Chemistry - GOC Question with Solution | TestHub

An $-\mathrm{OH}$ group ortho to –COOH stabilises the carboxylate ion by intramolecular hydrogen bonding, and the ortho effect also twists the –COOH out of the ring plane. A para $-\mathrm{OH}$ is electron-donating (+M) and lowers acidity.

Salicylic acid has one ortho $-\mathrm{OH}$ ($\mathrm{p}{K}_{\mathrm{a}}$ about 3.0). 2,6-Dihydroxybenzoic acid has two ortho $-\mathrm{OH}$ groups, so the effect is doubled ($\mathrm{p}{K}_{\mathrm{a}}$ about 1.3). 4-Hydroxybenzoic acid (about 4.5) is weaker than benzoic acid (4.2).

Answer: (B).