mm)marking pen microspatulamelting point capillary tubes 2 watch glasses

Reagents and Properties

molar masssubstance quantity (g/mol) mp (°C) bp (°C)

benzoic acid 10 mg 122.12 122–123 249mandelic acid 10 mg 152.15 120–122



Preview

• Measure the melting point of benzoic acid

• Measure the melting point of mandelic acid

• Measure the melting point range of four mixtures containing variousamounts of benzoic acid and mandelic acid

• Obtain a sample of an unknown compound

• Measure an orientation melting point and an accurate melting pointof your unknown compound

• Obtain a sample of each of two substances appearing in Table 1 thathave melting points similar to your unknown

• Prepare a mixture of your unknown compound and each of yourselected compounds

• Measure the melting point of each mixture

• Identify your unknown compound

Caution: Wear departmentally approved safety goggles at all timeswhile in the chemistry laboratory.

Always use caution in the laboratory. Many chemicals are poten-tially harmful. Prevent contact with your eyes, skin, and clothing.Avoid ingesting any of the reagents.

Caution: Benzoic acid is an irritant.

Place 2–3 mg of benzoic acid on a clean, dry watch glass. If the com-pound is not a fine powder, pulverize it using a microspatula.

Caution: Capillary tubes are fragile and easily broken.

Load a melting point capillary tube by pressing the open end of thetube into the powder. Pack the powder into the closed end of the tubeby tapping the closed end against the bench top. Repeat the cycle ofloading and packing until you can see 1–2 mm of benzoic acid throughthe tube. [NOTE 1]

To ensure good packing, drop the capillary tube with the open endup through a 1-m-long piece of glass tubing onto the bench top. Repeatseveral times. Place the capillary tube in the melting point apparatusprovided by your laboratory instructor.

Because pure benzoic acid melts at 122–123 °C, heat the capillarytube rapidly to 110 °C. Then slow the heating rate to 1–2 °C per min.[NOTE 2] Record the temperature at which liquid first appears in thebulk of the sample and the temperature at which the entire samplebecomes liquid.

Caution: The capillary tubes are hot. Allow them to cool enough toavoid burning your fingers.

When finished, remove the capillary tube. Place all used capillarytubes in the container labeled “Discarded Capillary Tubes”, providedby your laboratory instructor.



PROCEDURE

1. Measuring Melting Pointsof Benzoic Acid and

Mandelic Acid

NOTE 1: Make certain that no morethan 1–2 mm of compound is placed inthe capillary tube. A larger amount willgive a melting point range that is toolarge.

NOTE 2: Heating the capillary tubetoo quickly near the melting point willresult in an inaccurate melting pointmeasurement.

Obtain 2–3 mg of mandelic acid and measure the melting point fol-lowing the procedure described for benzoic acid. Pure mandelic acidmelts at 120–122 °C.

From your laboratory instructor, obtain four benzoic acid–mandelicacid mixtures of the following compositions:

percent percentbenzoic acid mandelic acid

mixture 1 80 20mixture 2 60 40mixture 3 40 60mixture 4 20 80

Using a marking pen, carefully label a capillary tube for each mix-ture. For example, near the top of the tube, mark the tube that will con-tain mixture 1 with one horizontal line. Similarly, mark the tubes formixtures 2–4 with two, three, and four lines, respectively. Load eachmixture into its capillary tube as previously described.

Place the capillaries containing mixtures 1 and 2 into the meltingpoint apparatus. [NOTE 3] Heat the samples rapidly to 80 °C. Then slowthe rate of increase to 1–2 °C per min. Carefully observe and record thetemperature at which the crystals first begin to melt and the tempera-ture at which the last trace of crystals melts. [NOTE 4]

Allow the apparatus to cool to 80 °C and repeat the melting pointmeasurements, using the capillaries containing mixtures 3 and 4.

Caution: Unknowns may be flammable, toxic, and irritating.

Obtain 10 mg of an unknown compound from your laboratory instruc-tor and record its identification code. Pulverize the sample, label andload a capillary tube, and take an orientation melting point. Cool theapparatus to 15 °C below its orientation melting point. Prepare a newsample, and accurately measure the melting point.

From Table 1 (on the next page), identify the two compounds thathave melting points closest to the melting point of your unknown com-pound. Obtain a few milligrams of each of these compounds. Place oneknown compound on a clean, dry, labeled watch glass. Add an approx-imately equal amount of your unknown compound.

Similarly, place the other known compound on a second watchglass and add an approximately equal amount of your unknown.Pulverize and mix each sample thoroughly, using a clean microspatulaeach time. Load the samples into separate, labeled capillary tubes. Also,load two capillary tubes with pure unknown.

Take the melting point of one of the mixtures and the pureunknown simultaneously. Quickly heat the samples to within 30 °C ofthe pure compound’s melting point. Then slow the heating rateincrease to 1–2 °C per min.

Repeat the procedure using the other mixture. Compare your dataand identify your unknown.

Procedure 7


2. Determining the EutecticTemperature and Composition

of a Benzoic Acid–Mandelic Acid Mixture

NOTE 3: If you are using a Thieletube, place the samples to the left andright of the thermometer bulb. Securethem in place with a small ring of rub-ber tubing, as shown in Figure 4. Makecertain the bottom of the capillary tubeis positioned vertically near the mid-point of the thermometer bulb. Also, becertain the rubber tubing and penmarks are 2–3 cm above the oil surfacebecause the oil expands when heated.

NOTE 4: If you are using a Mel-Tempapparatus, you will need to lift thesamples a few millimeters above thebase and slowly rotate the samples tosee the last trace of crystals melt. Becareful not to break the capillary tubes.

3. Identifying an UnknownCompound by Mixture

Melting Point

Figure 4 Attachment of two capillarytubes to a thermometer

compound mp (°C) compound mp (°C)

benzhydrol 65–67 trans-cinnamic acid 133–134

biphenyl 69–72 benzoin 135–137

phenanthrene 99–101 benzilic acid 150–153

o-toluic acid 103–105 adipic acid 152–154

acetanilide 113–115 benzanilide 164–166

fluorene 114–116 4-bromoacetanilide 167–169

(R,S)-mandelic acid 120–122 4-hydroxybenzoic 215–217acid

benzoic acid 122–123 anthracene 216–218

Use the labeled collection containers provided by your laboratoryinstructor. Wash your glassware with soap or detergent.

Caution: Wash your hands thoroughly with soap or detergent beforeleaving the laboratory.



4. Cleaning Up

Table 1 Melting points of com-pounds used as unknowns

1. Using the data from Parts 1 and 2 of the Procedure, plot on graphpaper the upper temperatures of the melting point ranges for benzoicacid and mandelic acid on the left and right vertical axes, respec-tively, as was done in Figure 1 for compounds X and Y. Plot theupper temperatures of the melting point ranges of the four mixtureson the same graph, using the proper mass percent of each com-pound on the horizontal axis. Use a temperature range of 80–130 °Con the vertical axis. From the graph, determine the approximateeutectic temperature and eutectic composition of a benzoicacid–mandelic acid mixture. [Note: Draw straight lines throughthe points, one straight line through the points for benzoic acid,mixture 1, and mixture 2; another straight line through the pointsfor mandelic acid, mixture 3, and mixture 4. Do not attempt to curvelines as shown in Figure 1.]

2. Using the melting point–mass percent composition diagram youdrew for Question 1, identify the approximate melting pointranges for benzoic acid–mandelic acid mixtures of the followingcompositions.

(a) 90:10

(b) 70:30

(c) 30:70

(d) 10:90

3. Describe in detail the melting point behavior of the 80:20 benzoicacid–mandelic acid mixture.

4. Devise a flowchart similar to the one in Figure 2 to show how youidentified your unknown.

5. Using your textbook or another appropriate resource, find the struc-tural formula for your unknown. Make a drawing of the formula.

6. Briefly explain why you were told to simultaneously measure themelting points of the mixtures and of the pure unknown in Part 3 ofthe Procedure.

Post-Laboratory Questions 9


Post-Laboratory Questions

NAME SECTION DATE

TECH 701/Measuring the Melting Points of Compounds and Mixtures

Pre-Laboratory Assignment

1. Briefly identify or explain

(a) two useful functions served by knowing the melting point of anorganic compound.

(b) why a finely powdered sample should be used in a melting pointmeasurement.

(c) why it is important to heat a sample slowly to obtain an accuratemelting point.

(d) two reasons why it is sometimes difficult to measure the temperatureat which the crystals first begin to liquefy.

(e) what two effects a soluble impurity usually has on the melting pointof a compound.

(f) what occurred when crystals began to disappear from the bottom ofthe capillary tube rather than turning to a liquid.

Pre-Laboratory Assignment 11


2. A sample has an experimental melting point of 100–101 °C. Can you con-clude that the sample is pure? Briefly explain your reasoning.

3. Using Figure 1, explain in detail the melting point behavior of a mixturecomposed of 60 mass percent X and 40 mass percent Y.

4. An unknown compound melted at 131–133 °C. It is thought to be one of thefollowing compounds (mp, °C): trans-cinnamic acid (133–134); benzamide(128–130); DL-malic acid (131–133); or benzoin (135–137). The mixture meltingpoints of the unknown compound with each of the test compounds are list-ed below. What is the unknown compound? Briefly explain your reasoning.

unknown plus mp range (°C)

trans-cinnamic acid 110–120

benzamide 130–132

DL-malic acid 114–124

benzoin 108–116

5. Using your textbook or another appropriate resource, find the structuralformula for benzoic acid and mandelic acid. Draw the structural formulasof these compounds.



ISBN 0-87540-701-3

Purifying Acetanilide byRecrystallizationprepared by Carl Wigal, Lebanon Valley College


Select an appropriate recrystallizing solvent. Separate and purifyacetanilide from a mixture by recrystallization. Compare the meltingpoints of impure and recrystallized acetanilide.

BACKGROUND REQUIRED You should know how to measure mass, in milligrams, and volume,in milliliters. You should know how to measure melting points.


Impurities often contaminate organic compounds that have beensynthesized in the laboratory or isolated from natural sources.Recrystallization is a purification process used to remove impuritiesfrom organic compounds that are solid at room temperature. Thisprocess is based on the premise that the solubility of a compound in asolvent increases with temperature. Conversely, the solubility of thecompound decreases as the solution cools, and crystals form.

Very pure compounds can be produced by recrystallization. As aheated solution of the desired compound cools, a small, pure seedcrystal of the compound forms in the solution. Layer by layer, addi-tional molecules attach to this crystal, forming a growing crystal lat-tice, as shown in Figure 1. The molecules in the crystal have a greateraffinity for other molecules of the same kind than they do for any im-purities present in the solution. In effect, the process of crystal forma-tion removes one kind of molecule from the solution.

TECH


Copyright © 1997 by Chemical Education Resources, Inc., P.O. Box 357, 220 S. Railroad, Palmyra, Pennsylvania 17078No part of this laboratory program may be reproduced or transmitted in any form or by any means, electronic or mechanical, including photo-copying, recording, or any information storage and retrieval system, without permission in writing from the publisher. Printed in the UnitedStates of America

00 99 — 15 14 13 12 11 10 9 8 7 6 5 4 3

Figure 1 (a) Identical molecules at-tach to one another, forming a crystallattice; (b) impurities have differentshapes or sizes and do not layer

Choosing a RecrystallizingSolvent

Selecting an appropriate recrystallizing solvent to use is probably themost difficult step of recrystallization. The primary consideration whenchoosing a recrystallizing solvent is the extent to which the compoundand impurities are soluble in the solvent at high and low temperatures.The graph in Figure 2 shows three possible scenarios for how the solu-bilities of the compound and the impurities depend on temperature.

Ideally, the compound to be recrystallized should be very solublein the chosen solvent at elevated temperatures, but almost insolublein the cold solvent, as shown by line A. Impurities should be solublein the chosen solvent at all temperatures so that impurities stay in so-lution, as shown by line B. Alternatively, impurities should be insolu-ble at all temperatures so they can be filtered from the hot solution, asshown by line C.

Experimentation is needed to select an appropriate recrystallizingsolvent. Typically, several solvents are used to test the extent of solubil-ity of the compound. A small amount of the compound is mixed with afew milliliters of each solvent. The compound’s solubility is observedat room temperature and near the solvent’s boiling point. If the com-pound is soluble in a solvent at room temperature, the solvent is notsuitable. If the compound is insoluble at room temperature and solublenear the solvent’s boiling point, the solvent is a suitable candidate.

“Insoluble” is a relative term. All compounds are soluble to someextent in every solvent. For example, benzoic acid in water has a solu-bility of 6.80 grams per 100 milliliters at 100 °C. However, benzoicacid has a solubility of only 0.34 gram per 100 milliliters in water at25 °C. Benzoic acid is typically listed as insoluble in 25 °C water.

When considering the solubility of an organic compound, a generalrule is like dissolves like. Polar organic molecules contain functionalgroups that can hydrogen bond, such as –OH, –NH2, and –CO2H.Polar molecules are generally most soluble in polar solvents. Manyorganic molecules are nonpolar. Nonpolar molecules are most solublein nonpolar solvents. A list of commonly used recrystallizationsolvents is shown in Table 1.

solvent bp (°C) solvent bp (°C)

water 100 ethyl ether 35methanol 65 dichloromethane 40ethanol (95%) 78 toluene 111acetone 56 petroleum ether 35–60ethyl acetate 77

The boiling point of the recrystallization solvent should be lowerthan the melting point of the compound to be recrystallized. If the sol-vent’s boiling point is higher than the compound’s melting point, thecompound will oil out. Oiling out occurs when a compound is insolu-ble in a solution at a temperature above the compound’s melting point.As a result, the compound is deposited as an oil, and not as crystals.

Another important criterion for selecting a recrystallizing solventrelates to recovery of the compound. An abundant quantity of crystalsmust be produced as the solution cools to room temperature or below.

The four major criteria for selecting a recrystallizing solvent aresummarized in Table 2.

2 TECH 703/Purifying Acetanilide by Recrystallization


Figure 2 Ideal solubility patterns of acompound, line A, and accompanyingimpurities, lines B and C, at varyingtemperatures

Table 1 Commonly used recrystall-ization solvents, in order of decreasingpolarity

(1) compound being purified must be insoluble insolvent at room temperature

(2) compound must be soluble in boiling solvent(3) solvent’s boiling point must be lower than the

compound’s melting point(4) an abundant quantity of crystals must be

recoverable from the cool solvent

Often, the requirements necessary for successful recrystallization arenot met by a single solvent. In these cases, a mixture of two solvents,called a solvent pair, is used. Two solvents are selected that are misciblewith each other, but have opposite abilities to dissolve the compound.The compound to be recrystallized should be soluble in one solvent (A)of the pair and should be relatively insoluble in the second solvent (B).

To determine the proper combinations of the two solvents, the com-pound is dissolved in a minimum volume of solvent A near the boilingtemperature of this solvent. Next, solvent B is added to the boiling mix-ture until the mixture becomes cloudy, indicating that the compound isprecipitating from solution. A few drops of solvent A are added to redis-solve the precipitate, producing a clear solution. Then the solvent pair istreated just like a single recrystallization solvent. Common solvent pairsare ethanol and water, acetone and ether, and acetic acid and water.

Dissolving the Compound Once a suitable solvent is found, the recrystallization process is contin-ued by dissolving the compound in a minimum volume of boiling sol-vent. Then a five percent excess of the solvent is added to the saturatedsolution to prevent premature crystallization. For example, if 10 mL of aboiling solvent is required to just dissolve a compound, five percent of10 mL or 0.5 mL would be added to bring the total volume to 10.5 mL.

Decolorizing the Solution Occasionally, a sample may contain a soluble impurity that produces acolored solution, and that solution colors crystals that would otherwisebe colorless. In that case, activated carbon, or decolorizing carbon, isused to remove these colored impurities from solution. Activated carbonhas a surface area that adsorbs dissolved organic substances. Adding anexcess of carbon must be avoided, because carbon can also adsorb thecompound that is being recrystallized, reducing the percent recovery.

The hot solution is filtered by gravity filtration through a funnelcontaining a fluted filter paper to remove any insoluble compound,including the carbon. If no undissolved impurities are present, or ifcarbon has not been added, the filtration step is omitted. A typicalgravity filtration apparatus is shown in Figure 3. The funnel, filterpaper, and collection flask are heated with boiling solvent prior tofiltering the solution to prevent premature crystal formation.

Using a fluted filter paper increases surface area inside the funneland speeds the filtering process. Figure 4 on the next page shows howto produce a fluted filter paper.

Recrystallizing PureCompound

After the compound is dissolved in a minimal amount of boiling solventand the solution is filtered, as necessary, the solution is allowed toslowly cool to room temperature. If crystal formation occurs too rapidly,impurities may become trapped in the crystals. Then the filtered solu-tion is cooled in an ice–water bath for a few minutes to maximize crystalformation. Crystals usually form as the solution temperature decreases.



Figure 3 A gravity filtration apparatusused to filter undissolved impurities

Table 2 Criteria for selecting a recrys-tallizing solvent

Sometimes, crystals do not form in the cooled solution. In thiscase, two methods can be used to induce crystallization. One methodinvolves scratching the inside of the flask with a glass stirring rod.The freshly scratched glass supplies sites for seed crystal formation.Alternatively, a seed crystal of the pure compound can be placed intothe solution to promote crystal growth.

Collecting, Washing, andDrying the Crystals

Vacuum filtration is the best method for separating the crystals fromthe mother liquor, or remaining solvent. A typical vacuum filtrationapparatus is shown in Figure 5 on the next page.

In vacuum filtration, a receiver flask with a sidearm, called a filterflask, is connected by heavy-walled vacuum tubing to a vacuumsource. A Büchner funnel is fitted to the filter flask with a rubberstopper or filter adapter.

The most common source of vacuum is a water aspirator. In a wa-ter aspirator, water moves past a small hole leading into a sidearmthat can be attached to a trap. A partial vacuum is created because ofthe reduced pressure at the point where the rapidly moving waterpasses the hole. At that point, air is pulled into the aspirator sidearm.This phenomenon is called the Bernoulli effect.

A trap can be used in tandem with a water aspirator to preventcontamination of the solution in the filter flask with water. Suddendrops in water pressure can cause water to be drawn into the filterflask. Fitting a trap between the filter flask and the aspirator preventsany reverse water flow from reaching the filter flask.

To recover the pure crystals, the perforated Büchner funnel plateis covered with a filter paper disk, which is moistened withrecrystallization solvent. With vacuum applied, the solution contain-ing the suspended crystals is poured onto the filter paper so that auniform thickness of crystals collects on the paper. After the motherliquor has been pulled through the filter, the crystals are washed withsmall portions of cold solvent. Then the crystals are dried and theirmass is measured.

Calculating Percent Recovery Percent recovery is calculated by dividing the mass of the recrystallizedcompound by the mass of the crude compound before recrystallization,as shown in Equation 1.



Figure 4 Folding a fluted filter paper

%recoverymass of recrystallized compound, g

mass o= ( )

f crude compound, g

100%

(Eq. 1)

Assessing Purity Purity of a recrystallized compound is assessed by observing its colorand by measuring its melting point range. If a compound is described inthe chemical literature as having white crystals, the recrystallized com-pound should appear white. If the compound has an off-white color, thecompound should again be recrystallized using activated carbon.

A pure compound melts over a narrow range of 1–3 °C near itsreported melting point. If a dry recrystallized compound has a meltingpoint range of four degrees or more, it should be recrystallized again.

Purifying Acetanilide by Recrystallization

Equipment

2 beakers, 100-mL microspatula250-mL beaker† Pasteur pipet, with latex bulbBüchner funnel, with filter paper sand bath*2 graduated Erlenmeyer flasks, screw clamp

25-mL stirring rod, glass11-cm fluted filter paper 2 support stands125-mL filter flask, 5 test tubes, 13 × 100-mm

with 1-hole stopper 2 utility clampsshort-stem filter funnel vacuum trap10-mL graduated cylinder 250-mL filter flask25-mL graduated cylinder 2-hole stopperhot plate 2 pieces glass or plastic tubinglabels vacuum tubing*or crystallizing dish on electric hot plate or electric heating well with heat controller†for ice bath



Figure 5 A typical vacuum filtrationapparatus


substance quantity molar mass (g/mol) mp (°C) bp (°C)

acetanilide 1 g 135.17 113–115acetone 2 mL 58.08 56carbon, activated 60 mgethanol 2 mL 46.07 78petroleum ether 2 mL * 35–60*mixture of hydrocarbons

Preview

• Check solubility of acetanilide in four solvents• Choose a recrystallizing solvent• Weigh the acetanilide• Dissolve the acetanilide in the hot recrystallizing solvent• Add activated carbon to remove dissolved impurities and filter the

hot solution• Recrystallize the pure acetanilide• Collect the crystals of acetanilide• Wash, dry, and weigh the crystals• Measure the melting points of crude and recrystallized acetanilide

PROCEDURE Caution: Wear departmentally approved safety goggles at alltimes while in the chemistry laboratory.

Always use caution in the laboratory. Many chemicals are po-tentially harmful. Prevent contact with your eyes, skin, and cloth-ing. Avoid ingesting any of the reagents.

1. Choosing a RecrystallizingSolvent

Caution: Acetanilide is toxic and irritating. Acetone and ethanolare flammable and irritating. Petroleum ether is flammable andtoxic. Use these compounds in a fume hood.

Label four 13 × 100-mm test tubes “acetone”, “water”, “ethanol”, and“petroleum ether”. Place approximately 100 mg of acetanilide intoeach test tube. Use a microspatula to pulverize the acetanilide.[NOTE 1] Place 2.0 mL of the appropriate solvent into each test tube.Thoroughly stir each mixture. Record whether the acetanilide issoluble or insoluble in each solvent at room temperature.

