The Synthesis and Analysis of Aspirin Mariam El-Magbri

Department of Chemistry, American University, Washington, D.C. 20016 Date of Publication: February 26, 2014

ABSTRACT: Acetylsalicylic acid commonly known as aspirin is the most widely used drug in the world today. Its analgesic, antipyretic, and anti-inflammatory properties make it a powerful and effective drug to relive symptoms of pain, fever, and inflammation. The purpose of this ex-periment was to synthesize aspirin by reacting salicylic acid and acetic anhydride in the pres-ence of phosphoric acid to form acetylsalicylic acid. After synthesis, the sample of acetylsalicyl-ic acid was purified by recrystallization and TLC analysis was utilized to check the purity of the sample. The actual yield of aspirin synthesized was 0.546 grams and the theoretical yield was 2.628 grams, thus the percent yield was 20.77% resulting in a percent error of 79.22%. The Rf values of crude product, salicylic acid, and re-crystallized product were 0.590 and 0.897, 0.818, and 0.606 and 0.879 respectively. Due to these Rf values, it was brought to the conclusion that the crude and recrystallized aspirin were composed of both salicylic acid and acetylsalicylic acid.

INTRODUCTION Aspirin, chemically known as acetylsalicylic ac-id, is the most commonly used anti-inflammatory drug. It is effective in relieving symptoms of pain (analgesic) due to headaches, injury, or arthritis, treating fever (antipyretic) and inflammation, and preventing blood clots (1). It was extracted by the Native Americans from willow and poplar tree bark about 2500 years ago. Native Americans used willow bark in teas to reduce fever. In 1763, Reverend Edward isolated and identified one of the compounds used to synthesize aspirin, which came to be known as salicylic acid. Large quanti-ties of salicylic acid became available; however,

it caused severe stomach irritation. In 1893, German chemist Felix Hoffman synthesized an ester derivative of salicylic acid, acetylsalicylic acid (“aspirin”). The acetyl group cloaks the acidity when ingested. The drug then passes through the small intestine where it gets convert-ed back to salicylic acid, and enters the blood-stream. Although, weaker than salicylic acid, as-pirin had medicinal properties without the bitter taste and harsh stomach irritation. The company Bayer patented aspirin in 1899, which has made aspirin one of the most widely used and commer-cially available drugs today (2).

Aspirin is a nonsteroidal anti-inflammatory drug (NSAID) which works to reduce levels of prosta-glandins, chemicals released due to inflamma-tion, pain, and fever. Prostaglandins are located on receptors of different cells types, thus having multiple effects. Cyclooxygenase is the enzyme that makes prostaglandins. NSAIDs inhibit the enzyme reducing the levels of prostaglandins, in turn reducing inflammation, fever, and pain. As-pirin is not only anti-inflammatory, but also anal-gesic and antipyretic. Prostaglandins carry out fever and pain by activating the hypothalamus, the portion of the brain that controls autonomic and endocrinal functions. Inhibiting prostaglan-dins suppresses fever and pain by stopping nerve signals that are sent to the brain. Suppression of prostaglandins also desensitizes the function of platelets and the ability of blood clots, thus aspi-rin’s antithrombotic effects have been approved to prevent heart attacks and strokes (2).

Despite the medicinal and wonderful properties of aspirin, negative effects have been associated with this drug. Symptoms associated with aspirin include nausea, vomiting, rashes, swelling, and

hives. Aspirin can still cause stomach irritation resulting in the risk of internal bleeding and ul-cers. Aspirin has also been known to interfere with platelet functioning and may cause Reyes syndrome in children (1).

