The structures of salicylic acid and benzoic acid differ due to the presence of an additional hydroxyl group (-OH) in salicylic acid. While both compounds share a benzene ring, salicylic acid has one of its hydrogen atoms replaced by a hydroxyl group. Benzoic acid, on the other hand, does not have any other functional groups and consists solely of a benzene ring with a carboxylic acid group (-COOH).

Here are the structural formulas of salicylic acid and benzoic acid to illustrate the difference:

Salicylic acid:

HO-C6H4-COOH

Benzoic acid:

C6H5-COOH

In salicylic acid, the hydroxyl group is directly attached to the benzene ring, giving it the characteristics of both an alcohol and a carboxylic acid. This difference in structure affects the properties and reactivity of salicylic acid compared to benzoic acid.