Caution: Heated test tubes containing solvent boil over easily. Becareful to avoid burns from the hot solvent.

Select the test tube(s) containing the solvent(s) in which acetanilidedid not dissolve at room temperature. Using a sand bath, heat themixture(s) to boiling. Record whether acetanilide is soluble or insolu-ble in each hot solvent.

Allow the heated solvent(s) to cool slowly to room temperature.Prepare an ice–water bath by half filling a 250-mL beaker with equalvolumes of ice and water. Place the tube(s) into the bath for 5 min, andobserve whether recrystallization occurs. Record your observations.



NOTE 1: Lumps of acetanilide may beslow to dissolve, interfering with thecorrect solvent selection.

Based on your observations, choose an appropriate solvent fromwhich to recrystallize acetanilide. Consult your laboratory instructorconcerning your solvent choice before proceeding to Part 2. Place thesolvents in the test tubes into appropriate containers labeled“Recovered Acetone”, “Recovered Ethanol”, “Recovered Water”, and“Recovered Petroleum Ether”, provided by your laboratory instructor.

2. Dissolving the Compound Weigh 500 mg of acetanilide and place it into a 25-mL Erlenmeyerflask. Place 15 mL of the appropriate recrystallizing solvent into asecond 25-mL Erlenmeyer flask. Add a boiling chip. Using a hot plate,heat the solvent to boiling.

Using beaker tongs, pick up the hot flask containing the boiling sol-vent. Use a Pasteur pipet to add 0.5–1 mL of boiling solvent to the flaskcontaining the acetanilide. Swirl the flask with each addition. Keep thesolvent in both flasks at boiling by placing the flasks on the hot plate.Continue the solvent additions until the acetanilide just dissolves.

Using beaker tongs, remove the flasks from the hot plate. Allow theacetanilide solution to cool below the solvent boiling point. Observethe solution color. Record your observations. Measure and recordyour solvent volume.

Calculate the additional solvent volume needed to have a 5% ex-cess. Measure and add that solvent volume to the acetanilide flask.

3. Decolorizing the Solution Caution: Activated carbon is an irritant. Prevent eye, skin, andclothing contact. Avoid inhaling dust and ingesting the carbon. Donot add carbon to a boiling solution. This addition will cause thesolution to boil over and burn your skin. Also, do not boil a solutioncontaining carbon too vigorously, or the solution may boil over.

Assemble a gravity filtration apparatus, as shown in Figure 3 earlier inthis module. Weigh 60 mg of activated carbon. Conduct the Procedure inParts A and B simultaneously. [NOTE 2]

A. Heating the Gravity Filtration Apparatus

Place 20 mL of the recrystallizing solvent into a 100-mL beaker. Adda boiling chip. Heat the solvent to boiling on a hot plate. Usingbeaker tongs, pick up the hot beaker containing the boiling solvent.Preheat the filtration apparatus by pouring the solvent through thefunnel containing a fluted filter paper. Do not allow the boiling chipto go into the funnel. Collect the solvent in another beaker. Place thegravity filtration apparatus on the hot plate to keep the solvent hot.

B. Adding the Activated Carbon

At the same time, add the 60 mg of activated carbon to the Erlenmeyerflask with the acetanilide solution. Reheat the solution to boiling.When you have completed Parts A and B, pour the boiling sol-

vent from the filtration apparatus beaker into the other 100-mLbeaker. While the gravity filtration apparatus is still hot from therecrystallizing solvent, filter the boiling solution containing the car-bon through the gravity filtration apparatus. Collect the liquid inthe 25-mL receiving flask. Observe the color of the filtered solution.Record your observations.

Procedure 7


NOTE 2: So that crystals will not formin the funnel, plan to filter the boilingsolution from Part B using the filter ap-paratus from Part A while the filter ap-paratus is still hot.


4. Recrystallizing PureAcetanilide

Allow the decolorized solution containing the acetanilide to cool toroom temperature. When the solution has reached room temperature,place the Erlenmeyer flask into an ice–water bath for 5 min to com-plete the crystallization.

5. Collecting, Washing, andDrying the Crystals

While the solvent and solution are cooling in the ice bath, assemblea vacuum filtration apparatus as shown in Figure 5 earlier in thismodule, using a 125-mL filter flask. Also prepare a washing solventby placing 5 mL of the recrystallizing solvent into a test tube. Cool thetube and its contents in the ice–water bath.

Weigh a filter paper and record its mass. Once crystallization iscomplete, turn on the water to the aspirator, and moisten the filter pa-per with a few drops of recrystallizing solvent. Swirl the flask con-taining the acetanilide, and pour the crystals and mother liquor intothe Büchner funnel, using a glass rod to direct the crystals to the mid-dle of the filter paper.

After the mother liquor has been pulled into the filter flask, releasethe vacuum by loosening the screw clamp on the trap. Remove theBüchner funnel from the filter flask and pour the mother liquor into abeaker. Tighten the screw clamp and reattach the Büchner funnel.

Use 4–5 mL portions of the mother liquor to rinse the remainingcrystals of acetanilide from the Erlenmeyer flask. Pour the rinses intothe Büchner funnel.

Wash the crystals in the Büchner funnel with the coldrecrystallizing solvent. Allow the crystals to dry by pulling airthrough the funnel for 10 min. Then disconnect the vacuum tubingand turn off the aspirator. Remove the filter paper and crystals. Dis-assemble the filtration apparatus.

Weigh your dried crystals and filter paper, and record the mass.Observe the color and shape of the crystals and record yourobservations.

Measure and record the melting point ranges of both crude andrecrystallized acetanilide. If your laboratory instructor directs you todo so, place your crystals into a labeled sample vial to turn in.

6. Cleaning Up Use the labeled collection containers provided by your laboratoryinstructor. Clean your glassware with soap or detergent.

Caution: Wash your hands thoroughly with soap or detergentbefore leaving the laboratory.


Post-Laboratory Questions 1. The solubility of benzoic acid in water is 6.80 g per 100 mL at 100 °Cand 0.34 g per 100 mL at 25 °C.(a) Calculate the minimum volume of water needed to dissolve 1.00g of benzoic acid at 100 °C.(b) Calculate the maximum theoretical percent recovery from therecrystallization of 1.00 g of benzoic acid from 15 mL of water,assuming the solution is filtered at 25 °C.

2. The solubility of acetanilide in your recrystallizing solvent is 5.0 mgper mL at 10 °C.(a) Calculate the maximum percent recovery in this experiment,assuming a 15.0-mL recrystallizing solution is filtered at 10 °C.(b) Calculate the percent recovery of the acetanilide produced inyour experiment.(c) How do your results compare to the maximum percent recov-ery? Briefly explain.

3. A student rushed through this experiment. Describe the effect thatthe following procedural changes would have on the percent recov-ery of acetanilide. Briefly explain the basis of each answer.(a) Rather than adding 0.5-mL portions of boiling solvent to theacetanilide, the student added 5-mL portions of boiling solvent.(b) The student did not pre-heat the gravity filtration apparatus inPart 3.(c) The student forgot to cool 5 mL of solvent in Part 5 and washedthe crystals with room-temperature solvent.




NAME SECTION DATE

TECH 703/Purifying Acetanilide by Recrystallization


1. Briefly explain why(a) you should not heat organic solvents over a Bunsen burner flame.

(b) you should add activated carbon to a cool solution and then heat themixture to boiling rather than add the carbon to a boiling solution.

2. Indicate a procedure to solve the following recrystallization problems.(a) oiling out

(b) lack of crystal formation

(c) presence of colored impurities

(d) premature recrystallization in the funnel stem during gravity filtration



3. Compound A, a white crystalline solid with a melting point of 75 °C, has thesolubility profile shown in the following table. Which of the solvents listedwould be a good recrystallizing solvent for Compound A? Briefly explain.The boiling points for these solvents are shown in Table 1 earlier in thismodule.

Compound A solubility profilesolubility at

solvent solubility at 25 °C boiling point

water I Smethanol I Sacetone S Sethyl ether S S

4. A student purified a 500-mg sample of phthalic acid by recrystallizationfrom water. The published solubility of phthalic acid in 100 mL of water is0.54 g at 14 °C and 18 g at 99 °C.(a) What is the smallest volume of boiling water the student could use todissolve 500 mg of phthalic acid?

Dissolution of phthalic acid in boiling water produced a dark-coloredsolution. The student allowed the solution to cool, added several spatulasfull of activated carbon, and heated the mixture to boiling. After gravityfiltration, the clear and colorless solution was allowed to cool to roomtemperature. Crystals formed, and the student isolated 380 mg of phthalicacid.(b) Calculate the percent recovery of phthalic acid in this experiment.

(c) Suggest one or more procedural errors the student made that could beresponsible for some loss of phthalic acid.


ISBN 0-87540-703-X

Separating Acids andNeutral Compoundsby Solvent Extractionprepared by Jerry Manion, University of Central Arkansas


Use solvent extraction techniques to separate a mixture consisting ofa carboxylic acid, a phenol, and a neutral compound.

EXPERIMENTAL OPTIONS Microscale Extraction . . . . . . . . . . . . . . . . . . . . . . . . . . . 5Macroscale Extraction . . . . . . . . . . . . . . . . . . . . . . . . . . 10

BACKGROUND REQUIRED You should be familiar with the experimental techniques used to de-termine melting points, to test for acidity using pH paper, and to sep-arate a solid from a solution using vacuum filtration. You shouldknow how to speed solvent evaporation using air or nitrogen.


Frequently, organic chemists must separate an organic compoundfrom a mixture of compounds, often derived from natural sources oras products of synthetic reactions. One technique used to separate themixture compounds is called extraction. Extraction is a process thatselectively dissolves one or more of the mixture compounds into anappropriate solvent. The solution of these dissolved compounds isoften referred to as the extract.

Extraction processes include removal of soluble compounds from asolid matrix, such as occurs in brewing coffee or tea or in decaffein-ating coffee with liquid carbon dioxide. In the organic chemistrylaboratory, however, extraction almost always refers to the transfer ofcompounds from one liquid solvent to another liquid solvent.

A compound can be separated from impurities in a solution byextracting the compound from the original or first solvent into asecond solvent. The compound must be more soluble in the secondsolvent than in the first solvent, and the impurities must be insolublein the second solvent.

Also, to effect the extraction, the two solvents selected must be im-miscible, or not soluble in one another, so that they produce twoseparate solvent layers. After dissolving the mixture in the first solvent,

TECH


Copyright © 1997 by Chemical Education Resources, Inc., P.O. Box 357, 220 S. Railroad, Palmyra, Pennsylvania 17078No part of this laboratory program may be reproduced or transmitted in any form or by any means, electronic or mechanical, including photo-copying, recording, or any information storage and retrieval system, without permission in writing from the publisher. Printed in the UnitedStates of America 00 99 98 — 15 14 13 12 11 10 9 8 7 6 5 4 3 2

the solution is added to the second solvent. The two layers are vigor-ously mixed to maximize the surface area between them. This mixingfacilitates the transfer of a dissolved compound from one layer toanother. Once the transfer process is complete, the layers are againallowed to form, as shown in Figure 1. Separation of the two layers thencompletes the separation of the desired compound from the impurities.

Washing is the reverse process, in which the impurities are re-moved to the second solvent, leaving the desired compound in theoriginal solvent, as shown in Figure 2.

Selecting theAppropriate Scale

The amount of compound to be extracted determines whethermacroscale or microscale techniques should be employed for theextraction. The chemical principles associated with the extractions areidentical, but the techniques are somewhat different.

Extractions using larger quantities of solvents, tens or hundredsof milliliters, require a separatory funnel, as shown in Figure 3. The sol-vent layers are mixed by shaking the separatory funnel. Then thelayers are allowed to reform. The bottom layer is drained through thestopcock; the top layer is poured from the top of the separatory funnel.

Microscale extractions can be conducted using a test tube or a cen-trifuge tube. Mixing and separating the layers can be done using aPasteur pipet.

Choosing Solvents The first requirement in the extraction process is to select two immis-cible solvents. One solvent, usually water, should be polar in nature.The second solvent should be nonpolar and might be a hydrocarbon,an ether, or a chlorinated solvent, such as dichloromethane. When thetwo immiscible solvents are placed into a container, two liquid layersresult. The more dense solvent is always the bottom layer.

It is important to identify the solvent in each layer. Hydrocarbonsand ethers are less dense than water or the dilute aqueous solutionsused in extractions. When one of these nonpolar solvents is used, thewater layer is the bottom layer, as shown in Figure 4.

However, dichloromethane is more dense than water. Whendichloromethane is used as the nonpolar solvent, the water layer willbe the top layer, as shown in Figure 5.

2 TECH 705/Separating Acids and Neutral Compounds by Solvent Extraction


Figure 1 Extraction occurs whenthe desired compound changes lay-ers, leaving impurities behind

Figure 2 Washing occurs when im-purities change layers, leaving the de-sired compound behind

Although the identity of each layer can be established from the den-sity of each solvent, their identities should be confirmed. To confirmthe identities of the layers, one or two drops of water are introducedjust below the surface of the top layer. If the drops of water mix with thetop layer, then the top layer is the water layer. If the drops of water fallthrough the top layer to the layer below, then the water layer is thebottom one. It is a good practice to save all layers in labeled containers untilthe experiment is complete and the desired product is isolated.

Often the two solvents will not completely separate after shaking,due to the formation of an emulsion at the interface between them.An emulsion is a suspension of small droplets of one liquid in an-other liquid. Emulsions are generally opaque or cloudy in appearanceand are often mistaken as a third layer.

The small size of the droplets in an emulsion causes the separation ofthe two solvents to take place very slowly. Several procedures may behelpful to facilitate this separation. For example, gentle swirling of thecontainer, addition of a few drops of saturated aqueous sodium chloride(NaCl) or ethanol, or addition of more solvent to dilute the solutionsmay help. In particularly difficult cases, it may be necessary to filter themixture to remove small solid particles that promote emulsion formation.

A simple, but useful, guide to solubility is like dissolves like. That is,nonpolar compounds, including most organic compounds, are moresoluble in nonpolar solvents than in polar solvents. On the otherhand, ionic and polar compounds are more soluble in polar solvents,such as water. These solubility differences can be exploited to sepa-rate nonpolar compounds from ionic or polar compounds.

For example, synthetic reactions often produce ionic, inorganic saltsas by-products of the desired nonpolar organic product. In such cases,these salts are removed by washing the nonpolar solvent with water.The organic compound remains dissolved in the nonpolar solvent.

Some organic compounds are sufficiently polar to be quite solublein water. Extraction of such polar compounds into a nonpolar solventis often difficult. The process can be facilitated by using the techniquecalled salting out. Inorganic salts, such as NaCl, are dissolved in wa-ter to reduce the solubility of the organic compound in the aqueouslayer. Under these conditions, the organic compound preferentiallydissolves in the nonpolar layer.

Extraction is a particularly effective means of separating organic com-pounds if one compound in the mixture can be chemically converted toan ionic form. The ionic form is soluble in an aqueous layer and can beextracted into it. Other non-ionized organic compounds in the mixturewill remain dissolved in the nonpolar solvent layer. Separation of thetwo layers results in the separation of the dissolved compounds.

Ionic forms of some organic compounds can be produced by react-ing them with aqueous acids or bases (see Figure 6 on the next page).Reacting organic acids with bases such as sodium hydroxide (NaOH)converts these acids to water-soluble anions. Reacting basic amineswith dilute aqueous acid solutions such as hydrochloric acid (HCl)converts the amines to water-soluble cations.

The extent to which an acid-base reaction proceeds to comple-tion depends upon the relative acidity and basicity of the reactantsand products. Reactions occur so that stronger acids and bases react toproduce weaker conjugate bases and acids. Recall that the pKa is a



Figure 4 Ethers and hydrocarbonsare less dense than water

Figure 3 A separatory funnel used formacroscale extractions

water

dichloromethane

Figure 5 Dichloromethane is moredense than water

measure of the acidity of an acid, as shown in Equation 1.

pKa = –log Ka (Eq. 1)

Stronger acids have smaller pKas and their conjugate bases are inher-ently weaker. The position of an acid–base equilibrium can then bepredicted from a knowledge of the pKas of the acids involved.Stronger acids, those with a smaller pKa, will react with the conjugatebases of weaker acids, those with a larger pKa.

An analysis of Figure 7 indicates that aqueous NaOH can be used toextract both p-toluic acid and p-tert-butylphenol from a nonpolar sol-vent, as shown in Equations 2 and 3. The stronger base, OH–, removesa hydrogen ion, H+, from p-toluic acid to form the salt, p-toluate. Thepolar salt is soluble in aqueous solution. Both OH– and p-toluate arebases. The pKa of 16 indicates that OH– is a stronger base than p-toluate,with a pKa of 4.2. The stronger base takes H+ from the weaker base.

4 TECH 705/Separating Acids and Neutral Compounds by Solvent Extraction


H3OH2O

H2O

H2CO3HCO3

H2O orOH

–

– or

HC

O

O– C

O

O

–OH –OC4H9O

HC4H9

NH3CH3NH2CH3

+

+

+

(Eq. 3)

(Eq. 4)

(Eq. 2)

+ HCO3

+ HCO3

NO REACTION

(Eq. 5)–OH

+ OH–

–

–

CH3

–OC4H9

CH3 C

O

OC

O

CH3

OHC4H9

OHC4H9

CH3 C

O

OH –C

O

O

+ H2CO3

+ H2O

pKa = 16

+ H2O

pKa = 16

pKa = 6.4p-toluic acid (pKa = 4.2)

(pKa = 10.2)p-tert-butylphenol

p-tert-butylphenol (pKa = 10.2)

p-toluic acid (pKa = 4.2) p-toluate anion

p-tert-butylphenoxide anion

p-toluate anion

+ OH–

Figure 6 Organic compounds can beconverted to ionic forms by reactionswith aqueous solutions of acid or base

Figure 7 The position of an acid–base equilibrium is determined by the relative acidity of the reactant acid and the product acid

Similarly, OH– is a stronger base than p-tert-butylphenoxide ion,with a pKa of 10.2. So OH– takes H+ from p-tert-butylphenol to formthe water soluble p-tert-butylphenoxide ion.

Sodium hydrogen carbonate (NaHCO3), with a pKa of 6.4, is aweaker base than p-tert-butylphenoxide ion, so HCO3

– will not takeH+ from p-tert-butylphenol, as shown in Equation 4. As a result,p-tert-butylphenol is not converted to a salt in aqueous sodium hy-drogen carbonate and does not become water soluble.

Although aqueous NaHCO3 is not sufficiently basic to react withp-tert-butylphenol, NaHCO3 will react with p-toluic acid to form thewater soluble p-toluate, as shown in Equation 5.

The p-toluic acid and the p-tert-butylphenol can be recovered byadding HCl to the aqueous solutions. The p-toluate and p-tert-butylphenoxide ions are stronger bases than is Cl–, so each one takesH+ from HCl. The acid forms are not water soluble, so they precipitatefrom solution.

The procedure you will use in this experiment exploits the differ-ences in these reactions to separate p-toluic acid and p-tert-butylphenolfrom the nonpolar solvent in which they are dissolved. First, you willextract only p-toluic acid into NaHCO3 solution. Then, you will extractp-tert-butylphenol into NaOH solution. Next, you will add HCl to eachof the extracts to precipitate the water-insoluble p-toluic acid andp-tert-butylphenol. You will isolate the precipitates from the solutionsby vacuum filtration, then air dry them. A flowchart for the separationsis shown in Figure 8 on the next page.

A third compound, acetanilide, does not react with either NaOH orNaHCO3 and remains dissolved in the nonpolar solvent. To recoveracetanilide, you will dry the nonpolar layer with anhydrous sodiumsulfate (Na2SO4) and evaporate the solvent in a fume hood. You willrecrystallize the acetanilide in an ice bath.

After you dry the compounds, you will measure the mass of eachisolated compound. Finally, you will measure the melting point ofeach compound and assess its purity by comparing the experimen-tally measured melting point with the literature value.

Microscale Extraction

Equipment

3 beakers, 50-mL hot plate2 beakers, 250-mL* melting point capillary tubes15-mL centrifuge tube, 5 Pasteur pipets, with latex bulb

with plastic cap pH test paperfilter paper, to fit sand bath†

Hirsch filter funnel thermometer, –10 to 110 °Cglass stirring rod 2 watch glasses10-mL graduated cylinder weighing paperHirsch filter funnel, with

50-mL filter flask and gasket*one for the ice bath, the other to support the centrifuge tube†sand in crystallizing dish on electric hot plate or sand in electric heating well with heatcontroller

Microscale Extraction 5


Parikh

Rectangle

Experimental Procedure:

Part A: Extraction of a Colored Polar Organic Solid from Water

1. Obtain 10 mL of 0.0001 % aqueous crystal violet and place it in your separatory funnel. 2. Extract the aqueous solution with either 10 mL of methylene chloride or two 5.0 mL portions of methylene

chloride (as assigned by your instructor). 3. Collect your methylene chloride layer or layers in a test tube. (How do you know which is the aqueous

layer and which is the methylene chloride layer?) If they appear cloudy, add some anhydrous magnesium sulfate to clear them.

4. Compare the color of your methylene chloride solution to that of a classmate who did the reverse of you in step 2. (If you cannot tell the difference between your two solutions, use a spectrophotometer set at 575 nm to compare their absorbances.) Record your observations.