Thin Layer Chromatography (TLC) is a tech-nique used in organic chemistry to separate a mixture of organic compounds. TLC is also used to identify and determine the purity of a com-pound. Through capillary action, compounds can separate due to their different affinities for the mobile and stationary phases. The stationary phase in TLC is the adsorbent, which is coated on a sheet of metal, plastic or glass. It is usually sili-ca or alumina; however, in this experiment silica was used. The mobile phase is the solvent which slowly rises due to capillary action and polarity. In this experiment the mobile phase was the 18:2 solution of ethyl acetate: methylene chloride, which is slightly polar. Different compounds travel at different rates and distance in respect to the solvent front. Polar stationary phases absorb polar compounds, thus silica being a polar sta-tionary phase will absorb polar compounds. Non-polar compounds will remain free and move with respect to the solvent front. The polarity of a compound is determined by its functional groups and masses. Salicylic acid is more polar than as-pirin. Acetylsalicylic acid has ester and acetyl functional groups and has a larger mass than sali-cylic acid. Salicylic acid has a hydroxyl function-al group. Hydroxyl groups are more polar than acetyl groups, thus salicylic acid will absorb to the silica more willingly than the acetylsalicylic acid due to hydrogen bonding. Furthermore, the acetylsalicylic acid will be in a free state and travel further because the ester and acetyl func-tional groups no longer have hydrogen bonds that bond to the polar silica plate. If a sample is com-posed of a mixture of compounds, they will sepa-rate into a series of sports along the TLC plate. If a sample is pure, only one spot will be displayed. The crude aspirin might have two different spots because it is not entirely pure. UV light is used to visualize the compounds on the plate. TLC re-sults are expressed in Rf values. Rf is the distance traveled by the sample over the distance traveled by the solvent front (3).

In this experiment, aspirin will be synthesized by reacting acetic anhydride with salicylic acid in the presence of phosphoric acid. The reaction equation is displayed below. After synthesis, the sample will be purified by recrystallization meth-ods. The purity of the sample will then be ana-lyzed using TLC. The results section will present all the numerical data necessary for this experi-ment (synthesis of aspirin masses, theoretical yield, percent yield and error, and TLC analysis). After synthesis and analysis, it will be deter-mined if the crude and recrystallized samples were composed solely of either salicylic acid, acetylsalicylic acid, or a combination of both (3).

Figure 1: Reaction of Aspirin (Snelling, 2013)

MATERIALS AND METHODS Goggles

Gloves

Lab coat

Salicylic Acid

50 mL Erlenmeyer Flask Acetic Anhydride

85% Phosphoric Acid

250 mL hot water bath

Stirring rod

Distilled Water

250 mL ice bath

Vacuum filter apparatus

1 110-mm Filter paper

2 pieces of round filter paper

2 Watch glasses

125 mL Erlenmeyer Flask

10 mL Ethanol

400 mL beaker

Silica gel coated TLC plate

Spotting pipettes

18 mL Ethyl acetate

2 mL methylene chloride

Ruler

UV light

Part 1: Synthesize Aspirin

Goggles, lab coats, and gloves were obtained for protection and used throughout the entire exper-iment. The reaction described below was con-ducted under a fume hood. 2.0 grams of salicylic acid was placed into a 50mL Erlenmeyer flask along with 5.0 mL of acetic anhydride and 5 drops of 85% phosphoric acid solution. The mix-ture was handled with care and then swirled to rinse off any pieces of solid. A 70-80°C hot water bath was prepared using a 250 mL beaker. The mixture previously prepared was kept in its flask and submerged in the water bath. Stirring occa-sionally, the mixture was heated for 15 minutes, until it started to release vapors. 2 mL of distilled water was added 10 minutes into the heating pro-cess. Once the reaction reached completion and no vapors appeared, it was removed from the hot plate and 20 mL of distilled water was added. The mixture was then cooled to room tempera-ture and then transferred into an ice bath for five minutes. Crystals of aspirin started to form as the mixture cooled. Once the mixture cooled, a vacu-um filtration was set up. Filter paper was massed and recorded to the nearest 0.001 g before the solid was filtered. The mixture was then filtered with vacuum suction. After most of the liquid was drawn through the funnel, suction was turned off, and the crystals were washed with 5mL of cold, distilled water. After about 15 seconds, suc-tion was turned back on and the crystals were washed with cold, distilled water two more times in the same procedure. After filtration, the dried recrystallized product along with the filter paper

was massed to 0.001g and recorded in the data table. The mass of the dry aspirin sample was calculated and recorded in the data table (3).

Part 2: Purification of Aspirin

1 g of crude crystals was set aside for TLC analy-sis in Part 3. The mass of the remaining crude was measured. The remaining crude was added to a 125 mL Erlenmeyer flask. Approximately 10 mL of hot solvent (ethanol/water) was added to the crude aspirin, which was then placed into a warm water bath until all crystals dissolved. Once the crystals dissolved, the mixture was tak-en out, covered with a watch glass and cooled slowly. When the solution reached room tem-perature, it was placed into an ice bath to com-plete the crystallization process. After about ten minutes in the ice bath, vacuum filtration was used again to filter the crystals. The crystals were rinsed with two 3mL portions of ice cold deion-ized water and one 2 mL portion of ice cold etha-nol. After rinsing, the crystals were placed onto a tared watch glass (3).

