Question

Sol. Key concept: Compare the stabilisation of each substituted benzoate conjugate base.

a) p-Hydroxybenzoic acid is weaker than benzoic acid: the para –OH group donates electron density by resonance, reducing the favourability of forming the carboxylate anion.

b) p-Methoxybenzoic acid is also weaker: resonance donation by para –OCH₃ outweighs its inductive withdrawal in this comparison.

c) m-Chlorobenzoic acid is stronger: chlorine withdraws electron density inductively, stabilising the carboxylate conjugate base. At the meta position, direct resonance donation does not offset this in the same way as at ortho/para positions.

d) o-Hydroxybenzoic acid (salicylic acid) is stronger than benzoic acid. The ortho arrangement permits strong intramolecular hydrogen bonding that stabilises the salicylate anion, along with the ortho/inductive effects. Therefore choose (c) and (d).