The conjugate
base of carboxylic acid, a carboxylate ion, is stabilised by two equivalent
resonance structures in which the negative charge is at the more
electronegative oxygen atom The conjugate base of phenol, a phenoxide
ion, has non-equivalent resonance structures in which the negative charge
is at the less electronegative carbon atom Therefore, resonance in
phenoxide ion is not as important as it is in carboxylate ion Further, the
negative charge is delocalised over two electronegative oxygen atoms in
carboxylate ion whereas it is less effectively delocalised over one oxygen
atom and less electronegative carbon atoms in phenoxide ion (Unit 7,
Class XII)