Synthesis of Aspirin and Flavoring Esters

Synthesis of Aspirin and Flavoring Esters
By: Leah Warkel and Matt Glattfelder

Introduction

An ester is an organic compound that is formed by combining a carboxylic acid and an alcohol.¹ It has the general formula R-CO-OR’.² If an alcohol undergoes a condensation reaction, an ester is formed. This is reversible however, so if an ester undergoes hydrolysis, an alcohol is formed.¹ A condensation reaction is when a molecule combines with another molecule to produce a larger molecule and a smaller molecule, which is usually water. Esterification usually requires a catalyst in order to take place, and H₂SO₄ is most commonly used to do so. Esters are commonly found to give things distinct fragrances, such as in various fruits. Aspirin also contains an ester functional group within its chemical structure. Aspirin is a common analgesic that can be used to relieve mild pain such as headaches, can prevent heart attacks, and relieve symptoms of arthritis.³

The purpose of this experiment is to prepare a common analgesic, aspirin, and see how versatile a single technique of synthesis can be. Class tests will be used to test for the presence of various substances and explore organic qualitative tests. The purity of our synthesized aspirin will be tested. Also, the aromas of different esters will be observed.

Procedure

1.127 grams of salicylic acid was weighed at the beginning of the procedure.  3.5 mL of acetic anhydride was then added to the test tube.  4-5 drops of 18M sulfuric acid were added to the test tube, and the contents of the test tube were mixed thoroughly.  The mixture in the test tube was left alone for 10 minutes.  After the ten minute period ended, the test tube was put into an ice water bath for approximately 5 minutes.  5mL of cold deionized water was then added to the test tube at a rate of 0.5mL per drop.  The test tube is then returned to the ice bath for approximately 15 minutes.  White crystals began to form. The test tube is then allowed to continue to cool for about 15 minutes after precipitation begins.  After this process is complete, the crystals were to be separated from the liquid using a Buchner filtration apparatus.

The synthesized aspirin was then compared with a stock aspirin and salicylic acid.  The first test of comparison involved to dissolving less than 0.25g of each solid in 5mL of absolute ethanol.  3 drops of 1% FeCl₃ (aq) were then added to each solution. In the second test, the three substances were dissolved in 1-2mL of sodium bicarbonate in a test tube.  The solution was mixed well and then was allowed to stand as it reacted.

Flavoring esters were prepared using the general procedure of adding approximately 2g (or 2mL if substance was a liquid) of the carboxylic acid and 2mL of the alcohol to a test tube.  2-3 drops of 18M sulfuric acid were then added and the solution was mixed thoroughly in a hot water bath of approximately 65 degrees Celsius for 10 minutes. The first test tube contained salicylic acid and methyl alcohol, the second one contained propanoic acid and butanol, and the third one contained 3-methyl-1-butanol and glacial acetic acid. The test tubes were then removed from the hot water bath poured into an evaporating dish.  1mL of saturated Na₂CO₃ was then added drop wise to neutralize any remaining acid.  The aroma produced by the product of each esterification was observed.

Results

Table 1: Class Tests

Table 1. Discusses the observations of the class tests on the various substances.

Table 2: Aromas of Flavoring Esters

Table 2. Discusses the aroma of different synthesized esters.

Discussion

In this experiment, sulfuric acid was used to act as a catalyst. A catalyst is something used to lower the activation energy of a reaction in order to start it. Water was added slowly to the acetic anhydride in small portions because if it is added to quickly, it will convert to acetic acid because it is not stable in water. The class tests help to determine purity of our aspirin because when the substances were added to it, no reactions took place. There was a change in solubility of aspirin and salicylic acid when conducting the sodium bicarbonate test because sodium bicarbonate is a neutralizing agent and therefore the salicylic acid and acetylsalicylic acid are neutralized. The first test tube in the fragrant ester reaction was kept to 65^o C because there was solid suspended in the liquid, so it was safer to keep it cool to avoid the solid from popping and coming out of the test tube.

References

1. Organic Chemistry Portal. Fischer Esterification. http://www.organic-chemistry.org/namedreactions/fischer-esterification.shtm. (Accessed 10/24/12).
2. JRank. Esters. http://science.jrank.org/pages/2572/Ester.html. (Accessed                 10/24/12).
3. Medline Plus. Aspirin. http://www.nlm.nih.gov/medlineplus/druginfo/meds/a682878.html

Results