Part 3: TLC Analysis

A developing chamber was prepared using a 400 mL beaker and watch glass. A piece of 110-mm filter paper with the bottom trimmed straight across was placed into the chamber in order to saturate the chamber with solvent vapors. 18 mL of ethyl acetate and 2 mL of methylene chloride were measured out and placed in to the 400 mL beaker and covered with the watch glass. Before introducing the TLC plate, the solvent traveled all the way to the top of the filter paper. In three separate small beakers, about 3mg each of sali-cylic acid, crude product, and recrystallized product was dissolved by placing 5-6 drops of TLC solvent in each beaker. The TLC plate was prepared by marking ½ inch from the bottom and marking 3 hashes evenly spaced for each of the spotted compounds. The TLC plate was spotted (no more than 1/8 in diameter) with the salicylic acid, crude product, and recrystallized product at each hash mark making sure a different spotting pipette was used for each compound. Once the TLC plate was prepared, it was placed into the developing chamber and the watch glass was re-placed on top. When the solvent front stopped

moving (approximately ½ inch from the top), the plate was removed and the solvent front was im-mediately marked. It then was dried and exam-ined under a UV light. Spots that were seen were circled and measured using the Rf calculation. The TLC solvent was discarded in the appropri-ate container and all materials were washed and placed back in their appropriate location (3).

RESULTS Table 1: Synthesis of Aspirin

Mass of Salicylic Acid Used (g)

2.015 g

Volume of Acetic An-hydride used (mL)

5.00 mL

Mass of Acetic Anhydride used

(1.08g/mL)

5.40 g

Mass of Aspirin and filter paper (g)

0.678 g

Mass of filter paper (g) 0.132 g

Mass of aspirin synthe-sized (g)

0.546 g

Theoretical yield of aspirin

2.628 g

Percent Yield 20.77%

Percent Error 79.22%

Table 2: TLC Analysis

Compound Distance of

Solvent (cm)

Distance Compound Traveled

(cm)

Rf

Crude

Product

3.9 cm 2.3 cm & 3.5 cm

0.590 &

0.897

Salicylic Acid 3.3 cm 2.7 cm 0.818

Recrystallized 3.3 cm 2.0 cm & 0.606 &

Product 2.9 cm 0.879

Calculations:

Equation 1: Mass of acetic anhydride used

=Volume (mL)xDensity(g/mL)

Mass of acetic anhydride

=(5.00mL)(1.08g/mL)=5.40g

Equation 2: Mass of aspirin synthesized

=(mass aspirin & filter paper)-(mass filter paper)

=(0.678g)-(0.132g)=0.546g

Equation 3: Limiting Reagent

2.015g  Salicylic  Acid x1  mol  salicylic  acid

138.12g

=0.01459 mol

5.40  g  Acetic  Anhydride x !  !"#  !"#$%"  !"#$%&'%(!"#.!"  !

=0.05289 mol

*Salicylic Acid is the limiting reagent*

Equation 4: Theoretical Yield

moles  of  Salicylic  Acid x1  mol    acetyl  salicylic  acid

1  mol  salicylic  acid

𝑥180.16  g

1  mol  acetyl  salicylic  acid

=  0.01459  mol  Salicylic  Acid x1  mol    acetyl  salicylic  acid

1  mol  salicylic  acid

𝑥 !"#.!"  !!  !"#  !"#$%&  !"#$%&#$%  !"#$

=2.628 g acetyl salicyl-ic acid

Equation 5: Percent Yield

Percent  Yield:  actual  yield

theoretical  yield x  100

Percent  Yield:   !.!"#$!.!"#$

x  100 = 20.77%

Equation 6: Percent Error

Percent Error: |!"#$!%  !"#$%!!"#$%#!&'()  !"#$%|!"#$%#!&'()  !"#$%

x  100

Percent Error: |  !.!"#  !  !!.!"#  !  |!.!"#  !

x  100= 79.22%

Equation 7: Rf

Rf= !"#$%&'(  !"#$%&  !"#$%&%'  (!")!"#$%&'(  !"  !"#$%&'  !"#$%  (!")

Rf (Crude product, spot 1)= !.!  !"!.!  !"

=0.590 cm

Rf (Crude product, spot 2)= !.!  !"!.!  !"

