Question -
Answer -
Resonance structures of phenoxide ion are:
It can be observed from the resonance structures ofphenoxide ion that in II, III and IV, less electronegative carbon atoms carry anegative charge. Therefore, these three structures contribute negligibly towardsthe resonance stability of the phenoxide ion. Hence, these structures can beeliminated. Only structures I and V carry a negative charge on the moreelectronegative oxygen atom.
Resonance structures of carboxylate ion are:
In the case of carboxylate ion, resonating structures I′and II′ contain a charge carried by a more electronegative oxygen atom.
Further, in resonating structures I′ and II′, the negativecharge is delocalized over two oxygen atoms. But in resonating structures I andV of the phexoxide ion, the negative charge is localized on the same oxygenatom. Therefore, the resonating structures of carboxylate ion contribute moretowards its stability than those of phenoxide ion. As a result, carboxylate ionis more resonance-stabilized than phenoxide ion. Hence, carboxylic acid is astronger acid than phenol.