Question

Why benzoic acid is more acidic than formic acid or its homologous derivative?

Answer 1

The pK_a value of formic acid is 3.77 while that of benzoic acid is 4.202. This clearly indicates that formic acid is a stronger acid than benzoic acid. We know that the acidity in carboxylic acids is basically due to the resonance stabilised carboxylate group in which the negative charge is well accommodated between the two electronegative oxygen atoms. However, in case of benzoic acid, the benzene ring is attached directly to the carbonyl group. Thus there is additional resonance between the carbonyl group and the benzene ring, which disturbs the resonance in the carboxylate ion. In other words, the negative charge in the carboxylate group of formic acid is accommodated in a much better manner than in carboxylate group of benzoic acid. Hence formic acid furnishes H⁺ ions more easily than benzoic acid.