=0.897 cm

Rf (Salicylic Acid)= !.!  !"!.!  !"

=0.818 cm

Rf (Recrystallized product, spot 1)= !.!  !"!.!  !"

=0.606 cm

Rf (Recrystallized product, spot 2)= !.!  !"!.!  !"

=0.879 cm

Table 1 displays all the raw and calculated data for the synthesis of aspirin. Salicylic acid was the limiting reagent and acetic anhydride was in ex-cess. The actual yield of aspirin synthesized was 0.546 grams and the theoretical yield was 2.628 grams, thus the percent yield was 20.77% result-ing in a percent error of 79.22%. Table 2 dis-plays all data for TLC analysis. The Rf values of crude product, salicylic acid, and recrystallized product were 0.590 and 0.897, 0.818, and 0.606 and 0.879 respectively.

DISCUSSION Esterification is the process of forming a carbox-ylate ester by reacting a carboxyl group with a hydroxyl group or phenol group. The formation of acetylsalicylic acid is an esterification reaction as well as an equilibrium process. Le Chatelier’s principle is also known as the equilibrium law and allows one to determine the effects on equi-librium due to pressure, temperature, and concen-tration. As the reactants are used, the concentra-tion of the reactions decreases and the concentra-tion of the products increases. Le Chatelier’s principle is used to favor products, because ex-cess acetic anhydride forces the equilibrium to

shift towards the desired product, aspirin. Le Chatelier’s principle also favors the reactants be-cause aspirin can be converted back to salicylic acid and acetic anhydride and once the changes are adjusted, a new equilibrium will be estab-lished (3).

Salicylic acid contains two acidic functional groups; a phenol group and a carboxylic acid. The phenol group on the salicylic acid causes stomach irritation. An ester is formed from the phenol group and carboxylic acid on the acetic acid. It is desirable to do away with one of these groups because the acids on the molecules are what cause irritation. By replacing one of the acid groups, the acid strength is reduced making it easier to digest (4).

Figure 2: Synthesis of Aspirin Mechanism

Above is the reaction occurring between salicylic acid, phosphoric acid, and acetic anhydride. Phosphoric acid attacks the carbon oxygen bond (C=O) of the acetic anhydride giving it a positive charge, thus acetic anhydride is more prone to nucleophilic attacks. The nucleophile in the reac-tion is salicylic acid. It was formed when the ox-ygen on the phenol group of salicylic acid at-tacked the partial positive charge of a carbon from acetic anhydride, transferring electrons to oxygen. Salicylic acid then attacks the acetic an-hydride because of its positive charge. The oxy-gen from the phenol group now has a positive charge and the carbonyl groups withhold a nega-tive charge. A tetrahedral intermediate is then formed. The electrophilic carbon is attached to the –OH group. The –OH group protonates the hydrogen and donates an electron to form a dou-ble bond between carbon and oxygen (C=O). The phosphoric acid is then reformed again as it gains the loss of the proton. Acetylsalicylic acid is thus formed (4).

In pure acetic anhydride, the synthesis of aspirin is slow, so a catalyst (phosphoric acid) was used to hasten the reaction. Phosphoric acid is present in the beginning and end of the reaction. Since it is water soluble it is easily removed by washing the crystals with cold distilled water. The phos-phoric acid also ensured that side reactions that could take place and increase the percent yield were avoided (4).

The salicylic acid and acetic anhydride mixture was heated at 70-80 degrees Celsius to increase solubility and collision of particles, thus speeding up the dissolving process of salicylic acid. The synthesis of aspirin is in endothermic reaction, and by adding heat it will push the reaction for-ward to favor formation of the products. After heating the mixture for 10-15 minutes, water was added to recrystallize the product. Acetic acid is very soluble in water, thus it can be easily sepa-rated from aspirin since it is not very soluble in water. Aside from the addition of water to recrys-tallize the product, the addition of water pro-motes nucleophilic substitution (4).

In the experiment the crude aspirin is dissolved in a mixture of hot water and ethanol. Aspirin has a solubility of 10mg/mL in water and a solubility of 50mg/mL in ethanol. Ethanol has a polarity of 5.2 and water has a polarity of 9, making ethanol a less polar solvent, thus substances like aspirin with low polarity dissolves in like less polar sol-vents. Additionally, aspirin should not be recrys-tallized solely from hot water because it yields impurities and allows the crude to get hydro-lyzed. Furthermore, aspirin should be recrystal-lized in hot ethanol to dissolve impurities and obtain a purified product. The crude aspirin was not clean from recrystallization because it was exposed to the environment (5).

