Carboxylic acid is stronger acid than …

A carboxylic acid can be represented by resonance structure and is stabilized (as shown below)

Fig. Resonance structure of carboxylate ion.

A carboxylate anion is more stabilized by resonance. Though phenoxide ion is resonance stabilized its stability is less compared to a carboxylate ion. This is because a carboxylate ion has two equivalent structures, which are equally favored when compared to the five resonance structure of phenoxide ion (refer the resonating structures as mentioned above), all of which are not equivalent. The pKa of carboxylic acid for example acetic acid is 4.5 whereas phenol has pKa of 9.9, which shows that a carboxylic acid is stronger acid than phenol.

Carboxylic acids dissociatein water to form carboxylate ion and hydronium ion. The carboxylate ion formed is stabilized through resonance by effective delocalization of the negative charge.carboxylate ion, the conjugate base of carboxylic acid is stabilized by two equivalent resonance structures in which the negative charge is effectively delocalized between two more electronegative oxygen atoms.

On the other hand in case of phenols,negative charge is less effectively delocalized over one oxygen atom and less electronegative carbon atoms in phenoxide ion. Therefore, the carboxylate ion exhibits higher stability in comparison to phenoxide ion. Hence, the carboxylic acids are more acidic than phenols.