A-Level Carboxylic acids, acyl chlorides and esters: worked solution

Question

In 1897, Felix Hoffmann, a German chemist then employed by the pharmaceutical firm Bayer, discovered that attaching an acetyl group to salicylic acid lessened its irritant effect. This led to the synthesis of aspirin, and Bayer took out a patent on the method. In a laboratory, a batch of aspirin crystals was prepared from salicylic acid, separated from the reaction mixture, and then dried. Using data from section 26 of the data booklet, suggest how infrared (IR) spectroscopy could be used to tell aspirin apart from salicylic acid.

Salicylic acid has both a carboxylic acid group and a free phenolic -OH; aspirin is formed by acetylating that phenol -OH into an ester, so aspirin has an ester group plus the carboxylic acid. Using section 26: salicylic acid shows a free/bonded phenol O-H absorption (around 3580-3650 cm-1, or broad hydrogen-bonded O-H) which is absent in aspirin because the phenol has been converted to an ester. Correspondingly, aspirin shows an additional ester C=O stretch (around 1735-1750 cm-1) not present in salicylic acid. Absence of the phenol O-H band (and/or appearance of the ester C=O band) confirms conversion to aspirin.