Question

Give reasons for the following observation:

p-Chloronitrobenzene reacts with (aq)NaOH at 443 K to give p-nitrophenol, whereas chlorobenzene reacts with the same reagent at 623 K and 300 atm.

Answer 1

p-Chloronitrobenzene reacts with aqueous NaOH at a lower temperature to form p-nitrophenol because the nitro group, being a strong electron-withdrawing group, increases the electrophilicity of the carbon bonded to chlorine. This enhances the susceptibility of the C–Cl bond to nucleophilic substitution, allowing the hydroxide ion to easily replace the chlorine atom.

In contrast, chlorobenzene reacts with aqueous NaOH only at high temperature (623 K) and high pressure (300 atm). These severe conditions are required to overcome the resonance stabilisation of the C–Cl bond, thereby enabling the substitution of chlorine by the hydroxide ion to form phenol.