Question

Dissociation constants of benzoic acid, p-nitrobenzoic acid and p-hydroxybenzoic acid are `6.3xx10^(-5)`, `36xx10^(-5)" and "2.5xx10^(-5)` respectively. Explain.

Answer 1

(i) `NO_(2)` group is an electron withdrawing group and tends to stabilise the carboxylate anion more than the benzoate ion after the release of `H^(+)` ion. Thus, p-nitrobenzoic acid is a stronger acid than benzoic acid.
(ii) OH group is an electron releasing group and tends to destabilise the carboxylate anion more than benzoate ion after the release of `H^(+)` ion. Thus p-hydroxybenzoic acid is a weaker acid than benzoic acid. The relative order of of acidic strengths is , thus, justified.