Question

Which acid of each pair shown here would you expect to be stronger?
(i) $Ph-COOH$ or $C{H}_{3}COOH$

Answer 1

(i) On Iosing a proton, carboxylic acids forms carboxylate ion and phenol forms phenoxide ion as follows: (ref. image 1)

Now, the negative charge is delocalized in both molecules as follows: (ref. image 2)

The conjugate base of carboxylic acid has two resonance structures in which negative charge in delocalized over two oxygen atoms (since $O$ is more electronegative than $C$) which stabilizes the carboxylate ion.

On the other hand, in phenoxide ion the charge is delocalized over entire molecule on the less electronegative atom (carbon), thus resonance of phenoxide is not important in comparison to resonance in carboxylate ion.

Further, in carboxylate ion the negative charge is effectively delocalized over two atoms whereas it is less effectively delocalized over one oxygen atom and less electronegative carbon atom.

Thus, Phenol is less acidic than carboxylic acids. In other words, carboxylic acids are stronger acids than phenol.