Question

Explain the acidic nature of carboxylic acids.

Answer 1

Acidic nature of carboxylic acids:

1. The carboxyl group (–COOH) imparts an acidic character to carboxylic acids.
2. A carboxyl group is made of –OH group bonded to a carbonyl group.
3. In aqueous solution, the H atom in OH of carboxyl group dissociates as proton and carboxylate ion is formed as the conjugate base,
   \[\begin{array}{cc}
   \phantom{.}\ce{O}\phantom{......................}\ce{O}\phantom{...........}\\
   \phantom{.}||\phantom{......................}||\phantom{...........}\\
   \ce{\underset{\text{Carboxylic acid}}{R - C -OH} + H2O ⇌ \underset{\text{Carboxylate ion}}{R - C - O-} + H3O+}
   \end{array}\]
4. Carboxylate ion is resonance stabilized by two equivalent resonance structures as shown below.
   
5. Carboxylate ion has two resonance structures (I) and (II) and both of them are equivalent to each other.
6. This gives good resonance stabilization to carboxylate ion, which in turn gives an acidic character to carboxylic acids.