Although phenoxide ion has more number of resonating structures than carboxylate ion, carboxylic acid is a stronger acid than phenol. Why?
Carboxylic acids are stronger acids than phenols because the carboxylate ion is more stabilised than the phenoxide ion. Although the phenoxide ion has a greater number of resonating structures, the resonance in the carboxylate ion is far more effective.
The carboxylate ion is stabilised by two equivalent resonance structures in which the negative charge is delocalised over two electronegative oxygen atoms. In contrast, the phenoxide ion has non-equivalent resonance structures where the negative charge is less effectively delocalised over one oxygen atom and less electronegative carbon atoms.
Since the negative charge is better accommodated by the more electronegative oxygen atoms in the carboxylate ion, it is more stable than the phenoxide ion, making carboxylic acids stronger acids.
Explanation
The textbook context highlights that the stability of the conjugate base determines acidity. While phenoxide has more resonance structures, the quality of resonance matters more. Carboxylate ions benefit from equivalent resonance structures where the negative charge resides on electronegative oxygen atoms, leading to greater stabilization. Phenoxide ions involve charge delocalization onto less electronegative carbon atoms, making the resonance less effective. This difference in conjugate base stability explains the higher acidity of carboxylic acids.
Solution Steps
Step 1: Compare the stability of the conjugate bases (carboxylate ion vs. phenoxide ion).
Step 2: Analyze the resonance structures; carboxylate has equivalent structures with charge on oxygen, while phenoxide has non-equivalent structures with charge on carbon.
Step 3: Conclude that greater stability of the carboxylate ion leads to stronger acidity of carboxylic acids.