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Q8.20:
Although phenoxide ion has more number of resonating structures than carboxylate ion, carboxylic acid is a stronger acid than phenol. Why?

Solution :

1. Analysis of Resonating Structures

The acidity of a compound is defined by the stability of its conjugate base. We compare the phenoxide ion ($C_6H_5O^-$) and the carboxylate ion ($RCOO^-$).

  • Phenoxide Ion: Exhibits 5 resonance structures. In 3 of these, the negative charge is located on the less electronegative carbon atoms (ortho and para positions). [Per the principles of Resonance Theory].
  • Carboxylate Ion: Exhibits 2 resonance structures. In both, the negative charge is delocalized over two highly electronegative oxygen atoms.

2. Comparative Charge Distribution Analysis

Stability is governed by the electronegativity of the atoms bearing the formal negative charge. Oxygen ($O$) is significantly more electronegative than Carbon ($C$).

Feature Phenoxide Ion ($C_6H_5O^-$) Carboxylate Ion ($RCOO^-$)
Number of Structures 5 2
Charge Location Delocalized on $O$ and $C$ Delocalized equally on two $O$ atoms
Stability Basis Lower due to $C^-$ contribution Higher due to $O^-$ contribution

3. Visualizing Electronic Delocalization

Carboxylate Resonance O-C-O Equivalent structures; charge on Oxygen.

4. Theoretical Justification

While the phenoxide ion possesses a higher number of resonating structures, the quality of resonance in the carboxylate ion is superior. In the carboxylate ion, the negative charge is effectively delocalized over two electronegative oxygen atoms [Per the Electronegativity Postulate of Resonance].

Conversely, in the phenoxide ion, the negative charge is pushed onto the carbon ring, which is less electronegative than oxygen. Consequently, the negative charge is not as effectively stabilized in the phenoxide ion compared to the carboxylate ion.

Final Solution: Because the negative charge in the carboxylate ion is delocalized over two highly electronegative oxygen atoms, it provides greater stabilization to the conjugate base than the phenoxide ion, where the charge is delocalized onto less electronegative carbon atoms. Therefore, carboxylic acids are stronger acids than phenols.


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