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Among the two isomers of hydroxybenzoic acid, the ortho isomer is a stronger acid compared to its para counterpart. Which of the following best explains this observation?

1. The lone pair on the –OH group participates in resonance with the aromatic ring
2. The carboxylate anion formed is stabilized through resonance involving the ring
3. The o-hydroxybenzoate anion is stabilized by intermolecular hydrogen bonding
4. The o-hydroxybenzoate anion is stabilized by intramolecular hydrogen bonding

Correct answer: The o-hydroxybenzoate anion is stabilized by intramolecular hydrogen bonding

Solution

In the ortho isomer, after deprotonation of –COOH, the resulting –COO⁻ is stabilized by an intramolecular hydrogen bond with the adjacent –OH group. This extra stabilization of the conjugate base makes o-hydroxybenzoic acid a stronger acid than the para isomer, where such intramolecular interaction is geometrically impossible.

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