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

B) C) and D) have acid groups ...so they will be more acidic than A)

among B) , C) and D)

C) will be most acidic due to hydrogen bonding .....among B) and D) ...B) will be more acidic as -OH is slighlty e- withdrawing

so correct order should be - (C) o-hydroxybenzoic > (B) p-hydroxybenzoic > (D) p-toluic acid > (A) p-nitrophenol

Answer 2

Explanation:

o-hydroxybenzoic acid

A mono-substituted benzoic acid is stronger than a mono-substituted phenol as the former being a carboxylic acid.

Among the given substituted benzoic acid, ortho-hydroxy acid is the 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 outweighs the electron-donating resonance effect of -OH.

The overall order of acid-strength of given four acids is:

ortho--hydroxybenzoic acid (pKa=2.98)> Toluic acid (pKa=4.37)> p-hydroxybenzoic acid (pKa=4.58)> p-nitrophenol (pKα=7.15)

Hence, the correct option is C.

Thank you