Synthesizing Aspirin: the Acetylation of Salicylic Acid

Emily Muller

Synthesizing Aspirin: The Acetylation of Salicylic Acid

3/25/13

Introduction:

The purpose of this lab was to create acetylsalicylic acid (aspirin) by esterification of salicylic acid. The reaction was assessed using the melt point, an iron (III) chloride test and TLC. The chemical reaction of the experiment was as follows, with H3PO4 as a catalyst in the reaction:

The reaction occurs due to the alcohol on the salicylic acid reacting with the acetic anhydride. Acetylsalicylic acid (aspirin) and an acetate ion are the products of the reaction. A test using iron (III) chloride can be used to determine the success of the reaction. This is because the FeCl3 reacts with alcohols but not the aspirin because the alcohol is part of the carboxylic acid. The aspirin product over time can react with water which causes the product to decompose back into the two starting products.

The experiment was conducted by placing the salicylic acid and the acetic anhydride in a round bottom flask, which was then placed in a hot water bath. The reaction was done in reflux for approximately 15 minutes. Next, the reaction mixture was cooled and then placed in an ice bath for crystallization. Cold distilled water was added to the crystals. The crystals were filtered out using vacuum filtration. The product was characterized using the melt point, an FeCl3 test and TLC. The TLC plate tested the product, Anacin, and caffeine against one another.

Data:

Table 2 – TLC Values

Table 1 – Experiment Values

Discussion:

The purpose of the lab was to synthesize aspirin from salicylic acid and acetic anhydride. The product was analyzed using the melt point, the FeCl3 test and TLC. These tests indicated whether or not the product was aspirin. Table 1 shows the results of the FeCl3 test. The salicylic acid produces a vibrant purple color due to the presence of the alcohol. The acetyl salicylic acid, a pure representation of the desired product, turned yellow. The created product, therefore, should have turned yellow as well. However, the test produced purple. This indicates the experiment was not successful. The product was not aspirin rather still remained salicylic acid. One potential reason for this occurring was the presence of water in the acetic anhydride. After an extended period of time, the acetic anhydride starts to become deactivated due to the presence of moisture in the environment. This would cause the acetic anhydride to not produce the desired effect in the reaction.

Also, in Table 1 the melt point was found to be 118-124oC. The literature value of the melt point for the desired product is 138oC and the literature value for the melt point of salicylic acid is 158oC. The observed melt point demonstrates the compound was not either of the compounds. While the observed temperature was closer to the ideal product, the wide range demonstrates the product was not very pure. Also, using the information from the iron (III) chloride test, it can be proposed the product was not acetylsalicylic acid. Even though the percent yield was 87.10%, the product made was not acetylsalicylic acid.

Lastly, using the TLC the product was compared with an industrial version of aspirin, Anacin, and caffeine, which is commonly found in Anacin. Table 2 shows the product produced one spot which had an Rf value similar to one of the Anacin. One potential reason for this was because the product had similar chemical compositions to the Anacin, even though previous results suggest the product was not aspirin. The TLC also verifies Anacin contains caffeine because the Rf values were very similar. In conclusion, the purpose of trying to create acetylsalicylic acid using salicylic acid and acetic anhydride was not achieved as evident by the iron (III) chloride test, the melt point, and TLC. Most likely this was due to the acetic anhydride containing some water, therefore, hindering the effects of it and the reaction to occur.

Post-Lab Questions in Lab Report