5. Examine the colors of the aqueous layers that you and your classmate obtained. Record your observations.

Part B: Washing an Organic Solid Contaminated with an Colored Impurity

1. Obtain a 30 mg sample of fluorene (a white solid) that has been contaminated with crystal violet. 2. Dissolve it in 25 mL of tert-butyl methyl ether and wash the ether layer with two 10 mL portions of water. 3. Record your observations. 4. Obtain a 50 mg sample of fluorene that has been contaminated with fluorenone (a yellow solid). 5. Dissolve it in 25 mL of tert-butyl methyl ether and wash the ether layer with two 10 mL portions of water. 6. Record your observations. Part C: Extraction of a Non-Polar Organic Liquid from Water

1. Place 5.0 g of pentadecane and 25 mL of water in your separatory funnel. 2. Shake the separatory funnel to mix the layers. 3. Allow the layers to separate. 4. Drain out the aqueous layer and collect the pentadecane in a tared container. (How do you know which

layer is which?) If any water droplets are present in the organic layer, remove them with a pasteur pipet. 5. Determine the yield and refractive index of the pentadecane and submit it in a labeled vial.

6. Rinse your separatory funnel out with acetone and repeat steps 1-3, except first dissolve the pentadecane in 15 mL of tert-butyl methyl ether.

7. Drain out the aqueous layer and collect the organic layer in a dry container. Do or skip step 8, as assigned by your instructor:

8. Dry the organic layer with anhydrous magnesium sulfate. Filter the solution to remove the drying agent and collect the filtrate in a tared container.

9. Evaporate the ether from the organic layer using an explosion proof hot plate. (How will you know when all the ether has evaporated?)

10. Determine the yield and refractive index of the product and submit it in a labelled vial. Obtain the results of a classmate who did the reverse of you for step 8.

11. Correct all refractive indexes to 20oC. Waste Disposal:

All aqueous phases go in an aqueous waste container. Methylene chloride solutions must be disposed of in a separate waste container for halogenated waste. All other organic solutions should go with regular organic waste. Discussion Questions:

Part A

1. Was one method of extraction more efficient than the other? Explain. 2. Was all of the crystal violet removed by each extraction? Explain.

Part B

1. How did the results for fluorenone differ from those of crystal violet? 2. Can the structural differences between these three molecules account for your results? Explain. 3. What general conclusion can you make as to what types of compounds can or cannot be washed away

when one is trying to purify a contaminated organic product? Part C

1. What conditions resulted in the highest yield of extracted material? 2. Why do you think this is the case? 3. What conditions resulted in the highest purity of extracted material? 4. Describe significant sources of error that could affect the results of this experiment

Separating Cyclohexane andToluene by Distillationprepared by Jerry Manion, University of Central Arkansas


Separate two miscible liquids, either by macroscale or microscale pro-cess, using simple and fractional distillation. Compare the efficienciesof simple and fractional distillation.

EXPERIMENTAL OPTIONS Macroscale Distillation. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5Microscale Distillations

A. Using Glassware with Elastomeric Connectors . . . . . . . . . . . . 7B. Using the Hickman Still. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10C. Using Test Tube Reflux . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 12

BACKGROUND REQUIRED You should be familiar with basic laboratory techniques for measur-ing volumes of chemical compounds. You should know how to pre-pare a bent-tip Pasteur pipet for microscale distillations. You shouldknow how to use a refractometer to measure refractive index.


Distillation is a technique widely used in organic chemistry for sepa-rating compounds based on differences in their boiling points. Manyorganic compounds are volatile; that is, they have relatively high va-por pressures and low boiling points. During distillation, such vola-tile compounds are heated to boiling in one container, called the pot.The vapors produced are then cooled and reliquefied by passingthem through a water-cooled condenser, and collected in a separatecontainer, called the receiver. This technique can be used to remove avolatile solvent from a nonvolatile product; to separate a volatileproduct from nonvolatile impurities; or to separate two or more vola-tile products that have sufficiently different boiling points.

When a liquid is placed in a closed container, some of the mole-cules evaporate into any unoccupied space in the container. Evapora-tion, which occurs at temperatures below the boiling point of a com-pound, involves the transition from liquid to vapor of only thosemolecules at the liquid surface. Evaporation continues until an equi-librium is reached between molecules entering and leaving the liquidand vapor states. The pressure exerted by these gaseous molecules onthe walls of the container is the equilibrium vapor pressure. Themagnitude of this vapor pressure depends on the physical character-istics of the compound and increases as temperature increases.

If the liquid is heated to its boiling point, quite a different phenom-enon occurs. The boiling point is the temperature at which the vapor

TECH



pressure of the liquid is equal to the external pressure applied to thesurface of the liquid. This external pressure is commonly atmosphericpressure. At the boiling point, bubbles of vapor are produced through-out the liquid, and the vapor pressure inside the bubbles is sufficientlyhigh to allow them to grow in size. The escape of these bubbles resultsin the characteristic chaotic motion of the liquid identified as boiling.

Liquid is converted to vapor more rapidly by boiling than by evapo-ration. If the heating rate is increased, the temperature of the boilingliquid does not change, but the rate at which vapor is produced fromthe liquid increases. This increase occurs because the energy that issupplied by the increased heating rate is absorbed as more liquid mole-cules overcome intermolecular interactions and enter the vapor phase.

When a mixture of two or more volatile compounds is heated, thevapor pressure of the mixture equals the sum of the vapor pressures ofeach compound in the mixture. The magnitude of the vapor pressureexerted by each compound is determined by the vapor pressure of thatcompound (P0) and the mole fraction of that compound present in themixture (X). For an ideal two-compound solution, the solution vaporpressure is expressed by Raoult’s law, shown in Equation 1.

PT = X1P10 + X2P2

0 (Eq. 1)

In this equation, PT is the total vapor pressure of the solution, P10 is

the vapor pressure of pure compound 1, X1 is the mole fraction ofcompound 1, P2

0 is the vapor pressure of pure compound 2, and X2 isthe mole fraction of compound 2.

When two liquids form a homogeneous solution, they are said tobe miscible. Such a homogeneous mixture will boil at a temperaturebetween the boiling points of the pure compounds. The exact boilingpoint of the mixture depends upon the relative amounts of thecompounds present. Figure 1 shows the relationship between boilingpoint and composition for a two-compound mixture of cyclohexaneand toluene.

When vapor is produced from such a liquid mixture, the composi-tion of the vapor mixture is different from the composition of the liq-uid mixture from which it forms, as shown in Figure 2. The vapor con-tains a larger percent of the more volatile compound of the mixture, inthis case cyclohexane. For example, a liquid composed of 50 percentcyclohexane and 50 percent toluene would boil at 90 °C and yield avapor composed of 70 percent cyclohexane and 30 percent toluene.

This composition change that accompanies the vaporization pro-cess is the basis for the separation of mixtures by distillation. As thevapors produced by the distillation move into the water-cooledcondenser, these vapors condense to a liquid, the distillate, which hasthe same composition as the vapor from which it is formed. Thedistillate collected in the receiver will contain more of the more vola-tile compound than was present in the original mixture.

If one compound is much more volatile than the other, the com-pounds can be separated in one vaporization step. Such a step iscalled simple distillation and uses an apparatus that consists of onlya pot, a distilling head, a condenser, an adapter, and a receiver, asshown in Figure 3.

When the boiling points of two compounds differ by less than40 °C, they cannot be separated by simple distillation. Fractional dis-tillation, a process that has the effect of many simple distillations,must be used. A fractional distillation apparatus includes a fraction-ating column placed between the pot and the distilling head, asshown in Figure 4. Typically, any one of a variety of materials, includ-ing glass beads and metal sponge, fill the fractionating column.

2 TECH 704/Separating Cyclohexane and Toluene by Distillation


Figure 1 The boiling point of a misci-ble mixture is between the boiling pointsof the pure compounds

Figure 2 Vaporizing a mixture of cy-clohexane and toluene produces a va-por that is enriched in cyclohexane

The vapors generated in the pot rise up the fractionating columnand encounter cooler surfaces, upon which they condense. The con-densed liquid is then reheated by rising hot vapors and revaporizes.This process of condensation and revaporization, shown graphically inFigure 5, may occur again and again as the vapors rise up the column.

Each vaporization is represented by a horizontal line connecting theliquid composition curve to the vapor composition curve. Each conden-sation is represented by a vertical line connecting the vapor curve to theliquid curve. For example, the 50:50 liquid mixture (A) vaporizes toproduce a 30:70 vapor mixture (A’). The 30:70 vapor mixture condensesto a 30:70 liquid mixture (B). The 30:70 liquid mixture, in turn, vaporizesto produce a 15™:85 vapor mixture (B’), and so on. Each condensa-tion–revaporization results in an increase in the concentration of the morevolatile compound. These composition changes are reflected by a decreasein boiling temperature as the mixture moves up the fractionating column.If the condensation–revaporization is repeated a sufficient number oftimes, the vapors of the more volatile compound reach the top of thefractionating column in a pure form. As these vapors move into thecondenser, the compound condenses and is collected as a liquid.

At the same time, the less volatile compound is enriched in theopposite direction. As the condensed liquid falls toward the pot, thepot gradually contains a higher and higher percent of the less volatilecompound. Thus, a separation of the two compounds is achieved.

Each condensation and revaporization that occurs on a fraction-ating column is called a theoretical plate. A fractionating columnwith a large number of theoretical plates accomplishes many conden-sation–revaporization steps and very efficiently separates the com-pounds in a mixture.

The fractionating column must be positioned vertically so thatcondensed liquid can percolate down through the rising hot vapors.This percolation promotes equilibrium between the liquid and vaporphases, a condition that allows the column to operate at maximumefficiency and provide an optimum separation.



Figure 3 An apparatus for macroscale simple distillation Figure 4 An apparatus for macroscale fractional distillation

Figure 5 Each condensation and reva-porization increases the concentrationof the more volatile compound

An equally important factor affecting separation of the compoundsis the distillation rate. If the distillation is conducted too rapidly,liquid–vapor equilibria will not be established in the fractionatingcolumn, and poor separation of the compounds will result.

As the liquid boils, a condensation line of vapor can be observedas it moves up the distilling head. Once these vapors reach the ther-mometer bulb, a dramatic temperature increase is observed.The tem-perature of the vapors in the distilling head provides information re-garding the progress of the distillation. Initially, the vapors are rich inthe more volatile compound, and the observed temperature is close tothe boiling point of that compound. In a distillation with an efficientseparation, the initial temperature remains relatively constant untilall of that compound is collected. After the compound with the lowerboiling point is completely distilled, the temperature rises sharply asthe vapors of the higher-boiling compound reach the thermometerbulb. At this time, the boiling point of the higher-boiling compound isobserved as it distills into the receiver.

When no fractionating column is used, or when the fractionatingcolumn is inefficient, mixtures of the distilled compounds are incom-pletely separated. This inefficiency is indicated by a very gradualincrease in the temperature measured during the distillation. Samplescollected at temperatures between the boiling points of the two com-pounds will consist of mixtures of the two compounds. A comparisonof the results of simple and fractional distillation is shown in Figure 6.

Microscale Distillation Distillation is a difficult organic laboratory technique to use whenseparating microscale volumes, because significant amounts of distil-late are commonly left adhering to the glass surfaces of the apparatus.However, specialized equipment has been designed to permit thesimple distillation of volumes less than one milliliter. One suchapparatus, the Hickman still, is shown in Figure 10 later in this module.Another apparatus for microscale distillations uses special glasswarewith elastomeric connectors, as shown in Figure 8 later in this module.Microscale distillations may also be conducted in a test tube using aPasteur pipet as a condenser and receiver.

Microscale distillations are especially useful when small volumesof a liquid must be purified for spectral or refractive index analyses.The relative amounts of cyclohexane and toluene present in a samplemay be determined by measuring the refractive index of the sample.Figure 7 shows a graph that correlates the refractive index of mixturesof cyclohexane and toluene with their composition.

Refractive index measurements are typically reported at 20 °C. Arefractive index measured at a temperature higher or lower than 20 °Cmust be corrected to 20 °C. To make this correction, Equation 2 is used,where n20 is the refractive index at 20 °C, nT is the refractive index at themeasured temperature, and T is the measured temperature.

n20 = n T + 0.00045(T – 20 °C) (Eq. 2)

For example, if the refractive index of a cyclohexane–toluene mix-ture is measured as 1.4752 at 26 °C, then the refractive index at 20 °Ccan be calculated:

n20 = 1.4752 + 0.00045(26 – 20 °C) = 1.4779

Locating the point on the graph in Figure 7 corresponding to n20 =1.4779 indicates that the sample contains 74 percent toluene and 26percent cyclohexane.



Figure 6 Distillation curves for simpleand fractional distillation

Figure 7 A correlation curve relatingrefractive index to the composition ofcyclohexane–toluene mixtures

Macroscale Distillation

Equipment

aluminum foil 2 support ringsboiling chips 2 support standscopper metal sponge* 2 utility clampselectric flask heater, with regulator wire gauze, ceramic center*50-mL graduated cylinderstandard taper glassware:

condenser, with adapter and rubber tubingdistilling headfractionating column*100-mL round-bottom flask–10 to 260 °C thermometer, with adapter†

*for fractional distillation†adapter may be required to hold thermometer in place


substance quantity (mL) mol mass (g/mol) bp (°C)

cyclohexane 25 84.16 80.7toluene 25 92.14 110.6

Preview

• Assemble macroscale simple distillation apparatus• Place cyclohexane–toluene mixture in pot• Distill the mixture, recording the temperature at 5-mL intervals• Assemble macroscale fractional distillation apparatus and repeat

as for simple distillation


Always use caution in the laboratory. Many chemicals are po-tentially harmful. Prevent contact with your eyes, skin, andclothing. Avoid ingesting any of the reagents.

General Considerations Exercise care when assembling a distillation apparatus. Support theflask heater with a support ring attached to a support stand so that theheater can be quickly lowered away from the apparatus, if necessary.

Use a utility clamp to attach the neck of the pot to the support standto support the apparatus in the event you remove the flask heater.Support the condenser with a second clamp and support stand.

Carefully adjust the angle of the clamp supporting the condenser.Lubricate the joints by using your finger to apply stopcock greaselightly along the interior joint section. Rotate the joint after connectionto distribute the grease uniformly. Check the joints immediately beforebeginning the distillation, and reconnect any joints that are loose.

Select a pot size so that the pot is one-half to two-thirds full of liq-uid. Add two boiling chips to the pot. [NOTE 1]

Insert the thermometer into a thermometer adapter so that the top ofthe thermometer bulb is even with or slightly below the bottom of theside arm on the distilling head, as shown in Figure 3 earlier in thismodule. [NOTE 2]

Macroscale Distillation 5


NOTE 1: Overfilling the pot can resultin bumping or foaming of material intothe receiver. Boiling chips provide asurface on which vapor bubbles canform. This bubble formation helpsprevent superheating and bumping ofthe liquid.

NOTE 2: Carefully positioning thethermometer ensures that the bulb issubmerged in any vapors that passthrough the distilling head and thatthe vapor temperature is measuredaccurately.

Use rubber tubing to attach the condenser to a water tap and todischarge water from the condenser to the drain. [NOTE 3]

Place the end of the adapter inside the receiver to minimize therelease of vapors into the room.

Caution: Heating a closed apparatus can cause the apparatus torupture. Make certain the distillation apparatus has an openingto the atmosphere. Do not heat a closed container.

Discontinue the distillation before all of the liquid is gonefrom the pot. Some organic compounds explode when heated todryness. Do not distill to dryness.

1. Conducting SimpleDistillation

Caution: Cyclohexane is flammable and irritating. Toluene isflammable and toxic. If possible, use a fume hood.

Do not add boiling chips to a hot liquid. The large surfacearea of the boiling chip can cause the hot liquid to foam out ofthe apparatus and cause burns.

Assemble the simple distillation apparatus shown in Figure 3 earlier inthis module, using a 100-mL round-bottom flask for the pot and a50-mL graduated cylinder for the receiver. Place two boiling chips and25 mL each of cyclohexane and toluene into the pot, taking care not tospill any chemicals onto the flask heater. Start the flow of waterthrough the condenser. Check the apparatus and reconnect any jointsthat are loose.

Heat the mixture to boiling. [NOTE 4] Adjust the heater to producedistillate at a rate that is no greater than one drop per s. Record thetemperature when you collect the first drop of distillate and again af-ter every 5 mL of distillate you collect. Continue the distillation untilthe temperature reaches 110 °C or until fewer than 5 mL of liquid re-mains in the pot.

Turn off the heater and lower it away from the pot. Allow the pot tocool for a few minutes. Then turn off the water to the condenser.

2. Conducting FractionalDistillation

Caution: Cyclohexane is flammable and irritating. Toluene is flam-mable and toxic. If possible, use a fume hood.

Do not add boiling chips to a hot liquid. The large surfacearea of the boiling chip can cause the hot liquid to foam out ofthe apparatus and cause burns.

Assemble the fractional distillation apparatus shown in Figure 4 earlierin this module, using a 100-mL round-bottom flask for the pot and a50-mL graduated cylinder for the receiver. Pack the fractionating col-umn with copper metal sponge, as directed by your laboratory in-structor. [NOTE 5]

Place two boiling chips and 25 mL each of cyclohexane and tolueneinto the pot. Start the water flow through the condenser. Check theapparatus and reconnect any joints that are loose.

Heat the mixture in the pot to boiling. Observe the condensationline as it moves up the fractionating column.

When the vapors reach the top of the column packing, reduce theheating rate so the vapor condensation line remains just above thecolumn packing and below the side arm of the distilling head. Main-tain the vapor condensation line in this position for 5 min to allow thevapor and liquid in the column to reach equilibrium.

Wrap the fractionating column and distilling head with aluminumfoil to minimize the temperature fluctuations during the distillation.Then adjust the heating rate to produce distillate at a rate no greaterthan 1 drop per s.



NOTE 3: Water should enter the con-denser at the bottom and exit from thetop so that no air remains in the cool-ing jacket. A moderate flow of water issufficient for cooling.

NOTE 4: As the liquid boils, watch forthe condensation line of vapor as itmoves up the distilling head. To ob-serve and record an accurate tempera-ture reading, the entire thermometer bulbmust be immersed in vapor.

NOTE 5: Be careful to position thefractionating column vertically to pro-mote mixing of the liquid and vaporphases. The fractionating column looksmuch like a condenser, but has indenta-tions in the inner jacket to support thecolumn packing. Be careful; these inden-tations are easily broken. The outerjacket insulates against heat loss fromthe inner jacket during distillation. Donot pass water through the fractionatingcolumn.

Record the temperature when you collect the first drop of distillateand again after every 5 mL of distillate you collect. Continue the dis-tillation until the temperature reaches 110 °C or until fewer than 5 mLof liquid remains in the pot.

Turn off the heater and lower it from the pot. Allow the pot to coolfor a few minutes. Then turn off the water to the condenser.



Microscale DistillationsA. Using Glassware with Elastomeric Connectors

Equipment

aluminum foil microspatula100-mL beaker 2 receiver vials, 5-mL,boiling chips with screw capscopper metal sponge* sand bath†

glassware with elastomeric 2 support ringsconnectors support stand

5-mL boiling flask –10 to 260 °C thermometer,distilling head with with adapter

air condenser 2 utility clampsdistilling column* wire gauze, ceramic center

*for fractional distillation†sand in crystallizing dish on electric hot plate or sand in electric heating well with heatcontroller



cyclohexane 1.5 84.16 80.7toluene 1.5 92.14 110.6

Preview

• Assemble microscale simple distillation apparatus• Place cyclohexane–toluene mixture in pot• Distill the mixture• Collect the distillate, recording the temperature as a function of the

number of drops• Assemble microscale fractional distillation apparatus and repeat as

for simple distillation

Using Glassware with Elastomeric Connectors 7




General Considerations Exercise care when assembling a distillation apparatus. Support theflask heater with a support ring attached to a support stand so thatthe heater can be quickly lowered away from the apparatus, ifnecessary.

Use a utility clamp to attach the neck of the pot to the support standto support the apparatus in the event you remove the flask heater.

Select a pot size so that the pot is one-half to two-thirds full of liq-uid. Add a boiling chip to the pot. [NOTE 1]

Insert the thermometer into a thermometer adapter so that the topof the thermometer bulb is even with or slightly below the bottom ofthe side arm on the distilling head–condenser, as shown in Figure 8.[NOTE 2]

Place the end of the distilling head–condenser side arm inside thereceiver to minimize the release of vapors into the room. Supportthe ice-filled beaker with wire gauze on a support ring.

Caution: Heating a closed apparatus can cause the apparatus torupture. Make certain the distillation apparatus has an openingto the atmosphere. Do not heat a closed container.

Discontinue the distillation before all of the liquid is gonefrom the pot. Some organic compounds explode when heated todryness. Do not distill to dryness.





NOTE 1: Overfilling the pot can resultin bumping or foaming of material intothe receiver. A boiling chip provides asurface on which vapor bubbles canform. This bubble formation helps pre-vent superheating and bumping of theliquid.

Figure 8 An apparatus using elastomeric connectors for Figure 9 An apparatus using elastomeric connectors formicroscale simple distillation microscale fractional distillation

NOTE 2: Carefully positioning thethermometer ensures that the bulb issubmerged in any vapors that passthrough the distilling head and thatthe vapor temperature is measuredaccurately.

Assemble the simple distillation apparatus as shown in Figure 8,using a 5-mL boiling flask for the pot and a 5-mL vial for the receiver.Place a boiling chip and 1.5 mL each of cyclohexane and toluene intothe pot.

Position the thermometer bulb below the side arm of the distillinghead–condenser, and place the end of the air condenser as deeply aspossible into the receiver. Place the receiver into a 100-mL beaker andsurround the receiver with ice. Check the apparatus and reconnectany joints that are loose.

