The hydroxyl (OH) group has both-I effect and +R effect. Whereas the former helps in the release of `H^(+)` from the carboxyl group, the latter tends to oppose the same because of resonance. At the para position, the +R effect dominates the -I effect. Therefore, p-hydroxybenzoic acid is a weaker aid `(K_(a)=2.5xx10^(-5))` than benzoic acid `(K_(a)=6.3xx10^(-5))`. But in case of m-hydroxybenzoic acid, the +R effect does not operate to the same extent as in para isomer. As a result, the acidic weakening effect is smaller and metahydroxybenzoic acid is a stronger acid `(K_(a)=8.5xx10^(-5))` than benzoic acid.
