Question

Carboxylic acids are more acidic than phenol and alcohol because of

• A. Intermolecular hydrogen bonding
• B. Formation of dimers
• C. Resonance stabilization of their conjugate base
• D. All of the above.

Answer 1

The correct option is C Resonance stabilization of their conjugate base

Carboxylic acids are more acidic than alcohols. Carboxylic acids react with $Na,NaOH,N{a}_{2}C{O}_3$ and $NaHC{O}_3$. But alcohols only reacts with Na. reason is, carboxylate ion is more stable than alkoxide ion due to resonance.
Carboxylate ion can show resonance structures. Therefore, it can spread it's negative charge by carbonyl group. So that brings a stability to the ion and equilibrium goes to the right side.
$C_2{H}_{5}OH+H_{2}O\rightleftharpoons C_2{H}_5{O}^{-}+H_3{O}^{+}$
Alkoxide anion can't show resonance structures. Therefore stability of alkoxide is low. So equilibrium goes to the left side.

In structures of carboxylate ion, the negative charge is on oxygen but in case of phenoxide ion, the negative charge is present on carbon (in 3 out of 4 structures). Thus, carboxylate ion is more stable than phenoxide ion. Thus carboxylic acid is stronger acid acid than phenol.