Heat the mixture to boiling. Adjust the heating rate by using a spat-ula to move the hot sand either around or away from the pot. Controlthe heating rate to produce distillate at a rate of about 2–4 drops permin. [NOTE 3]

Read and record the temperature when you collect the first drop ofdistillate and again after every 5 drops of distillate you collect. Con-tinue the distillation until the temperature remains constant at 110 °Cor until the pot is almost dry. Discontinue the heating before all of themixture distills and the pot becomes completely dry. Lower the heateraway from the pot.



Assemble the fractional distillation apparatus shown in Figure 9, us-ing a 5-mL boiling flask for the pot and a 5-mL vial for the receiver.Place a boiling chip and 1.5 mL each of cyclohexane and toluene intothe pot. Tightly pack the fractionating column with copper metalsponge.

Position the thermometer bulb below the side arm of the distillinghead–condenser, and place the end of the air condenser as deeply aspossible into the receiver. Place the receiver into a 100-mL beaker andsurround the receiver with ice. Check the apparatus and reconnectany joints that are loose.

Heat the mixture to boiling. Observe the condensation line as itmoves up the fractionating column. When the vapors reach the top ofthe column packing, reduce the heating rate so the vapor condensa-tion line remains just above the column packing and below the sidearm of the distilling head. Maintain the vapor condensation line inthis position for about 5 min to allow the vapor and liquid in the col-umn to reach equilibrium.

Wrap the fractionating column and distilling head with aluminumfoil to minimize the temperature fluctuations during the distillation.Then adjust the heating rate to produce distillate at a rate of about2–4 drops per min.

Read and record the temperature when you collect the first drop ofdistillate and again after every 5 drops of distillate you collect. Con-tinue the distillation until the temperature remains constant at 110 °Cor until the pot is almost dry. Discontinue the heating before the boil-ing flask becomes completely dry, and lower the heater away from thepot.





NOTE 3: As the liquid boils, watch forthe condensation line of vapor as itmoves up the distilling head. To ob-serve and record an accurate tempera-ture reading, the entire thermometer bulbmust be immersed in vapor.

Microscale DistillationsB. Using the Hickman Still

Equipment

boiling chips support stand2 conical vials, 3-mL –10 to 150 °C thermometer,Hickman still small size to fit Hickman stillmagnetic spinning band* –10 to 260 °C thermometer,microburner for sand bathmicrospatula tongs3 Pasteur pipets, with latex bulb 3 utility clampssand bath† 6 vials, 2-mL, with screw caps*for fractional distillation†sand in crystallizing dish on electric hot plate or sand in electric heating well with heatcontroller




Preview

• Prepare a bent-tip Pasteur pipet• Assemble Hickman apparatus and add cyclohexane–toluene mixture• Conduct the distillation, collecting samples from 80–90 °C,

90–100 °C, and 100–110 °C• Repeat the Procedure using a Teflon spinning band for fractional

distillation• Determine the percent composition of the samples, using refractive

index




1. Conducting a SimpleDistillation

Prepare a bent-tip Pasteur pipet by heating the pipet in a microburnerflame. Use tongs to bend the pipet to a 30° angle 1 cm from the tip. [NOTE 1]

Transfer 1.0 mL each of cyclohexane and toluene into a 3-mL conicalvial, and add a small boiling chip. Attach the Hickman still head andclamp the apparatus vertically in a sand bath, as shown in Figure 10.

Place a thermometer through the center opening of the still head sothat the thermometer bulb is positioned as shown in Figure 10. Raisethe sand-bath temperature to about 90 °C. Then gradually increasethe sand-bath temperature at a rate of 2 °C per min. Collect the mate-rial that distills when the Hickman still thermometer registers80–90 °C. Using a bent-tip Pasteur pipet, remove the distillate thatcondenses in the collar of the still head. Transfer the distillate to anappropriately labeled sample vial. [NOTE 2]



NOTE 1: A standard Pasteur pipet canbe used in a Hickman still model thathas a built-in side port.

NOTE 2: Cyclohexane and tolueneare quite volatile. Cap the vials to en-sure that the small samples do notevaporate.

Collect a second sample that distills in the range 90–100 °C and athird sample in the range 100–110 °C.


Transfer 1.0 mL each of cyclohexane and toluene into a 3-mL conical vial.Attach the Hickman still head containing a magnetic spinning band, andclamp the apparatus vertically in a sand bath, as shown in Figure 11.

Place a thermometer through the center opening of the still head sothat the thermometer bulb is positioned, as shown in Figure 11. Raisethe sand-bath temperature to 90 °C. When the mixture begins to boil,turn on the magnetic stirrer to a low setting to start the spinning band.Then gradually increase the sand-bath temperature at a rate of 2 °Cper min. As the vapor enters the bottom of the still column, increasethe spinning band rate to a middle range setting. Once liquid beginsto collect in the collar of the still, increase the spinning band rate tothe maximum setting.

Collect the material that distills when the Hickman still thermometerregisters in the range 80–90 °C. Using a bent-tip Pasteur pipet, removethe distillate that condenses in the collar of the still head. Transfer thedistillate to an appropriately labeled sample vial. [NOTE 3]

Collect a second sample that distills in the range 90–100 °C and athird that distills in the range 100–110 °C.

3. Measuring Refractive Index Using a refractometer, measure the refractive index of the compoundsin each vial. Correct the refractive indices for temperature, usingEquation 2.

Using the correlation curve shown in Figure 7 earlier in this experi-ment and the corrected refractive index for the solution in each collec-tion vial, determine the percent of cyclohexane and toluene in eachsample.

Using the Hickman Still 11


Figure 10 A Hickman still assembly for simple distillation Figure 11 A Hickman still assembly for fractional distillation

NOTE 3: Cyclohexane and tolueneare quite volatile. Cap the vials to en-sure that the small samples do notevaporate.



Microscale Distillations

C. Using Test Tube Reflux

Equipment

boiling chips support standcopper metal sponge* 2 test tubes, 13 × 100-mmmicrospatula –10 to 260 °C thermometer2 Pasteur pipets, with latex bulb utility clampsand bath† 6 vials, 2-mL, with screw caps*for fractional distillation†sand in crystallizing dish on electric hot plate or sand in electric heating well with heatcontroller




Preview

• Assemble apparatus and add cyclohexane–toluene mixture• Save sample of original simple distillation mixture for analysis• Distill approximately half of the mixture• Transfer residue to vial• Using refractive index, analyze the composition of the original

mixture, the distillate, and the pot residue• Repeat the Procedure for fractional distillation, using a test tube

packed with copper metal sponge





Place 1.0 mL each of cyclohexane and toluene into a 13 × 100−mm testtube. Mix well and add one small boiling chip. Using a Pasteur pipet,remove about 5 drops of the mixture, and place the drops into a smallvial labeled “Original Mixture–Simple”. [NOTE 1]

Clamp the test tube in a vertical position. Use a sand bath to heatthe liquid until the liquid boils and the condensation line for the va-por is about 2 cm from the top of the test tube, as shown in Figure 12.

Squeeze the bulb of a Pasteur pipet, place the pipet tip into the hotvapors, and very slowly draw the vapors into the cool pipet, where thevapors will condense. Transfer this distillate to a small vial labeled



Figure 12 Simple distillation of verysmall samples using a test tube and aPasteur pipet

NOTE 1: Cyclohexane and toluene arevolatile. Cap the vials to ensure thatthe small samples do not evaporate.

“Distillate–Simple”. Repeat the process until you collect about half ofthe mixture in the distillate vial.

Remove the test tube from the sand bath, allow it to cool, and trans-fer the remaining liquid into a vial labeled “Pot Residue–Simple”.


Place 1.0 mL each of cyclohexane and toluene into a 13 × 100−mm testtube, mix well, and add one small boiling chip. Using a Pasteur pipet,remove 5–10 drops of the mixture and place the drops into a smallvial labeled “Original Mixture–Fractional”. [NOTE 1]

Prepare a plug of copper sponge approximately 4 cm long. Tightlypack the copper plug into the test tube so that the bottom of the plugis about 1 cm above the top of the liquid and 3 cm below the mouth ofthe test tube, as shown in Figure 13.

Clamp the test tube in a vertical position and heat the liquid witha sand bath until the liquid boils. Observe the vapor condensationline as it moves through the copper sponge, and adjust the heat sothat the condensation line reaches a point about 1 cm above the top ofthe copper.

Squeeze the bulb of a Pasteur pipet, place the tip into the hot va-pors, and very slowly draw the vapors into the cool pipet, where theywill condense. Transfer this distillate to a small vial labeled “Distil-late–Fractional”, and repeat the process until you collect about half ofthe mixture in the distillate vial.

Remove the test tube from the sand bath. Cool the test tube, re-move the copper plug, and transfer the remaining liquid into a vial la-beled “Pot Residue–Fractional”.

3. Measuring Refractive Index Using a refractometer, measure the refractive index of the compoundsin each vial. Correct the refractive indices for temperature, usingEquation 2.

Using the correlation curve shown in Figure 7 earlier in this mod-ule and the corrected refractive index for the solution in each collec-tion vial, determine the percent of cyclohexane and toluene in eachsample. Compare your results for simple and fractional distillation.


Wash your hands thoroughly with soap or detergentbefore leaving the laboratory.

Using Test Tube Reflux 13


Figure 13 A test tube reflux apparatusfor conducting a fractional distillation

Post-Laboratory Questions 1. For macroscale distillations, or for microscale distillations usingglassware with elastomeric connectors, plot the data for simpledistillation and for fractional distillation on one graph. Plottemperature on the vertical axis and total volume of distillate onthe horizontal axis, as shown in Figure 6 earlier in this module.Draw a smooth curve through the data points for each distillation.

2. (a) At what temperatures were the first drop of distillate collectedin the simple and fractional distillations?(b) Using Figure 1, estimate the composition of these initial sam-ples of distillate. Based on the results, what conclusion can youdraw regarding the relative efficiencies of the two separations?

3. For macroscale distillations, or for microscale distillations usingglassware with elastomeric connectors, compare the plot from yoursimple distillation with that from your fractional distillation. Inwhich case do the changes in temperature occur more gradually?Which method is more effective in achieving separation? Brieflyexplain.

4. For Hickman still or for test tube microscale distillations, comparethe refractive index data for simple and fractional distillations. Dothe data suggest which distillation procedure is more efficient?Briefly explain.



NAME SECTION DATE

TECH 704/Separating Cyclohexane and Toluene by Distillation


1. Briefly explain why you should not add boiling chips to a boiling liquid.

2. (a) Briefly explain how and why you should position the thermometer inthe distillation head during a distillation.

(b) What is the purpose of the outer jacket on a fractionating column?

(c) How is the rate of heating adjusted when using a sand bath as a heatsource?

(d) How is the distillate collected in a test tube microscale distillation?

3. What effect does an increase in the heating rate have on the boiling temper-ature during a distillation?



4. As molecules move up a fractional distillation column, they condense andthen revaporize. During which of these steps is the concentration of themore volatile compound of the mixture increased? Briefly explain.

5. Using Figure 2, estimate the composition of a cyclohexane–toluene distil-late that is collected

at 85 °C;

at 95 °C;

at 105 °C.



ISBN 0-87540-704-8

E x p e r i m e n t 7 I s o l a t i o n o f E s s e n t i a l O i l s 49

Investigate the relationship between smell and three-dimensionalchemical structure, starting with either caraway seeds or orange peels.Optical activity of the compounds will be an important technique intheir analysis.

The first organic compounds to identified came from plant and animaltissues. Although most organic compounds used today are synthesizedfrom a variety of raw materials, living organisms continue to providesome industrial chemicals and a number of new drugs.

Essential oils make up one group of organic compounds stillobtained from plants. Essential oils are often characterized by very dis-tinctive odors, and odor depends to a certain degree on the stereochem-istry found in the particular molecule. As we shall see in the followingexperiments, essential oils of relatively low molecular weights can beobtained easily by steam distillation.

Many essential oils belong to the class of compounds called terpenes,compounds whose structures have been studied for many years andwhose carbon skeletons are composed of five carbon units called iso-prene units. For example, citronellol (oil of geranium) is composed of twoisoprene units.

Terpenes are classified as monoterpenes (which contain 10 carbons),sesquiterpenes (C15), diterpenes (C20), and so on. Vitamin A is a diter-pene:

Theories to explain the sense of smell have been the subject of scientificanalysis for many years. In fact, the Roman atomist Lucretius suggestedthat a substance could give off “atoms” of vapor of a given type and theodor perceived for that substance would depend on the nature of pores in

CH"CH9C"CH9CH"CH9C"CH9CH2OH

CH3 CH3

Vitamin A(a diterpene with four isoprene units)

2-Methyl-2-butene(isoprene)

CH3

OH

H3C

C"CHH3C CH3

Isoprene unit Citronellol(a monoterpene)

C

C9CC C

Experiment 7ISOLATION OF ESSENTIAL OILSAND THE THEORY OF ODOR

1

1

50 P a r t 1 E x p e r i m e n t s

the nose amenable to specific “atoms” of vapor. This idea was extended byR. W. Moncrief in 1949. He suggested that nasal pores had shapes andsizes such that they would act as receptor sites and that these sites wouldfit all molecular structures corresponding to a given primary odor.

J. E. Amoore continued to build the theory of primary odors. In the1950s he postulated seven primary odors: camphoraceous, ethereal, flo-ral, musky, pepperminty, pungent, and putrid. He concluded that therewas a general shape and size for all molecules in a primary odor cate-gory and that all structures in that odor group would fit into a certaincavity. Amoore also concluded that charge distribution on the moleculeand correspondingly in the receptor site is important for both the putridand the pungent classes.

The next level in odor theory involved stereochemistry. Specifically,the biological response of smell can be induced by one enantiomer(because of a proper fit in a receptor, which is composed of opticallyactive biopolymers), whereas the other enantiomer often shows noresponse (because of a lack of fit), or it may cause some very differentphysiological response (because of a fit with another enantiomeric recep-tor site).

Current theories of smell suggest that the situation may be muchmore complex and that there may be many different kinds of odor recep-tors. We still have much to learn about what controls our sense of smelland why different people smell the same compounds with distinctly dif-ferent results.

In Experiment 7.1 you will isolate the stereoisomer of carvone foundin caraway oil and analyze the optical properties of this isomer and itsmirror image isomer from spearmint oil. You will also prepare deriva-tives of each. It is worthy of note that these two enantiomers have verydifferent odors. In Experiment 7.2 you will isolate one enantiomer oflimonene from orange peels and analyze its optical properties.

Extract the essential oil from caraway seeds and analyze it by GC andpolarimetry.

CH3

CH3

O

C"CH2H

(S )-(�)-Carvonebp 230°C

density 0.965 g � mL�1

[ ]D �61.2°MW 150.2

Major component of carawayand dill seed oils

� �20

CH3

CH3

O

HH2C"C

(R )-(�)-Carvonebp 230°C


[ ]D �62.5°MW 150.2

Major component of spearmint oil

20

7.1Isolation of (S)-(�)-Carvone from Caraway Seeds

2 Isolation of Essential Oils and the Theory of Odor


It is remarkable that the distinctly different smells of caraway seedsand spearmint leaves come from two isomers that differ only in their chirality. The major compound in both caraway oil and spearmint oil is carvone, an unsaturated cyclic ketone with the formula C10H14O.One oil contains the (�) isomer and the other the (�) isomer. Currenttheories on smell hold that because odor receptors of the nose are chiral,the chiral carvone isomers, with their mirror image shapes, fit into different receptor sites. Curiously, it has been reported that about 1 person out of 10 cannot tell the difference between the odors of caraway and spearmint.

As far as we know, chiral (or “asymmetric”) compounds in natureexist only in living tissue or in matter that was once a part of living tissue. Chirality plays a major role in the mechanisms of biochemicalrecognition. Yet it is still a mystery why caraway plants, Carum carvi,produce (S)-(�)-carvone and spearmint plants produce its mirror image(R)-(�)-carvone. Other plants such as gingergrass produce racemic carvone.

Even more curious is the fact that the �-pinenes taken from differentpine trees in the same grove can have opposite optical activities. Naturegoes even one step further; some botanically indistinguishable plantsthat grow in different countries can carry out complete metabolicsequences of mirror image reactions. How such differences developed isstill unknown.

Caraway seed oil can be isolated by the steam distillation of groundcaraway seeds. The compound that is responsible for its characteristicodor, (S)-(�)-carvone, constitutes about 70–80% of the oil. The othermajor component is limonene, a terpene also found in spearmint oil andthe oils of oranges and lemons.

Caraway seeds are inexpensive when purchased in bulk and will beyour source of (�)-carvone. Although carvone boils at 230�C, far abovethe boiling point of water, it can easily be steam distilled from groundseeds because it is largely insoluble in water. Extraction of the distillatewith dichloromethane, followed by drying and evaporation, producescaraway seed oil.

In this experiment you will investigate the differences between the carvone from spearmint and that from caraway oil by using op-tical activity measurements, gas-liquid chromatography, and yoursense of smell. While the steam distillation of the caraway oil is bubbling along merrily, you can proceed with your analysis of (�)-carvone. (�)-Carvone is commercially available, and you will be comparing this enantiomer with the (�)-carvone you isolate from caraway seeds.

Because it is far easier to work with small amounts of solids than ofliquids, it is useful to make a solid derivative of your rather smallamount of (S)-(�)-carvone. Carvone is a ketone, and 2,4-dinitrophenyl-hydrazones are the most common derivatives of such carbonyl com-pounds. However, their deeply colored solutions make them impossi-ble to use for the polarimetric studies, because they do not transmitenough light. The solid derivative of choice here is the semicarbazone,which forms white crystals and colorless solutions.

H3C CH3

�-PineneCH3

CH3

CH3

C"CH2H

(R )-(�)-Limonene

Isolation of Essential Oils and the Theory of Odor 3


There are two possible diastereomers for the semicarbazones of (�)-carvone and (�)-carvone. They result from the restricted rotation aboutthe carbon-nitrogen double bond.

The � isomers of (�)- and (�)-carvone semicarbazone melt at162–163�C; the � isomers melt at 141–142�C. The �-semicarbazone formsunder our experimental conditions.

The preparation of carvone’s semicarbazone is doubly useful herebecause it lets us separate (�)-carvone from the other constituents of car-away seed oil. The other major component, limonene, has no carbonylgroup, so it will not form a semicarbazone and will be washed away inthe filtrate from the recrystallization.

R R�C

NNCNH2

H

O

R R�C

NH2NCN

H

O

Techniques Steam Distillation: Technique 7.6Extraction: Technique 4.2Melting Points: Techniques 6.3 and 6.4Gas-Liquid Chromatography: Technique 11Polarimetry: Technique 13

MMMMMacroscaleProcedure

Dichloromethane is toxic, an irritant, absorbed through the skin,and harmful if swallowed or inhaled. Use it in a hood, if possible,and wash your hands thoroughly after handling. Be sure to do theevaporation process in a hood.

(S)-(�)-carvone is highly toxic in concentrated form and rapidlyabsorbed through the skin. Avoid contact with skin, eyes, andclothing.

Methanol is toxic and flammable. Pour it only in a hood.

SAFETY INFORMATION

A bakery or a bakery supplieris the best source of fresh

caraway seeds. Seedspurchased in a supermarket

may have spent months on theshelf and still contain carvone

but tend to have little or nolimonene remaining in them.

Carvone

� H2NNCNH2 � HCl

CH3H

CH

H2O

CH3COO�Na�

O

O

CH3 CH2Carvone

-semicarbazonemp 141–142°C

MW 207.2

Semicarbazidehydrochloride

mp 175°CMW 111.5

NNCNH2

� H2O

CH3H

CH

O

CH3 CH2

�



Weigh 25 g of fresh caraway seeds and grind them in an electric blender ora coffee grinder. Put the ground seeds into a 500-mL round-bottomed flaskand add 250 mL of water. Set up a distillation apparatus as shown in Technique 7.6, Figure 7.14, but without the separatory (dropping) funneland round-bottomed receiving flask. Use a 100-mL graduated cylinder forthe receiver and close the second neck of the Claisen adapter with a glassstopper.

Boil the mixture vigorously, using a heating mantle or a sand bath asthe heat source, but be careful not to let any solid material bump overinto the condenser. Collect about 80 mL of distillate. Faster distillationseems to make for a smoother, less bumpy process, so do not decreasethe rate of heating once the distillation begins. Distill the mixture asrapidly as the cooling capacity of the condenser will permit. The distilla-tion should take approximately 30–40 min. During this time, prepare thesemicarbazone of (R)-(�)-carvone, spearmint oil (see p. 54).

Pour your distillate into a 125-mL separatory funnel. Add 5 g of sodiumchloride and shake the mixture to dissolve the salt. Then add a fewpieces of ice to make the solution distinctly cool before adding anyvolatile dichloromethane.

Obtain 20 mL of dichloromethane. Place a conical funnel in the top ofthe separatory funnel. Hold the condenser and the vacuum adapter usedto collect the distillate above the separatory funnel and rinse them with afew milliliters of the dichloromethane; let the dichloromethane drain intothe separatory funnel. Use the remaining dichloromethane to rinse the100-mL cylinder, and also add that solution to the separatory funnel. Thisrinse recovers all the caraway oil clinging to the glassware surfaces.

Extract the carvone from the aqueous layer by inverting the funneland shaking it back and forth gently [see Technique 4.2]. Repeat the gentleshaking and venting for about 2 min. Allow the layers to separate beforedraining the bottom (organic) layer into a dry 50-mL Erlenmeyer flask.Carvone is quite soluble in dichloromethane; any organic membranes thatmay form at the interface should be left with the aqueous layer.

