JEE Main · 2005 · 08 May 2005 · Offline · Aldehydes Ketones and Carboxylic Acids

The lowest $\mathrm{p}{\mathrm{K}}_{\mathrm{a}}$ belongs to the strongest acid, so we look for the one whose conjugate base is best stabilised. Alkyl groups donate electron density through their $+I$ effect, piling charge onto the carboxylate oxygen and destabilising it, which weakens the acid. Formic acid, $\mathrm{HCOOH}$, carries only a hydrogen in place of any alkyl group, so it lacks this destabilising push, its carboxylate is the most comfortable, and it stands as the strongest acid with the lowest $\mathrm{p}{\mathrm{K}}_{\mathrm{a}}$.