Salicylic acid

Salicylic acid is 2-hydroxybenzoic acid, ${\mathrm{C}}_7{\mathrm{H}}_6{\mathrm{O}}_3$: a benzene ring bears adjacent hydroxyl and carboxylic acid groups.^[ref]

The two groups supply distinct chemical sites. Acid–base reactions can change their protonation, while derivatization at the phenolic oxygen changes the molecular identity. Their adjacent arrangement also permits intramolecular hydrogen bonding, so moving one group to a different ring position can alter properties despite preserving elemental composition. This makes the compound a useful example of the difference between a formula and a positional isomer.