Question

The correct acidic order of following is

• A. I > II > III
• B. III > I > II
• C. II > III > I
• D. I > III > II

Answer 1

Answer: (B)

The acidic behaviour of phenols may be explained on the basis of two reasons value of dissociation constant of phenols $(K_a=10^{10})$ and alcohols $(K_{a}10^{6}to10^{18})$
(a) Due to resonance (which is not possible in alcohols), the oxygen atom of the - OH group acquires a positive charge which helps in the release of a proton.
(b) In the dissociation of phenol to phenoxide ion and a proton the equilibrium lies mainly towards the right hand side as the resulting phenoxide ion is more stabilised by resonance as compared to phenol. The acidic strength of phenols depends on the nature of substituents present in the benzene nucleus. Electron withdrawing groups like - $N{O}_2$, - CN, -CHO, - COOH, etc, when present at the ortho and para-positions with respect to phenolic group increases the acidity of phenol due to greater stabilisation of phenoxide ion. While the presence of electron releasing group like - $N{H}_2,-C{H}_3,$ etc, decrease the acidity of phenols. This explains the following order of acidity p-nitrophenol > phenol > p-cresol.