Repeat the extraction of the aqueous layer using 15 mL of freshdichloromethane. Combine the two dichloromethane extracts and drythe solution with anhydrous magnesium sulfate for at least 10 min.

Weigh a clean, dry 50-mL Erlenmeyer flask on an analytical balance.Filter half the dichloromethane solution through a conical funnel fittedwith a fluted filter paper [see Technique 5.3a, Figure 5.5] into the tared(weighed) Erlenmeyer flask. Add a boiling stick and evaporate thedichloromethane on a steam bath or hot water bath in a hood. Cool the

No additional water is neededfor this steam distillation;

replace the dropping funnelwith a Ts glass stopper.

Review Technique 4.2,Macroscale Extractions, before

doing this part of theexperiment.

The NaCl helps to minimizeemulsions during the

extractions by making theorganic layer less soluble in

the water layer.

Be sure to vent the separatoryfunnel frequently during the

extractions.

Isolation of Caraway Oil

Steam Distillation

Alternatively, thedichloromethane can beremoved with a rotary

evaporator [see Technique 4.8].



flask briefly and filter the remaining dichloromethane solution into it.Continue heating the flask until the solvent has completely evaporated.

Cool the flask and carefully wipe the outside dry with a tissue; thenweigh it on an analytical balance. Heat the flask several additional min-utes, cool, and weigh again. The two masses should agree within 0.05 g.If they do not, repeat the heating and weighing procedures. Record thefinal mass of your caraway oil to the nearest milligram. The residue inthe flask is caraway seed oil. Carefully compare the smell to that of (R)-(�)-carvone.

Use both nonpolar and polar columns at 170–200�C to find the retentiontime for a known sample of (�)-carvone on each column under yourconditions. If you are using a capillary column chromatograph, prepare asolution containing 0.5 mL of ether and 1 drop of the compound beingtested. After you have analyzed (R)-(�)-carvone, analyze your carawayseed oil using the same nonpolar and then polar GC columns at the sametemperature and flow rate. Also chromatograph samples of known (S)-(�)-carvone and limonene using identical GC conditions.

Calculate the retention times of (�)- and (�)-carvone and limonene.Also calculate the percentages of (�)-carvone and limonene in carawayseed oil by computing the areas under their respective peaks if you areusing a nonintegrating recorder (or peak heights, if the peaks are verynarrow) [see Technique 11.6].

Weigh 0.38–0.39 g of (R)-(�)-carvone (spearmint oil) into a 25-mLErlenmeyer flask and add 4.0 mL of 95% ethanol. Dissolve 0.40 g of semi-carbazide hydrochloride and 0.40 g of anhydrous sodium acetate or 0.64g of sodium acetate trihydrate in 2.0 mL of water in a mm testtube. Pour the resulting solution into the flask containing the carvonesolution and add a boiling stone or stick. Warm the mixture on a steambath set for a gentle steam flow or in an 80°C water bath for 15 min.

Add 2.5 mL of water to the warm solution; then set the reaction mix-ture aside to cool slowly to room temperature. Under these conditions,crystallization may take 30–45 min; slow crystallization gives nearlypure crystals that probably will not need to be recrystallized for the opti-cal activity studies. After crystallization is complete, cool the solution for5 min in an ice-water bath.

Collect the crystals by vacuum filtration, using a small Buchner fun-nel, and wash them on the funnel with a few milliliters of cold water.Allow the solid to dry for several hours or overnight before taking the

13 � 100

Gas ChromatographicAnalysis

Review Technique 11, Gas-Liquid Chromatography,

before you do this part of theexperiment.

Derivatives:Preparation of

Semicarbazones

Sodium acetate provides theproper pH for rapid carbonyl

addition-dehydrationreactions.



melting point [see Technique 6.3]. If it is necessary to recrystallize thesemicarbazone, you may do so from an ethanol/water mixture [see Tech-nique 5.2a].

Prepare the semicarbazone of the carvone in your caraway seed oil inthe Erlenmeyer flask containing the oil, using all of your sample exceptwhat you need for the GC analysis. Add 4.0 mL of ethanol to your car-away seed oil. Prepare the semicarbazide solution (as directed above) ina mm test tube and pour it into the ethanol solution of carawayseed oil in the Erlenmeyer flask. Follow the procedure given earlier forheating and crystallization. Adjust the proportions of all reagents if youhave less than 0.30 g of caraway seed oil. Again, determine the meltingpoint after the solid dries.

Also grind together a small amount of approximately equal portionsof the carvone semicarbazones from each source and take a melting pointof the mixture [see Technique 6.4].

If you are using 2-dm polarimetry tubes, use two 25-mL volumetricflasks and prepare a 25-mL solution of each of your dry carvone semicar-bazones in anhydrous methanol. If you are using 1-dm periscopepolarimeter tubes, prepare each solution in a 10-mL volumetric flask. Aconcentration of 1.50% works best if you have enough material (0.15 g/10 mL of solution), but if you do not have enough for this concentration,use what you have, saving only enough for the melting points. Weigh thedry carvone semicarbazone samples to the nearest milligram. After dis-solving the semicarbazone, stopper the flask and shake it a number oftimes to ensure a completely homogeneous solution.

If you see any undissolved particles such as paper fibers or pieces ofdust in the solution, filter the solution by gravity through a small plug ofglass wool or a small fluted filter paper, using a short-stemmed funnel. Ifyou are using a periscope polarimeter tube, filter the solution directlyinto the tube; if you are using a straight polarimeter tube, filter the solu-tion into a 25- or 50-mL Erlenmeyer flask; then fill the polarimeter tube.Keep the solutions tightly stoppered, except during transfer, to avoidevaporation of the solvent.

Compare the specific rotations of the carvone semicarbazones fromcaraway seed oil and from (R)-(�)-carvone. Are they of the same magni-tude and opposite in sign?

Cleanup: Filter the caraway seed residue from the aqueous liquidremaining in the distillation flask. (Do not put caraway seeds down thesink.) Wash the aqueous filtrate down the sink; dispose of the seed

13 � 100

Polarimetry

Read Technique 13,Polarimetry, before doing your

measurements of opticalactivity. Use a 5.0% or 10.0%sucrose solution in water as a

standard, to becomeacquainted with polarimetric

measurements. The specificrotation of sucrose, ,

equals �66.4� (H2O).Compare your calculatedspecific rotation with the

accepted value.

[�]D20

dm decimeter

If possible, make this derivative on the same day

that you isolate the carawayseed oil.



References1. Garin, D. L. J. Chem. Educ. 1976, 53, 105.

2. Glidewell, C. J. Chem. Educ. 1991, 68, 267–269.

3. Murov, S. L.; Pickering, M. J. Chem. Educ. 1973,50, 74.

Questions1. Critically evaluate your evidence on whether

the carvone isolated from caraway seed oil isthe mirror image of (�)-carvone.

2. What caused the melting point of the mixtureof (�)- and (�)-carvone semicarbazones to be

higher than the melting point of either purecompound?

3. Propose a method that you could use to isolatethe limonene in caraway seed oil.

Extract the essential oil from orange peels and analyze it by polarim-etry.

Limonene is a terpene whose stereochemistry depends on its source.Orange peels provide limonene that is virtually 100% (R)-(�)-limonene,whereas pine needles provide essentially 100% (S)-(�)-limonene. Thebiosynthetic pathway to limonene is quite complex, but we do know in

CH3

CH3

C"CH2H

(R )-(�)-Limonenebp 175.5–176°C


[ ] �125.6°MW 136.2

Major component of orange peels

CH3

CH3

HH2C"C

(S )-(�)-Limonenebp 175.5–176.5°C


[ ] �122.1°MW 136.2

Minor component of caraway seeds

��

residue as food garbage. Allow any residual dichloromethane to evapo-rate from the magnesium sulfate drying agent in a hood before placingthe solid in the container for inorganic waste or the container for solidhazardous waste. Pour the ether solutions used for GC analysis, the fil-trate from the semicarbazone preparations, and the methanol solutionsfrom your polarimetric measurements into the container for flammable(organic) waste.

7.2Isolation of (R)-(�)-Limonene from Orange Peels



principle how different plants produce one enantiomer or the other. Thestereocenter that differentiates the (�) and (�) isomers of limonenearises during the formation of the six-membered ring. The active site ofthe enzyme that promotes the formation of the ring in pine needles mustbe different from that in oranges.

You will need the peels from two oranges for this experiment. Yourinstructor will specify whether you are to provide the oranges orwhether they will be available in the laboratory.

Remove most of the white pulp from the orange peels with a knife orspatula before grinding the peels in a blender with 200–250 mL of waterto make a slurry that can be easily poured into a 500-mL round-bottomedflask. Add 4 drops of an antifoaming agent to the flask.

Assemble the steam distillation apparatus shown in Technique 7.6,Figure 7.14 but without the separatory (dropping) funnel. Use a 50-mLround-bottomed flask as the receiver. Before attaching the receiving flask,pour 35 mL of water into it and mark the level on the outside of the flask;then pour out the water (the flask does not have to be dried).

Boil the mixture at a moderately rapid rate, using a heating mantle asthe heat source, but be careful not to let any solid material bump overinto the condenser. Collect 35 mL of distillate.

Pour your distillate into a 125-mL separatory funnel and add 2 g ofsodium chloride. Stopper the funnel and shake it to dissolve the salt.Then add a few pieces of ice to make the solution distinctly cool beforeadding any volatile dichloromethane.

Obtain 15 mL of dichloromethane. Place a conical funnel in the topof the separatory funnel. Rinse the condenser and vacuum adapter usedto collect the distillate with a few milliliters of the dichloromethane byholding them above the funnel and letting the dichloromethane drain

Techniques Steam Distillation: Technique 7.6Extraction: Technique 4.2Polarimetry: Technique 13

MMMMMacroscaleProcedure

Dichloromethane is toxic, an irritant, absorbed through the skin,and harmful if swallowed or inhaled. Wear gloves. Use it in ahood, if possible, and wash your hands thoroughly after handling.

(�)-Limonene is an irritant and readily absorbed through the skin.

SAFETY INFORMATION

Steam Distillation

Peel the oranges just beforegrinding the peels to preventloss of the volatile limonene.

Because it is not necessary toadd more water during the

distillation, replace thedropping funnel with a Ts glass

stopper.

Isolation of Limonene

Review Technique 4.2,Macroscale Extractions, before

doing this part of theexperiment.



into the separatory funnel. Use the remaining dichloromethane to rinsethe 50-mL round-bottomed flask and add that solution to the separatoryfunnel. This rinse recovers any limonene clinging to the glassware surfaces.

Extract the limonene from the aqueous layer by inverting the funneland shaking it back and forth gently [see Technique 4.2]. Be sure to ventthe funnel frequently. Repeat the gentle shaking and venting for about 2min. Allow the layers to separate before draining the bottom organiclayer into a dry 50-mL Erlenmeyer flask. Limonene is quite soluble indichloromethane; any organic membranes that may form at the interfaceshould be left with the aqueous layer.

Repeat the extraction of the aqueous layer using another 10 mL ofdichloromethane. Combine the two dichloromethane extracts and drythe solution with anhydrous calcium sulfate for at least 10 min.

Weigh a dry 50-mL Erlenmeyer flask on an analytical balance to thenearest milligram. Filter the dichloromethane through fluted filter paperinto the tared flask [see Technique 4.7]. Carry out the evaporation ofdichloromethane in a hood. Put a boiling stick into the flask and evapo-rate the dichloromethane on a steam bath, using a gentle flow of steam,or in a hot-water bath. (Alternatively, the evaporation process may bedone with a stream of nitrogen or air and gentle warming of the flask ina beaker of hot tap water or with a rotary evaporator [see Technique4.8].) Cool the flask and carefully wipe the outside dry with a tissuebefore weighing it on an analytical balance. Heat the flask several addi-tional minutes, cool, and weigh it again. The two masses should agreewithin 0.05 g. If they do not, repeat the heating and weighing proceduresuntil this agreement is reached. Record the final mass of limonene to thenearest milligram. Describe its appearance and carefully note the odor(do NOT breathe the vapors of limonene).

The following directions for preparation of the polarimetry solution arefor 1-dm periscope polarimeter tubes. Consult your instructor if yourlaboratory is equipped with another type or size of polarimeter tubes.

Obtain 10 mL of 95% ethanol in a clean, dry graduated cylinder. Dis-solve your limonene in 3 mL of ethanol (use a Pasteur pipet to transferthe ethanol) and quantitatively transfer the resulting solution to a 10-mLvolumetric flask as follows: Set the volumetric flask in a small beaker sothat it will not tip, and place a very small funnel in the neck of the volu-metric flask. Carefully pour the limonene solution into the volumetricflask. Rinse the Erlenmeyer flask three times with approximately 1-mLportions of ethanol and add these rinses to the volumetric flask. Rinse

PolarimetryRead Technique 13,

Polarimetry, before you beginthis procedure. Use a 5.0% or

10.0% sucrose solution inwater as a standard, to become

familiar with polarimetricmeasurements. The specific

rotation of sucrose, ,equals �66.4� (H2O).

Compare your calculatedspecific rotation with the

accepted value.

[�]D20

The NaCl helps to minimizeemulsions during the

extractions by making theorganic layer less soluble in

the aqueous layer.


E x p e r i m e n t 8 N u c l e o p h i l i c S u b s t i t u t i o n R e a c t i o n s 59

the funnel with 1 mL of ethanol and remove it from the volumetric flask.Fill the volumetric flask to the calibration mark, using a Pasteur pipet;stopper the flask and invert it several times, until the contents are thor-oughly mixed.

If the solution is cloudy, filter it directly into a polarimeter tube,using a small funnel and fluted filter paper. If the solution is clear, it canbe poured directly into a polarimeter tube. Determine and record theobserved rotation for your limonene solution; also record the tempera-ture [see Technique 13.4]. Calculate the specific rotation [�]D of limoneneand its enantiomeric excess (optical purity) [see Technique 13.5].

Cleanup: Filter the orange peel residue from the aqueous liquidremaining in the distillation flask. (Do not put orange peels down thesink.) Wash the aqueous filtrate down the sink; dispose of the orangepeel residue as food garbage. Allow any residual dichloromethane toevaporate from the calcium sulfate drying agent in a hood before placingthe solid in the container for inorganic waste or the container for solidhazardous waste. Pour the ethanol solution used for your polarimetricmeasurements into the container for flammable (organic) waste.

Calibrate the polarimeterusing 95% ethanol as the

reference solvent.

Reference1. Glidewell, C. J. Chem. Educ. 1991, 68, 267–269.

Questions1. How would the presence of residual di-

chloromethane affect the observed rotation oflimonene? Explain.

2. The optical rotations of enantiomers are, inprinciple, equal in magnitude but opposite in sign. The rotations that we have provided

have opposite signs, but the magnitudes areunequal. Explain this inequality, keeping inmind that the same explanation probablyapplies to why the densities of the enantiomersare not equal, although—again, in principle—they should be.

Investigate the relationship of structure and reactivity in substitutionreactions, a topic that you already have or soon will be studying in theclassroom. The techniques encountered range from simple qualitativetests to NMR analysis to organic synthesis.

One of the most-studied and well-established mechanisms of or-ganic chemistry is that for nucleophilic substitution. In this reaction a

1 dm 1 decimeter

Experiment 8NUCLEOPHILIC SUBSTITUTIONREACTIONS


Separating a Mixture ofBiphenyl, Benzhydrol, andBenzophenone byThin-Layer Chromatographyprepared by Ronald J. Wikholm, University of Connecticut


Select a solvent to separate a mixture of biphenyl, benzhydrol, andbenzophenone by thin-layer chromatography. Identify the mixturecompounds by comparing Rf values with reference compounds. Usethin-layer chromatography to investigate solvent polarity effects onthe relative mobilities of these compounds in a mixture. Usethin-layer chromatography to identify the specific compounds in anunknown mixture containing any combination of biphenyl,benzhydrol, and benzophenone.

BACKGROUND REQUIRED You should know how to use a microburner or a Bunsen burner.


Thin-layer chromatography (TLC) is a simple and inexpensive ana-lytical technique that can quickly and efficiently separate quantitiesof less than ten micrograms (µg) of material. TLC has many applica-tions in the organic laboratory. TLC is used for the rapid analysis ofreagent and product purity, or to quickly determine the number ofcompounds in a mixture. Also, by comparing an unknown com-pound’s behavior to the behaviors of known standard compounds,mixture compounds can be tentatively identified.

Chemists frequently use TLC to follow the progress of a reaction bymonitoring the disappearance of a reactant or the appearance of aproduct. Also, TLC often is used to select a suitable solvent beforeattempting a larger scale column chromatography separation. Then,during the column chromatography experiment, TLC is frequentlyused to monitor the separation.

The term chromatography refers to several related techniques foranalyzing, identifying, or separating mixtures of compounds. All chro-matographic techniques have a two-part operation in common. In eachtechnique a sample mixture is placed into a liquid or gas, called a mobilephase. The mobile phase carries the sample through a solid support,

TECH



called the stationary phase, which contains an adsorbent or anotherliquid. The different compounds in the sample mixture move throughthe stationary phase at different rates, due to different attractions forthe mobile and stationary phases. Thus, individual compounds inthe mixture separate as they move through the stationary phase. Theseparate compounds can be collected or detected, depending on theparticular chromatographic technique involved.

In TLC, capillary action allows a liquid (mobile phase) to ascend asolid (stationary phase) coated on a support plate. A sample of thecompound mixture is applied near the bottom of a dry TLC plate, asshown in Figure 1(a). The plate is placed into a developing chamber,a covered container with a shallow layer of mobile phase liquid inthe bottom. As the mobile phase ascends the plate, the mixturecompounds dissolve in the mobile phase to different extents, due todifferences in their relative attractions for the mobile and stationaryphases. After the separation is complete, the TLC plate is called achromatogram, as shown in Figure 1(b).

During the TLC process, the solid stationary phase, called the ad-sorbent, adsorbs the mixture compounds. As the mobile phase, calledthe eluent, travels up over the adsorbent the compounds within themixture move at different rates. A reversible and continuous competi-tive attraction between the eluent and the adsorbent for the mixturecompounds causes this rate difference.

Compounds with less attraction for the adsorbent move rapidlywith the eluent. Compounds with more attraction for the adsorbentmove slowly with the eluent. Because TLC adsorbents are typicallyvery polar, the more polar is a compound in the mixture, the morestrongly it adheres to the adsorbent and the more slowly it moves.

Similarly, intermolecular attractions between the eluent and thecompounds determine the solubility of the compounds in the mobilephase. In general, the more polar the eluent, the more rapidly a givencompound moves. Polar compounds, which are strongly attracted tothe adsorbent, require polar eluents to attract them away from theadsorbent.

Determining a Retention Factor The ratio of the distance that a compound moves to the distance thatthe eluent front moves is called the retention factor, denoted as Rf. Acalculation for Rf is shown in Equation 1.


2 TECH 707/Separating a Mixture by Thin-Layer Chromatography

Figure 1 A TLC plate (a) labeled foridentification and spotted, and (b) as achromatogram

(a) (b)

R f =distance traveled by compound, mmdistance traveled by eluent front, mm

(Eq. 1)

For example, in Figure 2 the stock sample compound moved distanceA while the eluent front traveled distance S. If distance A is 25 milli-meters (mm) and distance S is 55 mm, then the Rf is calculated asshown in Equation 2.

RASf = = =

2555

0 45mmmm

. (Eq. 2)

The chromatographic behavior of individual compounds is repro-ducible as long as the stationary and mobile phases and the tempera-ture are kept constant. Therefore, an Rf can be used for identificationpurposes.

When a compound is strongly attracted to the adsorbent and doesnot travel very far from the origin, or point of application, the Rf issmall. An increase in eluent polarity would probably increase theattraction of the compound for the eluent. As a result, the compoundwould move farther up the plate, resulting in a larger Rf .

Identical Rf s for a known compound and an unknown compoundon the same chromatogram suggest that the known and the unknowncompounds are the same. However, two different compounds canhave the same Rf in a given eluent. Additional evidence that twosamples are the same compound can be obtained by comparing theirmobilities in several eluent systems of varying polarities. Two differ-ent compounds that have the same Rf in one eluent are unlikely tohave the same Rf in other eluents of different polarities, while two dif-ferent samples of the same compound will have the same Rf in everyeluent.

Choosing Adsorbentsand Eluents

Alumina (Al2O3) and silica gel (SiO2 • x H2O) are the most commonlyused adsorbents in TLC and column chromatography. However, foruse in TLC, a binder such as calcium sulfate is added to theseadsorbents to hold them onto the plate. For this reason, commerciallyprepared adsorbents may not be used interchangeably between TLCand column chromatography.

Alumina is generally suitable for chromatography of less polarcompounds. Silica gel gives good results with compounds containingpolar functional groups.

Water content affects adsorbent activity by occupying polar siteson the surface. The greater the water content, the lower the adsorbentactivity. For reproducible results, the plates are dehydrated by heat-ing in a drying oven and then stored in a desiccator.

The eluents are organic compounds of various structures and po-larities, as shown in Table 1 on p. 4. The more polar an eluent, thegreater is its eluting power, that is, its ability to move compoundsover the adsorbent surface.

Combining eluents of low polarity with those of high polarityallows the preparation of mixed eluents of practically any elutingpower. For example, the eluting power of a 1:1 mixture of hexane andethyl acetate would be between the eluting powers of pure hexaneand pure ethyl acetate. Eluent selection is usually a matter of trial anderror until a separation or desired mobility is achieved.



Figure 2 A chromatogram showingmeasurements for Rf calculations

least polar cyclohexanepetroleum etherhexane

increasing tolueneeluting dichloromethanepower ethyl acetate

ethanolacetone

most polar methanol

Using TLC in an Experiment A TLC experiment has three general stages: spotting, developing, andvisualizing.

