Question

Carboxylic acids are more acidic than phenol and alcohols because of -

• A. Intermolecular hydrogen bonding
• B. Formation of dimer
• C. Highly acidic hydrogen
• D. Resonance stabilization of their conjugate base.

Answer 1

The correct option is D

Resonance stabilization of their conjugate base.

The explanation for the correct option:

Although carboxylic acid is weak acid as compared to mineral acids $\mathrm{HCl},{\mathrm{H}}_2{\mathrm{SO}}_4,{\mathrm{HNO}}_3\mathrm{etc}.$ it is a stronger acid as compared to phenol and alcohol and the reason behind this is completely attributed to the delocalization of charges.

(D) resonance stabilization of their conjugate base: The carboxylic group of the carboxylic acid is considered a resonance hybrid structure in which due to resonance the oxygen of the hydroxyl group acquires the positive charge and attracts the electron pair of (-O-H) bond more towards itself.

Thus the displacement of the electron pair help in the release of a proton of the OH group.

The release of proton creates a carboxylate ion which is also a hybrid of a given structure.

Thus, both acid and ion are stabilised by resonance and are more stable because all the resonating structures are equivalent structures.

Phenol is also acidic and has resonating structures but the negative ion is dispersed on two oxygen atoms in the case of carboxylate ion while in the case of phenoxide ion negative ion is dispersed only on one oxygen atom that's the reason why carboxylic acid is more acidic than phenol.

The explanation for the incorrect option:

(A) Intermolecular hydrogen bonding: Hydrogen bonding between neighbouring molecules is called intermolecular hydrogen bonding. This is not the correct explanation for the increased acidity of carboxylic acids.

(B) Formation of dimers: Carboxylic acids can form dimers by hydrogen bonding.The formation of a compound by the interaction of two molecules or ions.

(C) Highly acidic hydrogen: The hydrogen that can be released easily from the compound to generate ions is called acidic hydrogen but this is not the correct explanation for the increased acidity of carboxylic acids.

Therefore option (D) is correct. i.e., the carboxylate ion is more acidic than the phenol and alcohol because there is a more stable conjugate base is present in the case of the carboxylate ion.