Experiment 2 

DISTILLATION AND GAS CHROMATOGRAPHY

OF ALKANES

Objectives To explore the relationship between

molecular structure, intermolecular forces and boiling points of various types of compounds.

To learn techniques of simple distillation and gas chromatography (GC).

To use these techniques to separate and identify compounds in an unknown mixture.

Before coming to lab…

It is *highly recommended* that you watch the following video prior to coming to lab. http://www.youtube.com/watch?

v=3JlIPnyrZMw

Structure, IMF, and Boiling Point

Alkanes are compounds which consist of ONLY carbon and hydrogen.

Straight chain alkanes will have a boiling point higher than that of a branched isomer due to greater surface area in contact, thus greater LDF between adjacent molecules.

Table 2.1

UnknownMolecular Formula

bp(oC)

Structure

Pentane(GC

SOLVENT)

Use www.chemexper.co

m

These can be drawn in Kekulé form OR skeletal!

n-hexane3-methylpentane

n-heptaneisooctane

(2,2,4-trimethylpenta

ne)

H CH

HCH

HCH

HCH

HCH

HH

Distillation Distillation is a laboratory technique

used for separating and purifying liquids. To separate two or more liquids by

distillation, you first heat them in a flask. The more volatile liquid (the liquid with

the lower boiling point) will typically evaporate first and the vapor will pass into a condensing column, where it can revert into a liquid (condense) on the cool glass where it trickles into a collection flask.

Heating further will cause the less volatile liquids to evaporate and distill at higher temperatures.

Distillation Involves vaporization,

condensation, and collection of the condensate (distillate).

The temperature of the distillate is measured by a thermometer placed in the path of the vapor.

Heating Mantle

to voltage regulator

water out

water in

iron ring

Collect four 5 mL fractions in small test tubes!

Simple Distillation Simple distillation is most effective when

the mixture contains only one volatile component.

More than one….most effective if the boiling points of the components differ by at least 70 ºC.

Chromatography Chromatography is the collective term

for a family of laboratory techniques for the separation of mixtures.

It involves passing a mixture dissolved in a "mobile phase" through a stationary phase, which separates the analyte to be measured from other molecules in the mixture and allows it to be isolated.

Gas Chromatography

In Gas chromatography, the MOBILE PHASE is helium gas.

For this reason, gas chromatography will be used only to separated volatile compounds in lab.

He

Carrier Gas

Injectionport

Detector

Column

Recorder-computer

Oven

Gas Chromatography

A A

A

AA

A

A

A

B

B

B

B

B

BB B

Liquid Stationary Phase

B A

A A

A

AA

A

AA

B BB

B

B

B

BB

Capillary Tube

He Carrier gas

Time0

Immediately after injection

He Carrier gas

After several minutes Resulting chromatogram

Lower boiling

component

Higher boiling

component

Typically, a nonpolar packing material is used as the STATIONARY PHASE.

For this reason, when analyzing compounds of similar polarity, the compounds will elute in order of volatility.

•The individual fractions collected are analyzed by GC during the course of the distillation. Based on peak size, it is evident that as the distillation proceeds, the later fractions contain mainly the higher boiling component of the mixture.

Original Sample

Fraction 2

Fraction 5

pentanehexane

Unknown X

Original Sample Mixture

Fraction 1

Fraction 5

Pentane( GC Sample

Solvent)Alkane #1

Alkane #2

•The individual fractions collected are analyzed by GC during the course of the distillation. Based on peak size, it is evident that as the distillation proceeds, the later fractions contains only the higher boiling component of the mixture.

Product Analysis(Quantification of Components…Adjusted Area

Percent) Most often you will dissolve your

compound or mixture in a low boiling solvent for GC analysis.

The relative areas of the components of interest must therefore be adjusted, to exclude the large % area of the solvent peak.

Refer to page 21 for further explanation.Adjusted area % =

area % of peak of interest X 100sum of area % of ALL peaks of interest*

*omit area % of solvent peak

Experimental Procedure(Simple Distillation)

Add the unknown mixture and 3 boiling chips to a round bottom flask.

Set up a simple distillation apparatus.

Begin water flow through condenser and apply heat to the flask.

Collect liquid distillate in small test tubes as it leaves the condenser.

Keep track of the temperature range for each fraction as it is collected.

Collect (4) 5mL fractions, leaving the final 5mL in the boiling flask.

Prepare GC sample for analysis.

Heating Mantle

to voltage regulator

water out

water in

iron ring

Product Analysis(GC Analysis)

• Analyze your FIRST and FOURTH fractions by GC. •SAMPLE PREP:

• Transfer 5 drops of your sample into an auto sampler vial using a plastic pipet. • Add 1 mL of GC solvent (pentane) from the solvent dispenser in balance room.• Be sure to record your GC vial slot # in your laboratory notebook. • You will receive an email to your UNCW account when GC results are ready for pick-up!

Table 2.3• You cannot complete this table until you obtain your GC results from the board outside of Dobo 203.•The standard chromatogram will be provided on the front of the envelope containing the chromatograms.• Use only the standard run for your lab section!

Peak #

Compound

Identity

StandardRt

(min)

Fraction #1 Fraction #4Sample

Rt (min)

Area%

Adjusted Area

%

Sample Rt

(min)

Area%

AdjustedArea %

123

SAFETY CONCERNS

All compounds used in today’s experiment are extremely flammable. Wear safety goggles at all times and use extreme caution!

WASTE MANAGEMENT

Pour all liquid waste into the bottle labeled “LIQUID ORGANIC WASTE”.

Do NOT pour any waste down the drain!

CLEANING Disassemble distillation apparatus in your

lab hood. In your lab hood, rinse all glassware with

wash acetone ONLY directly into a beaker from your drawer (DO NOT REMOVE DISTILLATION APPARATUS FROM YOUR HOOD AT ANY TIME.) Return distillation glassware to case in your hood. Return all other glassware to your lab drawer.

LABORATORY NOTEBOOK(Pre-lab)

o OBJECTIVE (Must clearly state…)oWhat is the goal of the experiment?o What technique will you used to accomplish it?o How will you determine if it worked?

o TABLE OF PHYSICAL DATA (Complete the following table using MSDS sheets from http://hazard.com/msds/ ONLY. Wikipedia is unacceptable)

o REFERENCE TO PROCEDURE (Must include…)o full titleo Editiono authorso page numbers where actual procedure can be found

Compound MW (g/mol) bp (Co) d (g/mL) HAZARDSPentanen-hexane3-methylpentanen-heptaneisooctane

LABORATORY NOTEBOOK(In-lab)

• DATA/CALCULATIONS • Record which unknown was used• Record distillation range of each fraction collected• Record GC vial slot number for fraction #1• Record GC vial slot number for fraction #4• Record GC sample solvent used• Show one EXAMPLE of an adjusted area percent calculation

• EXPERIMENTAL PROCEDURE• In paragraph form, briefly describe the procedure that you

actually followed during the lab. • Paragraph must be written in PAST TENSE, PASSIVE VOICE.• Include any volumes of chemicals used during the experiment• Include any mistakes, accidents, or observations if necessary.

For Next Lab… Experiment 4 Pre-lab Notebook Entry due! It is *highly recommended* that you watch

the following link prior to coming to lab! (must cut/paste address)

http://www.youtube.com/watch?v=yoYECMP_1iI&feature=related

This video is not exactly like the procedure we will use, however will provide a good example of the experiment!