Chemistry - GOC Question with Solution | TestHub

In benzoic acid the ring donates electron density toward the COOH group by resonance, which destabilises the carboxylate anion. An ortho substituent forces the COOH group out of the ring plane (ortho effect / steric inhibition of this resonance), so that donation is cut off and the anion is less destabilised; the acid becomes stronger. Methyl groups at meta or para positions only donate and weaken the acid. Hence o-toluic acid (II) is the most acidic (pKa : II 3.91 $<$ I 4.20 $<$ III 4.27 $<$ IV 4.37).