Question

Last week, you separated aspirin from the unknown analgesic. Even if this separation did not go well, you would be able to easily distinguish aspirin from acetanilide or phenacetin by IR spectroscopy. Clearly explain how you would distinguish aspirin from your unknown analgesic, giving specific wavenumbers/peak ranges to look for.

Answer 1

Unlike acetanilide or phenacetin, aspirin has a carboxylic acid group

Carboxylic acid O-H stretching appears at 3000 - 2500 cm^-1 which is strong and very broad.

Along with a carboxylic acid group aspirin also has an ester group. The C=O stretching frequency for both carboxylic acid and ester appear above 1710 cm^-1 (range 1710 - 1780 cm^-1) in the IR spectrum whereas the amide C=O stretch for acetanilide or phenacetin will appear below 1700 cm^-1 (range 1630 - 1690 cm^-1).