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:



In $II,III$ and $IV,$ less electronegative carbon atoms carry a negative charge which is observed from the resonance structures of phenoxide ion.

Therefore, the resonance stability of the phenoxide ion gets a negligible contribution from these three structures.

Hence, these structures can be eliminated. The more electronegative oxygen atom contains negative charge in the structures $I$ and $V.$

Resonance structures of carboxylate ion are given below-



In the case of carboxylate ion, resonating structures $I\text{'}$ and $II\text{'}$ contains a charge carried by a more electronegative oxygen atom.

Also, the negative charge is delocalized over two oxygen atoms, in resonating structures $I\text{'}$ and $II\text{'}.$

In the resonating structures, $I$ and $V$ of the phenoxide ion, the negative charge is delocalized on the highly electronegative oxygen and less electronegative carbon atoms.

But in the resonating structures, $I$ and $II$ of the carboxylate ion the negative charge delocalised only on high electronegative oxygen and both are the equivalent structure.

Since we know, negative charge is stable on high electronegative element and positive charge on less electronegative element. So, phenoxide ion is less resonance-stabilized than carboxylate ion.

Hence, carboxylic acid is a stronger acid than phenol.