Question

Although phenoxide ion has more number of resonating structures than carboxylate ion,carboxylic acid is a stronger acid than phenol.Why?

Answer 1

Resonance structures of phenoxide ion are:

It can be observed from the resonance structures of phenoxide ion that in II,III and IV,less electronegative carbon atoms carry a negative charge.Therefore,these three structures contribute negligibly towards the resonance stability of the phenoxide ion.Hence,these structures can be eliminated.Only structures I and V carry a negative charge on the more electronegative oxygen atom.

Resonance structures of carboxylate ion are:

In the case of carboxylate ion,resonating structures I”² and II”² contain a charge carried by a more electronegative oxygen atom.

Further,in resonating structures I”² and II”²,the negative charge is delocalized over two oxygen atoms.But in resonating structures I and V of the phexoxide ion,the negative charge is localized on the same oxygen atom.Therefore, the resonating structures of carboxylate ion contribute more towards its stability than those of phenoxide ion.As a result,carboxylate ion is more resonance-stabilized than phenoxide ion. Hence,carboxylic acid is a stronger acid than phenol.