Pure aspirin was not made in this experiment. The Rf values of crude product, salicylic acid, and recrystallized product were 0.590 and 0.897, 0.818, and 0.606 and 0.879 respectively. It can be concluded that the crude and recrystallized prod-uct were composed of both acetylsalicylic acid and salicylic acid. Since the crude and recrystal-lized product were not pure, two spots appeared one correlating with salicylic acid and the other correlating with acetylsalicylic acid. Polar sta-tionary phases absorb polar compounds, thus sili-ca being a polar stationary phase will absorb po-lar compounds. Nonpolar compounds will remain free and move with respect to the solvent front, which is slightly polar. Acetylsalicylic acid has ester and acetyl functional groups and has a larg-er mass than salicylic acid. Salicylic acid has a hydroxyl functional group. Hydroxyl groups are more polar than acetyl groups, thus salicylic acid will absorb to the silica more willingly as dis-played by an Rf value of 0.818 due to hydrogen bonding. The acetylsalicylic acid traveled further as displayed by Rf values of 0.897 in the crude product and 0.879 in the recrystallized product because the ester and acetyl functional groups no longer have hydrogen bonds that bond to the po-lar silica plate.

The actual yield of aspirin synthesized was 0.546 grams and the theoretical yield was 2.628 grams, thus the percent yield was 20.77% resulting in a percent error of 79.22%. It is difficult to obtain a percent yield of 100% because products can react to produce the reactants. The percent yield is

relatively low because of possible sources of er-ror. One possible source of error could result from loss of product due to exposure to atmos-pheric air causing prolonged air drying. Another possible source of error could be due to insuffi-cient heating, which would also yield a loss of product. A lower yield could also result from an incomplete conversion of reactants to products due to decomposition of acetic acid in the solu-tion (5).

CONCLUSION All in all the actual yield of aspirin was 0.546 g. Given that 2.015 g of aspirin was used, the theo-retical yield was 2.628. The percent yield and percent error were 20.77% and 79.22% respec-tively. Because the percent yield was low, a lot of the product was lost. The Rf values of crude product, salicylic acid, and recrystallized product were 0.590 and 0.897, 0.818, and 0.606 and 0.879 respectively. Furthermore, we can con-clude that acetylsalicylic acid traveled further because it is less polar than salicylic acid. The crude product and recrystallized product were composed of both salicylic acid and aspirin, as displayed by two spots on the TLC plate. Salicyl-ic absorbed to the silica more willingly than the acetylsalicylic acid as displayed by an Rf value of 0.818, thus the acetylsalicylic acid traveled fur-ther as displayed by Rf values of 0.897 in the crude product and 0.879 in the recrystallized product. To prevent error in the future, the water bath could be sufficiently heated. Also less expo-sure to atmospheric air could be prevented by conducting the entire experiment under a fume hood.

REFERENCES

(1) Ogbru, Omudhome. MedicineNet. Acetylsali-cylic Acid. http://www.medicinenet.com/acetylsalicylic_acid/article.htm (accessed Feb 23, 2014).

(2) Snelling. Volstate.edu. Synthesis of Aspirin. http://www2.volstate.edu/chem/1110/Synthesis_of_Aspirin.htm (accessed Feb 23, 2014).

(3) Williamson, K.; Masters, K. Macroscale and Microscale Organic Experiments, 6th ed.; Cen-gage Learning: Belmont, 2011.

(4) Esobel. UPLB College Student. Synthesis of Aspirin. http://theuplbcollegestudent.blogspot.com/2011/05/full-report-synthesis-of-aspirin.html (accessed Feb 23, 2014)

(5) Lewis. Aspirin: A Curriculum Resource for Post-16 Chemistry Courses. 2nd ed.; Royal Socie-ty of Chemistry: London, 2003.

ACKNOWLEDGMENTS

We thankfully acknowledge support from The American University in Washington D.C, Medi-cine.Net. and Volstate. We are also grateful to Omudhome Ogbru, C.R. Snelling, Kenneth L. Willamson, Esobel, David Lewis and to the stu-dents at American University for their participa-tion in the experiment.

This assignment is my own work and I have cited all material used in its preparation. This assign-ment has not previously been submitted at any other time or any other course. I have not copied in part or whole the work of other students or person.