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)

A monosubstituted benzoic acid is stronger than a monosubstituted 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 decreases acid strength. It is due to very high stabilisation of conjugate base by intramolecular $H$-bond which outweight the electron donating resonance effect of $-OH$.

The overall order of acid-strength of given four acids is ortho-hydroxybenzoic acid $(p{K}_a=$ 2.98) $>$ Toluic acid $p{k}_a=4.37)>$ $p$-hydroxybenzoic acid $\left(p{k}_a=4.58\right)>p-$ nitrophenol $\left(p{k}_a=7.15\right)$.