Spotting a Plate

The origin is marked, usually by drawing a thin line across the bot-tom of the plate with a pencil, as shown in Figure 3. The sample com-pound or mixture should be dissolved in a volatile solvent such as ace-tone or dichloromethane. A glass capillary tube is used to apply a smallamount of sample solution onto the plate, keeping the sample in as smallan area as possible. With practice, spots with diameters of 1–2 mm canbe produced.

After the solvent evaporates, additional sample solution can beapplied to the same spot. Application of too much sample can lead to“tailing” and poor separation, as shown in Figure 3. Varying amountsof a sample can be spotted on the same plate to determine whichapplication gives the best results.

Developing a Plate

To develop the chromatogram, a piece of filter paper is placed alongthe walls of the developing chamber, which contains a shallow layerof the appropriate eluent. The paper acts as a wick that absorbs theeluent and ensures that, when the chamber is closed, its atmosphere issaturated with eluent vapor, minimizing evaporation from the plate.

When the spotted plate is placed into the chamber, the originmarked on the plate must be higher than the level of the eluent, toprevent the sample from dissolving from the plate into the eluentlayer. When the eluent reaches a point approximately 10 mm from thetop of the plate, the plate is removed from the chamber. The point thatthe eluent has reached is called the eluent front and is immediatelymarked with a pencil, as shown in Figure 3. The plate is dried byallowing the eluent to evaporate from the plate.

If the eluent front is allowed to reach the top of the plate, the mix-ture compounds may continue to move along the plate. An Rf mea-sured under these circumstances is not valid.

Visualizing the Compound

Upon development, a successful separation of colored compoundswill reveal distinct spots, indicating that the mixture compoundshave separated, as shown in Figure 3. To make separated colorlesscompounds observable to the eye, the spots are treated in some wayto make them visible. The process is called visualization.



Figure 3 A completed chromatogram

Table 1 Approximate order of polarityof eluents used in chromatography

Some compounds fluoresce. Such compounds can be visualized byviewing the TLC plate under an ultraviolet (UV) lamp. Frequently,the adsorbent contains a chemically inert fluorescent material. Whenviewed under UV light, compounds that absorb the UV light appearas dark spots that may be outlined with a pencil.

Another simple method for visualizing organic compounds is toplace the chromatogram in a chamber containing iodine (I2) crystals andvapor. The I2 vapor forms a colored complex with many compoundsand allows detection of their spots. The spot location must be markedimmediately because the I2 will eventually sublime from the plate.

In some instances, a reagent such as phosphomolybdic acid solu-tion is sprayed on the plate. This reagent forms a colored productwith the compound of interest.

Equipment

5 beakers, 250-mL, with plastic 5–6 melting point capillary tubeswrap to cover each microburner or Bunsen burner

12-cm filter paper,* cut to fit pencildeveloping chamber 5 rubber bands

glass stirring rod ruler10-mL graduated cylinder 9 thin-layer chromatographylabels plates, silica gel, 2.5 × 7.5-cm,marking pen with fluorescent indicator*or paper toweling


substance quantity molar mass (g/mol) mp (°C) bp (°C)

benzhydrol 184.24 65–67benzophenone 182.22 49–51biphenyl 154.21 69–72dichloromethane 5 mL 84.93 40ethyl acetate 5 mL 88.11 77hexane 5 mL 86.18 69iodine 253.81 113 184methanol 5 mL 32.04 65toluene 5 mL 92.14 111

Preview

• Prepare micropipets from capillary tubes• Prepare five developing chambers, each containing one of the

eluents to be investigated• Label and mark TLC plates• Spot stock solution on TLC plate• Develop TLC plate and mark eluent front• Visualize and mark chromatogram under UV light• Repeat spotting, developing, and visualizing in each of the other

eluents• Using eluent that separates the three compounds, spot known

compound alongside the mixture• Repeat with other compounds



• Use iodine chamber to visualize one chromatogram• Analyze an unknown mixture

PROCEDURE Chemical Alert

benzhydrol in acetone—flammable and irritantbenzophenone in acetone—flammable and irritantbiphenyl in acetone—flammable and irritantdichloromethane—toxic and irritantethyl acetate—flammable and irritanthexane—flammable and irritantiodine—toxic and corrosivemethanol—flammable and toxictoluene—flammable and toxic

Caution: Wear departmentally approved safety goggles at alltimes while in the chemistry laboratory.

1. Preparing Micropipetsfor Spotting

Caution: Before lighting any flame in the laboratory, check forthe presence of any flammable solvents nearby. Extinguish allflames before preparing the developing chambers, which containflammable eluents.

Prepare micropipets for spotting the TLC plates by drawing outmelting point capillary tubing. Draw out the tubing using a smallflame from a microburner or a Bunsen burner to heat the midpoint ofa melting point capillary. Slowly rotate the tubing until a yellowflame indicates the tube is softened. Remove the tubing from theflame and immediately draw out the ends of the tubing about 5–10 cmto form a very fine open capillary.

Break the capillary into two micropipets. Prepare 5–10 micropipets.Use a new micropipet for each solution.

2. Preparing the DevelopingChamber

Caution: Ethyl acetate and hexane are flammable and irritating.Dichloromethane is toxic and irritating. Methanol and tolueneare flammable and toxic. Do not use these compounds nearflames or other heat sources. Use a fume hood. Prevent eye, skin,and clothing contact. Avoid inhaling vapors and ingesting thecompounds. Do not use these eluents until all students have preparedtheir micropipets and all burners have been removed.

Obtain five 250-mL beakers and label each beaker with the name ofone of the eluents: “ethyl acetate”, “hexane”, “methanol”, “dichloro-methane”, and “toluene”. [NOTE 1] Obtain five rubber bands and fivepieces of plastic wrap, each large enough to cover a 250-mL beaker.

Cut a piece of filter paper into a rectangle wide enough so that thepaper extends nearly to the top of the slide and only long enough sothe paper covers three-quarters of the beaker wall, as shown inFigure 4. Cut papers to fit the other four beakers.

Using a glass stirring rod to direct the flow, pour 5 mL of the appro-priate eluent into each labeled beaker to moisten the filter paper linerand to form a layer 3–4 mm deep. Cover each developing chamberwith plastic wrap and set aside.



NOTE 1: Your laboratory instructorwill tell you if you are to coordinateyour work with other students.

Figure 4 Developing chamber for TLC

rubberband

plasticwrap

thinlayerplate

eluent

filter paper

3. Spotting the TLC Plates Obtain five 2.5 × 7.5-cm TLC silica gel plates. [NOTE 2] With a pencil,label each plate at the top with the name of one of the five eluents.

Mark the origin on each of the five plates by making a very faintpencil line across the plate 1 cm from the bottom. [NOTE 3] Faintlymark two cross-hatch lines on the origin line to indicate where the so-lution will be spotted, as shown in Figure 1(a) earlier in this module.

Caution: Benzhydrol, benzophenone, and biphenyl in acetonesolutions are flammable and irritating. Do not use thesecompounds near flames or other heat sources. Prevent eye, skin,clothing, and combustible materials contact. Avoid inhalingvapors or ingesting these compounds.

Obtain a vial of the stock solution mixture containing biphenyl,benzhydrol, and benzophenone. Place the drawn-out end of amicropipet into the stock solution and allow the liquid to rise bycapillary action. Spot the solution onto one TLC plate by quickly andlightly touching the end of the micropipet to the surface of the adsor-bent at each cross-hatch. Transfer an amount of liquid to the plate thatproduces a spot with a diameter less than 2 mm. [NOTE 4]

After the sample solvent has evaporated, make a second applica-tion to one of the two spots to increase the sample amount. Allow thesolvent to evaporate.

4. Developing TLC Plates Check to be certain that the eluent level in each developing chamber isbelow the point where the samples have been spotted. Place the spottedTLC plate into the ethyl acetate developing chamber. Use the chamberwall to support the plate, as shown in Figure 4. [NOTE 5] Cover thechamber with a piece of plastic wrap, secured with a rubber band.

When the eluent front rises to within 1 cm of the top of the plate,remove the plate from the chamber and immediately mark the eluentfront with a pencil. [NOTE 6] Allow the eluent to evaporate from theplate under a fume hood.

Caution: Ultraviolet radiation can cause severe damage to theeyes. Wear goggles. Do not look directly into the UV lamp.

Examine the developed TLC plate under an ultraviolet light. Use apencil to circle any visualized spots.

Using each of the four other eluents and new TLC plates, repeat theprocedures for spotting the stock solution and developing thechromatogram. Use the same micropipet to spot the stock solution oneach plate.

Compare the chromatograms from each eluent chamber. Comparethe spots from both single and double sample applications. Recordthe name of the eluent that gives the best separation of the mixture.Use this eluent in Parts 5 and 6. Record whether a single sample appli-cation or a double sample application gives better results.

Use a ruler to measure the distance from the origin to the center ofeach spot on the plate developed with the chosen eluent. Measure thedistance from the origin to the eluent front. Use Equation 1 to calcu-late the Rf for each spot.


Procedure 7

NOTE 2: Avoid touching the coatedsurface of the TLC plate with your fin-gers. Hold the plate at the top or by thesides. Use a pencil to mark on the TLCplate. Inks dissolve in the eluents.

NOTE 3: Do not bear down with thepencil when marking the origin. Thepencil lead can cut completely throughthe adsorbent, forming a gap that maystop the flow of eluent.

NOTE 4: Allow the solvent to drycompletely between applications to thesame spot. Otherwise, the spot will be-come too large.

NOTE 5: Avoid leaning the plateagainst the filter paper. Eluent on thefilter paper can be adsorbed by the ad-sorbent on the plate and interfere withthe ascending eluent. Such extraneouseluent may prevent the mixture com-pounds from moving up the plate in astraight line.

NOTE 6: Mark the eluent front imme-diately after removing the plate fromthe chamber. Many eluents evaporatequickly and leave no trace.

5. Identifying the Compoundsin a Mixture

Obtain a solution of biphenyl from your laboratory instructor. Usinga new micropipet for each new solution, spot the biphenyl alongsidethe stock solution mixture on a new TLC plate. [NOTE 7]

Develop the plate in the eluent identified in Part 4 that gives themost efficient separation. Visualize the chromatogram using ultra-violet light. Use a pencil to outline the spots.

Repeat this procedure for benzhydrol and benzophenone. Calcu-late the Rf for each compound. Record each Rf and the identity of eachcompound.

Caution: Iodine (I2) is toxic and corrosive. Avoid inhaling vaporswhen using the I2 chamber.

Place one of the developed chromatograms into an I2 chamberprepared by your laboratory instructor. Allow the chromatogram toremain in the chamber for 5 min. Describe the appearance of the plateafter visualization with I2.

6. Analyzing an UnknownMixture

Obtain an unknown solution and a new TLC plate. Spot the unknownsolution and the stock solution mixture onto the plate. Develop theplate in the eluent identified in Part 4. Visualize the spots with eitherultraviolet light or I2.

Calculate Rf s for each compound in the mixture. Record thecompounds present in the unknown solution.

7. Cleaning Up Use the labeled collection containers provided by your laboratoryinstructor. When you are finished with the developing chambers,pour any remaining eluent into the appropriate containers labeled“Recovered Dichloromethane”, “Recovered Ethyl Acetate”,“Recovered Hexane”, “Recovered Methanol”, and “RecoveredToluene”. Place your used TLC plates and micropipets into thecontainer labeled “Used TLC Plates and Micropipets”.




NOTE 7: Your laboratory instructormay have you spot all three com-pounds and the stock solution mixtureon one TLC plate.

Post-Laboratory Questions 1. (a) List the five eluents in order of increasing polarity.(b) With regard to your experimental results, briefly describe theeffect of eluent polarity on the Rf of each stock solution mixturecompound.(c) In the Pre-Laboratory Assignment, you listed biphenyl,benzhydrol, and benzophenone in order of increasing polarity.What conclusion can you draw about the effect of compoundpolarity on Rf in the chosen eluent? Briefly explain.

2. Briefly explain why you selected the eluent you used to separateyour unknown mixture, and why you rejected each of the othereluents.

3. Briefly describe the effect of the following procedural errors on athin-layer chromatogram.(a) Applying too large a sample when spotting the plate.(b) Placing the spotted plate into a developing chamber contain-ing an eluent level above the level of the origin.(c) Allowing the plate to remain in the developing chamber untilthe eluent front reaches the top of the plate.(d) Removing the plate from the developing chamber when theeluent front has moved only half-way up the plate.(e) Developing the plate in an uncovered chamber.

4. Suppose you prepared benzhydrol by the reduction of benzophe-none with sodium borohydride. Briefly describe how you coulduse TLC to decide when the reaction was complete.



NAME SECTION DATE

TECH 707/Separating a Mixture by Thin-Layer Chromatography


1. (a) Briefly describe why all burners must be turned off before any devel-oping chambers are prepared.

(b) Briefly explain why it is important to work in a fume hood when youpour the eluents used in this experiment.

2. Briefly define or describe each of the following terms as they pertain to thisexperiment.

(a) spotting a plate

(b) developing a plate

(c) visualizing

(d) Rf



3. Use your textbook or an appropriate reference to determine the structuralformulas of biphenyl, benzhydrol, and benzophenone.

(a) Draw the structural formulas of these compounds.

(b) Based on these structures, list the three compounds in order ofincreasing polarity.



ISBN 0-87540-707-2

11

Column Chromatography: Isolation of Pigments From Spinach

63

u

Online edition for students of organic chemistry lab courses at the University of Colorado, Boulder, Dept of Chem and Biochem. (2009)

Experiment 11


Techniques:

Column and Thin Layer Chromatographies (read or review the sections on these techniques in the

Handbook for Organic Chemistry Lab

).

Column chromatography is an isolation and purification technique used extensivelyby organic chemists to obtain pure samples of chemicals from natural sources orfrom reaction sequences.

In column chromatography, a mixture of compounds is applied to the top of a verti-cal column of solid adsorbent packed (usually) in a tall, glass cylinder. As solvent isadded to the top of the column, the compounds move down the column at differentrates. If the compounds are colored (as is the case for the pigments to be isolated inthis experiment) then the progress of the separation can simply be monitored visu-ally. If the compounds to be isolated from a column chromatography are colorless,several means for monitoring the separation progress are available. One of the sim-plest of these involves the collection of relatively small fractions of the eluent inlabeled tubes and the analysis of the component(s) of these fractions by the tech-nique of thin layer chromatography (see Prelab Question #1).

This experiment employs a gravity column; you will be introduced to flash columnsin later experiments in this course. The methods for the packing and elution of thistype of column are not detailed in the

Handbook

, therefore, the procedure is given inthe section below. Uniform packing of the chromatography column is critical to thesuccess of this technique.

Column Chromatography of Spinach Pigments

Two categories of molecules are primarily responsible for photosynthesis in plants:the chlorophylls and the carotenoids. Chlorophylls, the green pigments, absorb cer-tain wavelengths of light that are then converted into chemical energy. The struc-tures of chlorophyll a and b are given in Figure 11.1: the only difference between thetwo is the substitution of an aldehyde group in chlorophyll b for one of the methylgroups in chlorophyll a (the different group is marked with an asterisk). On exposure to air, water, and/or light, the chlorophylls degrade to the pheophytins.Pheophytin a and b are identical to chlorophyll a and b, respectively, except for theloss of the magnesium ion (see Figure 11.1).

Carotenoids, the yellow pigments found in spinach, are also involved in photosyn-thesis.

!

-Carotene differs from

"

-carotene in the placement of the double bond onone of the rings. Xanthophylls are oxygen-containing derivatives of the carotenes; asample xanthophyll, lutein, is illustrated in Figure 11.1.

Carotene is the compound that gives butter and margarine their characteristic yellowcolor. Cows eat the carotene-containing green grass, but do not metabolize the caro-

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11


64

tene entirely, and it ends up in their milk and, thus, the butter made from the milk. Atcertain times of the year, the food that cows eat does not contain carotene, and thebutter made from this milk is white. Since the consumer prefers yellow butter (andmargarine), the dairy companies used to add a dye called Butter Yellow to white but-

Mg

H3CCOOCH3

O

CH2CH2COO-Phytyl

CH3

H3CH2C

H3C

H2C=C CH3

N

N N

N

chlorophyll a

Phytyl = –CH2CH=C(CH3)-(CH2CH2CH(CH3)CH2)2-CH2CH2CH(CH3)2

Mg

H3CCOOCH3

O

CH2CH2COO-Phytyl

CH3

H3CH2C

H2C=C CH3

N

N N

N

chlorophyll b

H

O

HH

H3CCOOCH3

O

CH2CH2COO-Phytyl

CH3

H3CH2C

H3C

H2C=C CH3

HN

N N

NH

pheophytin a

H3CCOOCH3

O

CH2CH2COO-Phytyl

CH3

H3CH2C

H2C=C CH3

HN

N N

NH

pheophytin b

H

O

HH

ß-Carotene

CH3 CH3

CH3 CH3

CH3

CH3

CH3

CH3

CH3

CH3

HO

OH

a xanthophyll (this one is lutein)

* *

Figure 11.1 The chemical structures of several spinach pigments.

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11


65


ter. However, Butter Yellow (an azo dye) was found to be carcinogenic. Currently,butter and margarine are colored with synthetic carotene.

In this column chromatography experiment, you will separate the green and yellowpigments found in spinach. The spinach “extract” is prepared by first grinding freshspinach with magnesium sulfate and sand, and then adding acetone to the ground-upmixture and agitating for several minutes. (Your TA or the Coordinator will preparethe extract while you prepare the column.) The pigments in the extract will separateinto main bands on the column: the fastest-moving band (yellow) contains the car-otenes and the slower-moving band (green) contains the chlorophylls.

Before you run the column, run a TLC of the extract. The crude extract might con-tain all of the following:

Other pigments are sometimes observed. These components arise from air oxida-tion, hydrolysis, and other chemical reactions.

Safety Precautions

Hexanes and acetone are flammable.

Procedure Section

A. Column Chromatography

Obtain a chromatography column (actually, a 10 mL disposable pipet), place a pieceof glass wool in the bottom of the column, and gently tamp it down with a glass rod(Figure 11.2). Clamp the bottom of the column with a pinch clamp. Place a funnel inthe column and add about 5 mL of hexanes to the column. Weigh 8 g of alumina intoa beaker. Place about 15 mL of hexanes in a 125 mL Erlenmeyer flask and slowlyadd the alumina powder, a little at a time, while swirling. Swirl the flask, thenquickly pour some of the slurry into the funnel. Place a flask under the column, openthe pinch clamp, and allow the liquid to drain into it. Continue to transfer the slurryto the column until all the alumina is added. Add more hexanes as necessary; thehexanes collected in the flask under the column can be re-used. (Any hexanesremaining at the end of the packing procedure should be placed in the “RecycledHexanes” recovery bottle.)

carotenes (1 spot; yellow orange)pheophytin a (grey, can be as intense as chlorophyll b; might not see this band)pheophytin b (grey; might not see this band)chlorophyll a (blue-green, more intense than chlorophyll b)chlorophyll b (green)xanthophylls (possibly 3 spots; yellow)

decr

easin

g R f

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66

This slurry method of pouring a column usually eliminates air bubbles from formingin the alumina. When finished packing, drain the excess solvent until it just reachesthe top level of the alumina. Close the pinch clamp.

Obtain about 0.5 mL of spinach extract from your TA. Use a Pasteur pipet to transferit onto the column, being careful not to disturb the surface of the adsorbent. Openthe pinch clamp and allow the colored liquid to pass onto the column. When thelevel of the extract is just even with the top of the column, carefully add a few milli-liters of hexanes to wash down the sides of the column.

When the level of the solvent is again even with the top of the column, add the firsteluting solvent, hexanes/acetone (70:30), to the top of the column, adding the firstfew milliliters carefully. Continue to add this eluting solvent to the column whileyou collect the eluate in flasks under the column. This process is called “eluting thecolumn”. You should observe that the yellow carotenes should move rapidly throughthe column, while the green chlorophylls and xanthophylls move more slowly.Change collection flasks when the yellow band begins to come off the column.

When the yellow band has been eluted from the column, change the eluting solventto the second eluting solvent: acetone (100%). Also change the collection flask. Thegreen band will now begin to move more quickly down the column. When it beginsto come off the column, again change the collection flask.

You should end up with flasks of yellow and green pigments, as well as some inter-mediate flasks that contain clear liquid. It’s not necessary to analyze these fractionsfurther; it’s enough to simply visualize your success in separating the yellow andgreen pigments.

Figure 11.2 Column for gravity chromatography.

fill column withalumina

small plugof glass wool

use a pinch clampon this tubing to control thesolvent flow

Do not allow the column to run dry during

the packing pro-cedure!

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67


B. Analysis by Thin Layer Chromatography

Run a TLC of the spinach extract (the pre-column extract). The TLC eluting solventis: petroleum ether/cyclohexane/ethyl acetate/acetone/methanol (60:16:10:10:4).

*

If time allows, you can also run a TLC of the different column fractions. The columnfractions might need to be concentrated (ask your TA for advice). If you do need toconcentrate them, use the vacuum system to remove solvent rather than evaporationon a steam bath because the pigments are heat-sensitive. Be aware that the highlyunsaturated hydrocarbon pigments are subject to rapid photochemical auto-oxida-tion, although they are somewhat protected when in solution. You might see morespots than expected when you TLC the column fractions due to these degradationreactions.

Wastes

Organic Waste:

Column fractions and the TLC eluting solvent.

Recycled Hexanes Recovery Bottle:

For clean hexanes, used only to pack the chro-matography column.

