Question

Among the following compounds, the most acidic is

• A. $p$-nitrophenol
• B. $p$-hydroxybenzoic acid
• C. $o$-hydroxybenzoic acid
• D. $p$-toluic acid

Answer 1

Answer: (C)

$o$-hydroxybenzoic acid

Answer 2

A mono-substituted benzoic acid is stronger than a
mono-substituted phenol as former being a carboxylic acid.
Among the given substituted benzoic acid, ortho-hydroxy acid
is strongest acid although -OH causes electron donation by
resonance effect which tends to decrease acid strength.
It is due to a very high stabilisation of conjugate base by
intramolecular H-bond which outweigh the electron donating
resonance effect of -OH.
The overall order of acid-strength of given four acids is
ortho--hydroxybenzoic acid $(pK_\alpha = 2.98) >$ Toluic acid
$(pK_\alpha = 4.37) >$ p-hydroxybenzoic acid $(pK_\alpha = 4.58) >$
p-nitrophenol $(pK_\alpha = 7.15)$