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JEE Main levelSingle correct optionDifficulty 2 of 3g2147

Salicylic acid (2-hydroxybenzoic acid, $p K_{a} = 2.97$ ) is a much stronger acid than 4-hydroxybenzoic acid ( $p K_{a} = 4.58$ ). The best explanation is

Athe $+ M$ effect of the ortho $OH$ stabilises the carboxylate
Bthe phenolic $OH$ is more acidic than $COOH$ in salicylic acid
Cintramolecular hydrogen bonding between the phenolic $OH$ and the carboxylate stabilises the salicylate ion
Dthe ortho $OH$ increases the electron density on the carboxylate oxygen atoms

Answer: C

In the salicylate anion the ortho phenolic

OH

forms an intramolecular hydrogen bond with the carboxylate oxygen, which stabilises the conjugate base and gives the lower

p K_{a}

(2.97). In the 4-isomer this is impossible, and the

+ M

effect of

OH

actually weakens the acid.

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