Used alumina columns:

Place your entire column, still packed (but drained of sol-vent), in the container in the main hood.

Used disposable glassware:

Used microcaps go in the small trash receptacle labeled“used microcaps, mp capillaries, pipets”.

Used TLC plates:

TLC plates go in the small trash receptacle labeled “used TLCplates” (in the main hood).

Study Questions

1) As an example of how TLC can be used to monitor the progress of a separationby column chromatography consider the following hypothetical situation. Achemist found that a mixture of four components (A–D) could be separated on asilica gel TLC plate using 10% diethyl ether in hexanes as the eluting solvent(see far left plate in Figure 11.3 below). The mixture was then chromatographedon a silica gel column eluted with this same solvent mixture and 11 fractions of15 mL each were collected. Thin-layer chromatographic analysis of the variousfractions under the conditions stated above gave the results shown in the figurebelow.

a) According to these results which fractions should be combined to give puresamples of A, B, C, and D?

b) What fraction numbers still contain more than one component? Indicate forthese ‘mixed’ fraction numbers what components of the original mixture arepresent.

* From “An Improved Method for the Extraction and Thin-Layer Chromatography of Chloro-phyll a and b from Spinach,” Quach, Steeper, and Griffin,

J Chem Ed

, 81, pp. 385-387 (2004) for the TLC conditions and the method of preparing the spinach extract used in this experiment.

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68

2) Why do you think that the carotenes move faster in both thin-layer and columnchromatography than do the chlorophylls?

1 2 3 4 5 6 7 8 9 10 11

original mixture

Figure 11.3 Hypothetical TLC analysis of 11 column fractions.

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Dehydrating Cyclohexanolprepared by Carl T. Wigal, Lebanon Valley College


Dehydrate cyclohexanol to prepare cyclohexene. Characterize cyclo-hexene by using ammonium cerium (IV) nitrate test, bromine test, infra-red spectroscopy and/or refractive index.

EXPERIMENTAL OPTIONS Semi-Microscale Dehydration . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3Microscale Dehydration

Using Glassware with Elastomeric Connectors . . . . . . . . . . . . . . . . . 6Using a Hickman Still Assembly. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8

Product Characterization . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11

BACKGROUND REQUIRED You should be familiar with basic laboratory techniques for measuringvolumes and masses. You should know how to conduct a simpledistillation. For product characterization, you should know how tomeasure refractive index and/or obtain and interpret an infrared spectrum.


Elimination reactions involve the loss of a small molecule (H–X) fromadjacent carbon atoms, resulting in pi-bond formation. Consequently,elimination reactions are good synthetic methods for producing alkenesor alkynes. These reactions occur through a process called heterolyticbond cleavage. Heterolytic bond cleavage occurs when one atom leavesa compound with both electrons of the original bond, resulting in theformation of ions. For example, elimination of H–X from an organicmolecule involves the loss of a proton (H+) and a leaving group (X–), asshown in Figure 1. The leaving group departs with both electrons fromthe original C–X bond. The electrons in the adjacent C–H bond form thenew pi bond of the alkene, with loss of the proton.

The elimination of water (H–OH) from alcohols was one of theearliest organic reactions studied. This reaction, still widely used, iscalled a dehydration reaction. In many cases, alcohol dehydration is anacid-catalyzed reaction that proceeds by an elimination mechanism

REAC

712m o d u l a r · l a b o r a t o r y · p r o g r a m · i n · c h e m i s t r ypublisher: H.A. Neidig organic editor: Joe Jeffers

Copyright 1998 by Chemical Education Resources, Inc., P.O. Box 357, 220 South Railroad, Palmyra, Pennsylvania 17078No part of this laboratory program may be reproduced or transmitted in any form or by any means, electronic or mechanical, including photo-copying, recording, or any information storage and retrieval system, without permission in writing from the publisher. Printed in the UnitedStates of America 02 01 00 99 — 15 14 13 12 11 10 9 8 7 6 5 4 3 2

Figure 1 Elimination of HX from an or-ganic molecule

C C

X

H

+ H+X–

alkene

called E1. The E1 mechanism for the dehydration of 2-methyl-2-butanol isshown in Figure 2.

The first step of dehydration is a proton transfer from the acidcatalyst to the oxygen atom of the alcohol. This protonation forms anoxonium ion, the conjugate acid of the alcohol. Weak bases are goodleaving groups, so changing the leaving group from hydroxide to waterfavors the reaction.

The second step of the dehydration reaction is loss of water from theoxonium ion forming a positively charged carbocation. This step of themechanism is rate-determining.

Not all alcohols dehydrate at the same rate. Alcohols are classifiedaccording to the number of alkyl groups attached to the carbon bearingthe hydroxyl group. The terminology used to describe the degree ofsubstitution is tertiary (3°), secondary (2°), and primary (1°).Experimental evidence shows that the ease of alcohol dehydrationfollows the trend 3° > 2° > 1°. This reactivity directly relates to thestability of the carbocation intermediate formed during the rate-determining step of the reaction.

In the third and final step, a proton is released from a carbon atomadjacent to the positively charged carbon. The electrons previouslycomprising the C–H bond form the new carbon–carbon pi bond.

The formation of two isomeric alkenes is possible in eliminationreactions where a proton can be lost from either of two different carbonatoms. Saytzeff’s rule states that the orientation of the double bondfavors the more thermodynamically stable alkene; that is, the alkenewith the greatest number of alkyl groups bonded to the carbons of the

2 REAC 712/Dehydrating Cyclohexanol

1998 Chemical Education Resources

Figure 2 E1 mechanism for the dehydration of 2-methyl-2-butanol

Step 1

Step 2

Step 3

H3CC

OH

H3C CH2CH3 H+

H3CC

OH

H3C CH2CH3

H

+

H3CC

OH

H3C CH2CH3

H

+

–H2O

CH3C+ CH3

CH2CH3

CH2C+ CH3

CHCH3HH

–H+

CH3

CCH3C

H

CH3 +

CH3

CCH2

H2C

CH3

2-methyl-2-butanol oxonium ion

carbocation

2-methyl-2-butene(major product)

2-methyl-1-butene(minor product)

double bond. Thus, dehydrating 2-methyl-2-butanol producesprimarily 2-methyl-2-butene, a trisubstituted alkene, rather than2-methyl-1-butene, a disubstituted alkene.

In this experiment, you will dehydrate cyclohexanol to formcyclohexene. Because cyclohexene has a lower boiling point thancyclohexanol, the cyclohexene can be distilled away as it forms. You willisolate and characterize the cyclohexene by performing qualitative testsfor alcohols and alkenes. Your laboratory instructor will tell youwhether to further characterize the cyclohexene by measuring itsrefractive index and/or by generating infrared spectra for bothcyclohexanol and cyclohexene.

Qualitative Tests The presence of the hydroxyl group of an alcohol can be determined byobserving the reaction of an alcohol with ammonium cerium(IV) nitrate,(NH4)2Ce(NO3)6. A positive test for an alcohol is indicated as the yellow(NH4)2Ce(NO3)6 solution turns red when complexed with an alcohol, asshown in Figure 3. Even small contaminating amounts of alcohol cancause a slight color change.

The presence of a carbon–carbon double bond of an alkene can bedetermined by observing the reaction between bromine and an alkene,as shown in Figure 4. Bromine is a reddish-brown color. A positive test isindicated by the decolorization of the bromine solution.

Semi-Microscale Dehydration

Equipment

100-mL beaker magnetic stir bardistillation apparatus 2 Pasteur pipets, with latex bulb

condenser, with tubing 5-mL sample vialdistilling head sand bath†

10-mL round-bottom flask 13 × 100-mm test tube‡

receiver flask* support ringthermometer, –10 to 260 °C, 2 support stands

or equivalent, with adapter 3 utility clampsvacuum adapter wire gauze

10-mL graduated cylinder*10-mL vial or 10-mL round-bottom flask†stirring hot plate with crystallizing dish filled with sand or magnetic stirrer and electricflask heater filled with sand‡or centrifuge tube

Semi-Microscale Dehydration 3


R R

R R+ Br2

Br

R

R

Br

R

Ralkene red-brown colorless

(NH4)2Ce(NO3)6 + OHR [alcohol + reagent]yellow alcohol red complex

Figure 3 Reaction of ammonium ce-rium(IV) nitrate with an alcohol

Figure 4 Reaction of bromine with analkene


substance quantity molar mass bp d(g/mol) (°C) (g/mL)

calcium chlorideanhydrous 0.25 g 110.99

cyclohexanol 2.84 g 100.16 160 0.948

cyclohexene* 82.15 83 0.811

phosphoric acid, 85% 4.0 mL 98.00 1.685

sulfuric acid,concentrated 0.2 mL 98.08 1.840

*product

Preview

• Assemble the distillation apparatus

• Add cyclohexanol, sulfuric acid, and phosphoric acid to the flask

• Distill the reaction mixture and collect the distillate in a receiver

• Transfer the distillate to a test tube

• Remove the bottom layer of the distillate

• Dry the top layer with anhydrous calcium chloride

• Tare a sample vial

• Transfer the cyclohexene to the sample vial

• Weigh the cyclohexene


anhydrous calcium chloride—irritant and hygroscopic

cyclohexanol—irritant and hygroscopic

cyclohexene—flammable and irritant

phosphoric acid—corrosive

sulfuric acid—toxic and oxidizer


1. Using Distillation toDehydrate Cyclohexanol

Caution: Concentrated sulfuric acid (H2SO4) is toxic and an oxi-dizer. Phosphoric acid (H3PO4) is corrosive. Prevent eye, skin,clothing, and combustible materials contact. Cyclohexanol is an ir-ritant and hygroscopic. Avoid inhaling vapors and ingesting thesecompounds. Use a fume hood.

Assemble the distillation apparatus shown in Figure 5. If necessary, usesubstitute glassware as directed by your laboratory instructor. Removethe 10-mL round-bottom flask from the apparatus. Place 4.0 mL of 85%H3PO4 and 2.84 g (3.0 mL) of cyclohexanol in the round-bottom flask. Add5 drops of concentrated H2SO4 to the flask and add a magnetic stir bar.



Reattach the round-bottom flask to the distillation apparatus. Startthe flow of water through the condenser.

Turn on the magnetic stirrer. Heat the reaction mixture whilestirring until the product starts to distill. Continue the distillation,collecting the product in the receiver flask until no more liquid distills oruntil the temperature of the thermometer rises above 85 °C.

Turn off the heat. Allow the apparatus to cool. Turn off the magneticstirrer.

Remove the receiver flask from the distillation assembly. Use a Pasteurpipet to transfer the distillate into a centrifuge tube or small test tube.

2. Isolating Cyclohexene Caution: Cyclohexene is flammable and irritating. Keep away fromflames or other heat sources. Prevent eye, skin, and clothing con-tact. Avoid inhaling vapors. Use a fume hood.

Notice that as the distillate in the tube cools, two layers form. Use thePasteur pipet to remove the majority of the bottom layer. Place thebottom layer into the container labeled “Recovered Acid Layer”,provided by your laboratory instructor.

Caution: Anhydrous calcium chloride is irritating and hygroscopic.Avoid inhaling dust.

Dry the top organic layer by placing about 0.25 g of anhydrouscalcium chloride into the test tube. Let the test tube stand for 5 min.

Weigh a clean 5-mL sample vial. Using a clean dry Pasteur pipet,remove the liquid from the test tube, and transfer the liquid to the taredsample vial. Weigh your product.

Semi-Microscale Dehydration 5


Figure 5 Distillation apparatus forsemi-microscale dehydration

Characterize your cyclohexene using the methods in the ProductCharacterization section designated by your laboratory instructor.

3. Cleaning Up Place your recovered materials in the appropriate labeled collectioncontainers as directed by your laboratory instructor. Clean yourglassware with soap or detergent.

Caution: Wash your hands thoroughly with soap or detergent be-fore leaving the laboratory.

Microscale DehydrationUsing Glassware with Elastomeric Connectors

Equipment

100-mL beaker 2 Pasteur pipets, with latex bulbdistillation apparatus receiver vial

elastomeric connectors 5-mL sample vialdistilling head–air condenser sand bath*5-mL round-bottom flask support ringthermometer, –10 to 260 °C, support stand

or equivalent 10 × 75-mm test tube†

10-mL graduated cylinder 2 utility clampsmagnetic stir bar wire gauze*stirring hot plate with crystallizing dish filled with sand or magnetic stirrer and electricflask heater filled with sand†or centrifuge tube



calcium chloride,anhydrous 0.25 g 110.99

cyclohexanol 1.422 g 100.16 160 0.948




*product

Preview


• Add cyclohexanol, sulfuric acid, and phosphoric acid to the flask

• Distill the reaction mixture and collect the distillate in a receiver


















Assemble the distillation apparatus shown in Figure 6. Remove the5-mL round-bottom flask from the apparatus. Place 1.5 mL of 85%H3PO4 and 1.422 g (1.5 mL) of cyclohexanol into the flask. Add 3 dropsof concentrated H2SO4 to the flask and add a magnetic stir bar. Reattachthe round-bottom flask to the distillation apparatus.

Turn on the magnetic stirrer. Heat the reaction mixture while stirringuntil the product starts to distill. Continue the distillation, collecting theproduct in the receiver vial until no more liquid distills or until thetemperature of the thermometer rises above 85 °C.



Figure 6 Distillation apparatus for mi-croscale dehydration using glasswarewith elastomeric connectors

Turn off the heat. Allow the apparatus to cool. Turn off themagnetic stirrer.

Remove the receiver vial from the distillation assembly. Use a Pasteurpipet to transfer the distillate into a centrifuge tube or small test tube.


Notice that as the distillate in the tube cools, two layers form. Use thePasteur pipet to remove the majority of the bottom layer. Place thebottom layer into the container labeled “Recovered Acid Layer”,provided by your laboratory instructor.


Dry the top organic layer by placing about 0.25 g of anhydrouscalcium chloride in the test tube. Let the test tube stand for 5 min.

Weigh a clean 5-mL sample vial. Using a clean dry Pasteur pipet,remove the liquid from the tube and transfer the liquid to the taredsample vial. Weigh your product.




Microscale DehydrationUsing a Hickman Still Assembly

Equipment

5-mL conical vial support stand10-mL graduated cylinder 10 × 75-mm test tube†

Hickman still thermometer, –10 to 150 °C,magnetic spin vane with adapter‡

2 Pasteur pipets, with latex bulb thermometer, –10 to 260 °C, or3-mL sample vial equivalentsand bath* 2 utility clamps*stirring hot plate with crystallizing dish filled with sand or magnetic stirrer and electricflask heater filled with sand†or centrifuge tube‡to fit Hickman still





calcium chlorideanhydrous 0.25 g 110.99

cyclohexanol 1.422 g 100.16 160 0.948




*product

Preview


• Add cyclohexanol, sulfuric acid, and phosphoric acid to the vial

• Distill the reaction mixture into the still collar
















Assemble the distillation apparatus shown in Figure 7 on the next page.

Using a Hickman Still Assembly 9


Remove the 5-mL conical vial from the apparatus. Place 1.5 mL of85% H3PO4 and 1.422 g (1.5 mL) of cyclohexanol into the vial. Add 3drops of concentrated H2SO4 to the vial and add a magnetic spin vane.Reattach the vial to the distillation apparatus.

Turn on the magnetic stirrer. Heat the reaction mixture while stirringuntil the product starts to distill. As the product starts to collect in the still,use a Pasteur pipet to remove the liquid from the still. [NOTE 1] Transfer theliquid into a centrifuge tube or a small test tube. Continue the distillationand collection until no more liquid distills or until the temperature of thethermometer rises above 85 °C.

Turn off the heat. Allow the apparatus to cool. Turn off the magneticstirrer.


Notice that as the distillate in the test tube or vial cools, two layers form.Use the Pasteur pipet to remove the majority of the bottom layer. Placethe bottom layer into the container labeled “Recovered Acid Layer”,provided by your laboratory instructor.


Dry the top organic layer by placing about 0.25 g of anhydrouscalcium chloride into the test tube. Let the test tube stand for 5 min.

Weigh a clean 3-mL sample vial. Using a clean dry Pasteur pipet,remove the liquid from the test tube and transfer the liquid to the taredsample vial. Weigh your product.




Figure 7 Distillation apparatus formicroscale dehydration using a Hick-man still

NOTE 1: If your Hickman still is notequipped with a side port, prepare abent-tip Pasteur pipet as directed byyour laboratory instructor.



Product Characterization

Equipment

5 Pasteur pipets, with latex bulb white spot plate3 test tubes, 10 × 75-mm


substance quantity molar mass bp(g/mol) (°C)

ammonium cerium(IV)nitrate test reagent 0.6 mL

bromine test reagent 0.24 mL

cyclohexanol 0.16 mL 100.16 160

cyclohexene 0.16 mL 82.15 83

dichloromethane 2.4 mL 84.93 40

1,4-dioxane 0.36 mL 88.11 100–102

Preview

• Test standards and product for presence of alcohol using ammo-nium cerium(IV) nitrate reagent

• Test standards and product for presence of alkene using brominereagent

• Compare cyclohexanol and cyclohexene product using infraredanalysis

• Measure refractive index of cyclohexene product


ammonium cerium(IV) nitrate—irritant and oxidizer

bromine—highly toxic and oxidizer



dichloromethane—toxic and irritant

1,4-dioxane—flammable and suspected carcinogen


Product Characterization 11


1. Using AmmoniumCerium(IV) Nitrate to

Test for Alcohols

Caution: Ammonium cerium(IV) nitrate, (NH4)2Ce(NO3)6, is irri-tating and an oxidizer. 1,4-Dioxane is flammable and a suspectedcarcinogen. Keep 1,4-dioxane away from flames or other heatsources. Prevent eye, skin, and clothing contact. Avoid inhaling va-pors. Use 1,4-dioxane in a fume hood.

Cyclohexanol is irritating and hygroscopic. Cyclohexene isflammable and irritating. Keep cyclohexene away from flames orother heat sources. Prevent eye, skin, and clothing contact. Avoidinhaling vapors.

Place 5 drops of the (NH4)2Ce(NO3)6 test reagent and 3 drops of1,4-dioxane in each of three wells of a white spot plate. [NOTE 1] Add 2drops of cyclohexanol to the first well and stir. Observe any color changeand record your results.

Add 2 drops of cyclohexene from the reagent bottle to the secondwell and stir. Observe any color change and record your results.

Add 2 drops of your cyclohexene product to the third well and stir.Observe any color change and record your results.

2. Using Bromine toTest for Alkenes

Caution: Bromine (Br2) is highly toxic. Dichloromethane is toxicand irritating. Cyclohexanol is irritating and hygroscopic.Cyclohexene is flammable and irritating. Keep cyclohexene awayfrom flames or other heat sources. Prevent eye, skin, and clothingcontact. Avoid inhaling vapors. Use these reagents in a fume hood.

Label three small test tubes “cyclohexanol”, “cyclohexene”, and “product”.Place 2 drops of cyclohexanol into the tube labeled “cyclohexanol”. Place 2drops of cyclohexene into the tube labeled “cyclohexene”. Place 2 drops ofyour product into the tube labeled “product”.

Add 20 drops of dichloromethane into each test tube, and stir. [NOTE 2].

Add 2 drops of the Br2 test reagent to each tube and stir. Observe any colorchange. Record your results.

3. Using Infrared Analysis toCompare Cyclohexanol and Your

Cyclohexene Product

Obtain the operating instructions for using the infrared spectrometerfrom your laboratory instructor. Obtain a set of KBr, NaCl, or AgCl saltplates and a holder. [NOTE 3] Place 1 drop of your cyclohexanol betweenthe salt plates. Gently press the plates together to remove any airbubbles. Place the plates in the holder and secure the plates. Run andplot the IR spectrum according to your operating instructions.

Repeat this procedure for your cyclohexene product. Examine theregion of the spectrum above 1500 cm–1. Assign the bonds that give riseto these absorptions.

4. Using Refractive Indexto Characterize Your

Cyclohexene Product

Obtain the operating instructions for the refractometer from yourlaboratory instructor. Measure the refractive index for your cyclohexeneproduct. Measure the laboratory temperature in °C. Make temperaturecorrections, if necessary. [NOTE 4] Compare your refractive index to theliterature values shown in Table 1.



NOTE 1: 1,4-Dioxane is used as asolvent.

NOTE 2: Dichloromethane is used as asolvent.

NOTE 3: Salt plates are fragile and hy-groscopic. Do not use water to wipe theplates. Even moisture from your fingerswill attack the plates. Use gloves andonly handle the plates by the edges.

Table 1 Refractive indices (20 °C)

water 1.3329

cyclohexanol 1.4650

cyclohexene 1.4460



Product Characterization 13


NOTE 4: The refractive index at 20 °Cis calculated by using the followingequation, where T is the ambient tem-perature in degrees Celsius and n T

Dis the

refractive index measured at ambienttemperature.

n n T CD

T

D

20 0 00045 20= + °. ( – )

Post-Laboratory Questions 1. Calculate the percent yield that you obtained from this reaction.2. Did your product cause a color change with (NH4)2Ce(NO3)6 test

reagent? Explain your results.3. Did your product cause a color change with the Br2 test reagent? Ex-

plain your results.4. Compare the IR spectra for cyclohexanol and your product. What IR

evidence do you have that your product is cyclohexene and not cy-clohexanol? Briefly explain.

5. (a) Calculate the refractive index of your product at 20 °C.(b) Compare the refractive index of your product to the data of Ta-ble 1. Does the result indicate that your product is pure? Brieflyexplain.(c) If the refractive index of your product differs from the listedvalue, what is the most likely contaminant in your product, as indi-cated by the refractive index? Briefly explain.

