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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. Electronic Structure Analysis

To determine the relative acidity, we must examine the stability of the conjugate bases: the carboxylate ion ($RCOO^-$) and the phenoxide ion ($C_6H_5O^-$).

Feature Carboxylate Ion ($RCOO^-$) Phenoxide Ion ($C_6H_5O^-$)
Resonating Structures Two equivalent structures Five non-equivalent structures
Charge Distribution Negative charge on highly electronegative oxygen Negative charge delocalized onto less electronegative carbon atoms

2. Comparative Stability Analysis

A. Delocalization of Charge: In the carboxylate ion, the negative charge is delocalized over two highly electronegative oxygen atoms [Per the principle of electronegativity]. Because the two oxygen atoms share the negative charge equally, the resonance hybrid is exceptionally stable.

B. Energy Content of Resonating Structures: In the phenoxide ion, while there are more resonating structures (five total), three of these structures place the negative charge on the carbon atoms of the benzene ring. Since carbon is significantly less electronegative than oxygen, these structures are inherently higher in energy and less stable [Per the Electronegativity-Stability Criterion].

3. Visual Representation of Resonating Hybrids

Carboxylate Ion (Equivalent Resonance) COO⁻ Phenoxide Ion (Non-equivalent Resonance) O⁻ Negative charge delocalized on C

4. Thermodynamic Justification

Acidity is directly proportional to the stability of the conjugate base. By comparing the two ions:

  • The carboxylate ion stability is governed by the equivalence of its resonance structures. The negative charge remains on the oxygen atoms, which are more electronegative and thus better able to accommodate the negative charge.
  • The phenoxide ion stability is undermined by the non-equivalence of its resonance structures. The delocalization onto carbon atoms involves structures with higher potential energy, making the phenoxide ion less stable than the carboxylate ion.

Consequently, the carboxylate ion is a weaker base (more stable) than the phenoxide ion. By the Brønsted-Lowry Theory, a weaker conjugate base indicates a stronger corresponding parent acid.

Final Solution: Carboxylic acids are stronger acids than phenols because the negative charge in the carboxylate ion is delocalized over two equivalent, highly electronegative oxygen atoms, whereas the negative charge in the phenoxide ion is delocalized onto less electronegative carbon atoms, rendering the phenoxide ion less stable.


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