6. What would be the major product obtained from the E1 dehydrationof 2-methylcyclohexanol?

7. Outline a mechanism for the dehydration of 1-methyl-1-cyclohexanol.Would you predict this reaction to be faster or slower than the reactionyou performed?



NAME SECTION DATE

REAC 712/Dehydrating Cyclohexanol


1. What safety precautions must be observed when using concentrated H2SO4and H3PO4?

2. (a) Write the chemical equation for the dehydration of cyclohexanol.

(b) Using the following information, calculate the theoretical yield for thedehydration of 3.0 mL of cyclohexanol.

substance molar mass d(g/mol) (g/mL)

cyclohexanol 100.16 0.948

cyclohexene 82.15 0.811

3. When 2-butanol undergoes E1 dehydration, three alkenes are obtained.Draw the structures for these alkenes. Which alkene would you predict to beformed in greatest abundance?



ISBN 0-87540-712-9


Brominating Alkenesprepared by Carl T. Wigal, Lebanon Valley College


Synthesize vicinal dihalides by brominating alkenes. Characterize vici-nal dihalides by using the silver nitrate test and by using melting pointmeasurement to determine the relative stereochemistry.

EXPERIMENTAL OPTIONS Using Semi-Microscale Techniques to Brominate Alkenes . . . . . . . . . . . 4Cinnamic Acid . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6cis-Stilbene . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6trans-Stilbene . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6

Using Microscale Techniques to Brominate Alkenes . . . . . . . . . . . . . . . . 7Cinnamic Acid . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8cis-Stilbene . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9trans-Stilbene . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9

BACKGROUND REQUIRED You should consult your textbook for the Cahn–Ingold–Prelog Systemfor assigning the configuration of a chiral center. You should be famil-iar with techniques for reflux, for vacuum filtration, and for meltingpoint measurement.


The halogenation of alkenes is an important reaction in the chemical in-dustry. For example, over 8 million tons of 1,2-dichloroethane per year areproduced by the addition of chlorine (Cl2) to ethylene. This product isused both as a solvent and in the preparation of polyvinyl chloride, PVC, acommon organic polymer used in household plumbing. The products ob-tained from alkene halogenation are called vicinal dihalides because thetwo halogen substituents are attached to adjacent carbon atoms.

When the halogen used is either bromine (Br2) or chlorine (Cl2),halogenation of alkenes occurs rapidly at room temperature, and the re-sulting vicinal dihalides are stable. Fluorination is a violent reaction thatis difficult to control and is accompanied by several side reactions. Iodin-ation is an endothermic process, resulting in vicinal diiodides that tendto revert to alkenes. Consequently, the most common applications ofalkene halogenation are chlorination and bromination.

Typically, alkenes undergo reactions through electrophilic addition,a process in which the alkene pi (π) bond is replaced with two sigma (σ)bonds. The general mechanism of electrophilic addition involves twosteps, as shown in Figure 1 on the next page.

m o d u l a r · l a b o r a t o r y · p r o g r a m · i n · c h e m i s t r ypublisher: H.A. Neidig organic editor: Joe Jeffers

Copyright 1998 by Chemical Education Resources, Inc., P.O. Box 357, 220 South Railroad, Palmyra, Pennsylvania 17078No part of this laboratory program may be reproduced or transmitted in any form or by any means, electronic or mechanical, including photo-copying, recording, or any information storage and retrieval system, without permission in writing from the publisher. Printed in the UnitedStates of America 02 01 00 99 — 15 14 13 12 11 10 9 8 7 6 5 4 3 2

SYNT

719

The first step involves reaction of an electron deficient species, called anelectrophile (E+), with the electron-rich πbond of the alkene. The two elec-trons of the π bond shift toward the electrophile, forming a new car-bon–electrophile σ bond. This step results in formation of a positivelycharged intermediate. In most instances, the positive charge centers on acarbon atom, so the electron-deficient intermediate is called a carbocation.

In the second step, the electrophilic carbocation reacts with anelectron-rich species called a nucleophile (Nu–). The nucleophile do-nates an electron pair to the positively charged intermediate forming acarbon–nucleophile σ bond.

Alkene bromination follows the same general mechanism with a fewimportant modifications, as shown in Figure 2. In the first step, the prox-imity of the π electrons of the alkene to Br2 polarizes the bromine–bro-mine bond. This polarization induces a bond dipole that allows Br2 to actas an electrophile. Electrons flow from the π bond to the polarized Br2,forming a carbon–bromine bond and breaking the bromine–brominebond, producing a bromide ion (Br–). The positively charged intermedi-ate in bromination is not a carbocation, but a bromonium ion. This cyclicintermediate results from a nonbonding electron pair from bromine thatstabilizes the positive charge on carbon. The bromonium ion is more sta-ble than a simple carbocation because all atoms of the bromonium ionhave an octet of electrons.

In the second step, Br– acts as a nucleophile, attacking the elec-tron-deficient bromonium ion. The second carbon–bromine σ bond of thevicinal dihalide forms in this step. An important feature of this step is theresulting stereochemistry. Bromide ion adds to the side opposite the car-bon–bromine bonds of the bromonium ion. This process is called anti-addi-tion. Anti-addition of Br– occurs because Br– is blocked from one face of thebromonium ion by the bromine atom. The consequence of anti-addition ofBr2 to cyclic alkenes is the formation of trans vicinal dihalides.

Chiral carbon atoms, or chiral centers, are generated in many or-ganic reactions. A chiral carbon is a carbon atom that is bonded to fourdifferent substituents. As a consequence, two different configurationsare possible for a chiral center: rectus (R) or sinister (S).

2 SYNT 719/Brominating Alkenes


Figure 1 Electrophilic addition to analkene

Figure 2 Bromination of cyclohexene

CCE+ ENu

carbocation

E+ Nu– Nu–

Br

Br

Br

δ+ δ–bromonium ion

BrBr +

Br–

In the case of alkene bromination, the formation of the two car-bon–bromine σ bonds could result in two new chiral centers in the vici-nal dihalide. Using the 2n rule, where n is the number of chiral centers ina molecule, a maximum of four stereoisomers could result from alkenebromination. However, not all stereoisomers are formed during a singlebromination. Bromonium ion formation preserves the stereochemicalintegrity of the starting material. Therefore, trans alkenes form trans bro-monium ions and cis alkenes form cis bromonium ions.

Consider the bromination of maleic and fumaric acids, as shown inFigure 3. When maleic acid, the cis isomer, is brominated, the bro-monium ion formed has cis configuration. The product is a mixture oftwo stereoisomers. These stereoisomers are enantiomers having the ab-solute configurations of 2R,3R and 2S,3S. Enantiomers are moleculesthat contain chiral centers and are non-superimposable mirror images.

Anti-addition of Br– to the cis bromonium ion can take place by ei-ther path a or path b in Figure 3. Addition by path a results in 2R,3R,while addition via path b results in 2S,3S. Addition occurs at the samerate by either path; therefore, the two enantiomers are produced in equalamounts. A mixture containing equal amounts of a pair of enantiomersis called a racemic mixture. A racemic mixture is often designated byplacing (+) at the front of the name.

Unlike bromination of maleic acid, bromination of fumaric acid re-sults in a single stereoisomer. The bromination of fumaric acid results ina trans bromonium ion intermediate. Addition of Br– to the trans brom-onium ion by either path a or path b yields the same compound. The ab-solute stereochemistry of the product is 2R,3S, which is identical to2S,3R. This compound is an example of a meso compound, which is a



HO2C CO2H

H H

BrHO2C CO2H

HH

+

Br –

a b

HO2C

CO2HH

HBr

Br

2R, 3R

Br

BrHO2C

CO2H

H

H

2S, 3S

a

b

2R, 3S

Br

BrHO2C

H

H

CO2H

2S, 3R

BrHO2C H

CO2HH

+

Br –

a b

HO2C H

H CO2H

a

b

(meso)

(meso)

HO2C

HH

CO2HBr

Br

maleic acid

fumaric acid

Br2

Br2

Figure 3 Bromination of maleic acid and fumaric acid

molecule that contains chiral centers, but also contains an internal planeof symmetry. As a result, meso compounds have superimposable mirrorimages. Meso dibromides result from the bromination of symmetricallydisubstituted trans alkenes.

If the alkene is not symmetrically substituted, a pair of enantiomerswill result. The relative stereochemistry of the enantiomers could haveboth chiral centers having the same configuration (R, R or S,S) or the op-posite configuration (R,S or S,R). The prefixes erythro and threo are usedto differentiate these stereoisomers. Erythro refers to a pair of enanti-omers having a configuration similar to the sugar erythrose. The erythroform is often described as “meso-like” because the molecule would havea plane of symmetry if the two dissimilar groups were equivalent. Threorefers to a pair of enantiomers having a configuration similar to thesugar threose. These configurations are shown in Figure 4.

In this experiment, you will brominate an alkene using pyridiniumtribromide, a comparatively safe, convenient source of bromine. Youwill characterize your product by measuring its melting point and byconducting a silver nitrate test. Vicinal dihalides react with alcoholic sil-ver nitrate within five minutes to form a precipitate of the correspondingsilver halide. This reaction can serve as a simple test for the presence ofbromine or chlorine atoms. You will determine the relative stereochem-istry of your product by melting point measurement because the stereoi-somers’ melting points differ significantly.

Using Semi-Microscale Techniques to Brominate Alkenes

Equipment

250-mL beaker 2 Pasteur pipets, with latex bulb25-mL filter flask, reflux assembly

with vacuum tubing condenser, with tubingfilter paper 25-mL round-bottom flask10-mL graduated cylinder thermometer, –10 to 260 °CHirsch funnel, with adapter sand bath*magnetic stir bar spatulamagnetic wand 2 support standsmelting point capillary tubes 13 × 100-mm test tubemicropipet, 100 to 1000-µL 2 utility clamps*stirring hot plate with crystallizing dish filled with sand or magnetic stirrer and electricflask heater filled with sand



H OHCHO

H OHCH2OH

HO HCHO

HHOCH2OH

H OHCHO

HO HCH2OH

HO HCHO

H OHCH2OH

(R,R) (S,S) (R,S) (S,R)

(±)-erythrose (±)-threose

(a) (b)

Figure 4 Configurations of (a) erythroseand (b) threose


substance quantity molar bp mp dmass (°C) (°C) (g/mL)

(g/mol)

acetic acid*, glacial 6 mL 60.05 118 1.049

cinnamic acid 450 mg 148.16 133

1,2-dibromo-1,2-diphenylethane† 340.07

2,3-dibromo-3-phenyl-propanoic acid† 307.97

ethanol*, 95% 0.5 mL

pyridiniumtribromide* 0.6–1.155 g 319.84

silver nitrate*,2% in ethanol 0.5 mL

cis-stilbene 300 µL 180.25 14513mm 1.011

trans-stilbene 300 mg 180.25 124*amount for one bromination†product

Preview

• Assemble the reflux apparatus

• Add the alkene, pyridinium tribromide, and acetic acid

• Reflux the reaction mixture

• Allow the reaction mixture to cool to room temperature

• Remove the stir bar

• Add water and cool the reaction mixture in an ice-water bath

• Use vacuum filtration to isolate the product

• Dry and weigh the product

• Measure the melting point of the product

• Test the product with silver nitrate reagent


acetic acid—corrosive

cinnamic acid—irritant

ethanol—flammable and irritant

pyridinium tribromide—corrosive and lachrymator

silver nitrate—toxic and oxidizer


Using Semi-Microscale Techniques to Brominate Alkenes 5


Figure 5 Semi-microscale refluxapparatus

Figure 6 Vacuum filtration apparatus

1. Assembling the Apparatus Assemble the reflux apparatus shown in Figure 5 on the previous page.

2. Brominating the Alkenes[NOTE 1]

Caution: Acetic acid is corrosive. Pyridinium tribromide is corro-sive and a lachrymator. Prevent eye, skin, and clothing contact.Avoid inhaling or ingesting these compounds. Use a fume hood todispense these reagents.

Cinnamic Acid

Caution: Cinnamic acid is irritating. Prevent eye, skin, and clothingcontact.

Remove the 25-mL round-bottom flask from the apparatus. Place 450mg of cinnamic acid, 6.0 mL of acetic acid, and 1.155 g of pyridiniumtribromide in the round-bottom flask. Add a magnetic stir bar.[NOTE 2] Proceed to Part 3.

cis-Stilbene

Remove the 25-mL round-bottom flask from the apparatus. Place 300 µLof cis-stilbene, 6.0 mL of acetic acid, and 600 mg of pyridiniumtribromide in the round-bottom flask. Add a magnetic stir bar. [NOTE 2]

Proceed to Part 3.

trans-Stilbene

Remove the 25-mL round-bottom flask from the apparatus. Place 300mg of trans-stilbene, 6.0 mL of acetic acid, and 600 mg of pyridiniumtribromide in the round-bottom flask. Add a magnetic stir bar. [NOTE 2]

Proceed to Part 3.

3. Refluxing the Reaction Reattach the round-bottom flask to the reflux apparatus. Start the flow ofwater through the condenser. Heat the reaction mixture to reflux whilestirring. Reflux for 20 min. After 20 min, remove the flask from the heat.Allow the reaction mixture to cool for 5 min. Turn off the water.

Remove the condenser and use a magnetic wand to remove the stirbar. Add 8.0 mL of distilled or deionized water to the flask. Prepare anice-water bath by half filling a 250-mL beaker with equal volumes of iceand water. Place the flask in the ice-water bath for 15 min.


While the reaction mixture is cooling in the ice bath, assemble a vacuumfiltration apparatus, as shown in Figure 6 on the previous page. Turn onthe water to the aspirator and moisten the filter paper with a few dropsof water. Filter the crystalline solid using the vacuum filtration appara-tus. Wash the crystals with 3.0 mL of water. Allow the crystals to dry inthe Hirsch funnel by pulling air through the funnel for 15 min. Weighyour dried product and record its mass.

5. Identifying the Product Caution: Ethanol is flammable and irritating. Keep away fromflames or other heat sources. Silver nitrate is toxic and oxidizing.Prevent eye, skin, and clothing contact. Avoid inhaling fumes andingesting these compounds.



NOTE 1: Your laboratory instructorwill designate which alkenes you willbrominate.

NOTE 2: The solid materials are notsoluble at room temperature in aceticacid.

Measure and record the melting point of the product. Using a small testtube, dissolve approximately 10 mg of the product in 0.5 mL of 95%ethanol. To this test tube, add 0.5 mL of 2% ethanolic silver nitrate.Allow the test tube to stand for 5 min. Record the presence or absenceof a precipitate.

6. Cleaning Up Place your recovered materials in the appropriate labeled collection con-tainers as directed by your laboratory instructor. Clean your glasswarewith soap or detergent.


Using Microscale Techniques to Brominate Alkenes

Equipment

250-mL beaker 10-mL graduated cylinderconical vial reflux assembly* Hirsch funnel, with adapter

condenser, with tubing magnetic stir bar or spin vane5.0-mL conical vial melting point capillary tubes

elastomeric connector 100-µL micropipetreflux assembly* 2 Pasteur pipets, with latex bulb

condenser, with tubing sand bath‡

elastomeric connector spatula5.0-mL round-bottom flask 2 support stands

25-mL filter flask, 13 × 100-mm test tubewith vacuum tubing thermometer, –10 to 260 °C

filter paper 2 utility clampsforceps†

*use reflux assembly indicated by your laboratory instructor†or a magnetic wand‡stirring hot plate with crystallizing dish filled with sand or magnetic stirrer and electricflask heater filled with sand


substance quantity molar bp mp dmass (°C) (°C) (g/mL)

(g/mol)

acetic acid*, glacial 2 mL 60.05 118 1.049

cinnamic acid 150 mg 148.16 133

1,2-dibromo-1,2-diphenylethane† 340.07

2,3-dibromo-3-phenyl-propanoic acid† 307.97

ethanol*, 95% 0.5 mL

pyridiniumtribromide* 200–385 mg 319.84

Using Microscale Techniques to Brominate Alkenes 7


silver nitrate*,2% in ethanol 0.5 mL

cis-stilbene 100 µL 180.25 14513mm 1.011

trans-stilbene 100 mg 180.25 124*amounts for one bromination†product

Preview

• Assemble the reflux apparatus

• Add the alkene, pyridinium tribromide, and acetic acid

• Reflux the reaction mixture

• Allow the reaction mixture to cool to room temperature

• Remove the stir bar

• Add water and cool the reaction mixture in an ice-water bath

• Use vacuum filtration to isolate the product

• Dry and weigh the product

• Measure the melting point of the product

• Test the product with silver nitrate reagent


acetic acid—corrosive

cinnamic acid—irritant

ethanol—flammable and irritant

pyridinium tribromide—corrosive and lachrymator

silver nitrate—toxic and oxidizer


1. Assembling the Apparatus Depending upon your glassware, assemble the reflux apparatus shownin Figure 7(a) or 7(b).

2. Brominating the Alkenes[NOTE 1]

Caution: Acetic acid is corrosive. Pyridinium tribromide is corro-sive and a lachrymator. Prevent eye, skin, and clothing contact.Avoid inhaling or ingesting these compounds. Use a fume hood todispense these reagents.

Cinnamic Acid

Caution: Cinnamic acid is irritating. Prevent eye, skin, and clothingcontact.

Remove the 5.0-mL conical vial (or round-bottom flask) from theapparatus. Place 150 mg of cinnamic acid, 2.0 mL of acetic acid, and 385mg of pyridinium tribromide in the conical vial (flask). Add a magneticstir bar. [NOTE 2] Proceed to Part 3.



NOTE 1: Your laboratory instructorwill designate which alkenes you willbrominate.

NOTE 2: The solid materials are notsoluble at room temperature in aceticacid.

cis-Stilbene

Remove the 5.0-mL conical vial (or round-bottom flask) from theapparatus. Place 100 µL of cis-stilbene, 2.0 mL of acetic acid, and 200 mgof pyridinium tribromide in the conical vial (flask). Add a magnetic stirbar. [NOTE 2] Proceed to Part 3.

trans-Stilbene

Remove the 5.0-mL conical vial (or round-bottom flask) from theapparatus. Place 100 mg of trans-stilbene, 2.0 mL of acetic acid, and 200mg of pyridinium tribromide in the conical vial (flask). Add a magneticstir bar. [NOTE 2] Proceed to Part 3.

3. Refluxing the Reaction Reattach the conical vial (flask) to the reflux apparatus. Start the flow ofwater through the condenser. Heat the reaction mixture to reflux whilestirring. Reflux for 15 min. After 15 min, remove the vial (flask) from theheat. Allow the reaction mixture to cool for 5 min.

Remove the condenser and use forceps or a magnetic wand to re-move the stir bar. Add 2.5 mL of distilled or deionized water to the coni-cal vial (flask). Prepare an ice-water bath by half-filling a 250-mL beakerwith equal volumes of ice and water. Place the vial (flask) in the ice-water bath for 15 min.


While the reaction mixture is cooling in the ice bath, assemble a vacuumfiltration apparatus, as shown in Figure 8.

Turn on the water to the aspirator and moisten the filter paper with afew drops of water. Filter the crystalline solid using the vacuum filtra-tion apparatus. Wash the crystals with 3 mL of water.

Allow the crystals to dry in the Hirsch funnel by pulling air throughthe funnel for 15 min. Weigh your dried product and record its mass.

Using Microscale Techniques to Brominate Alkenes 9


Figure 7 Microscale reflux apparatuswith (a) conical vial or (b) round-bottomflask and elastomeric connectors

Figure 8 Vacuum filtration apparatus

5. Identifying the Product Caution: Ethanol is flammable and irritating. Keep away fromflames or other heat sources. Silver nitrate is toxic and oxidizing.Prevent eye, skin, and clothing contact. Avoid inhaling fumes andingesting these compounds.

Measure and record the melting point of the product. Using a small testtube, dissolve approximately 10 mg of the product in 0.5 mL of 95%ethanol. To this test tube, add 0.5 mL of 2% ethanolic silver nitrate.Allow the test tube to stand for 5 min. Record the presence or absence ofa precipitate.

6. Cleaning Up Place your recovered materials in the appropriate labeled collection con-tainers as directed by your laboratory instructor. Clean your glasswarewith soap or detergent.


Post-Laboratory Questions 1. (a) Compare the melting point of your product(s) to the data pro-vided. In each case, identify the product you produced.

compound mp (°C)

2,3-dibromo-3-phenylpropanoic acid

(±)-threo 94

(±)-erythro 203

(±)-1,2-dibromo-1,2-diphenylethane 110

meso-1,2-dibromo-1,2-diphenylethane 238

(b) Draw your product in its correct stereochemical configuration.(c) Compare your results with your predictions for Pre-LaboratoryAssignment question 4.

2. Calculate the percent yield that you obtained from your alkene bro-minations.

3. (a) When silver nitrate solution was added to your product, whatdid you observe?(b) Explain your observations.

4. Write reactions for the brominations you performed, in each caseshowing the intermediate bromonium ion that formed.



NAME SECTION DATE

SYNT 719/Brominating Alkenes


1. What safety precautions must be observed when using(a) pyridinium tribromide?

(b) acetic acid?

2. Calculate the theoretical yield for the bromination of both stilbenes and cin-namic acid, assuming the presence of excess pyridinium tribromide. Notethe theoretical yields here and in your laboratory notebook.

3. Draw the mechanism, including the intermediate bromonium ion, generatedin the bromination of trans-2-butene.

4. (a) Look up and draw structures for cinnamic acid, cis-stilbene, andtrans-stilbene.(b) Predict the relative stereochemistry of each product and draw the pre-dicted structures.



ISBN 0-87540-719-6


chm 2005 lab manaul summer 2010 - peter

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