Question

2,4-dinitrochlorobenzene is much more reactive than chlorobenzene towards hydrolysis reaction with NaOH

Answer 1

The lone pair of electrons on chlorobenzene are delocalised on the benzene ring.
As a result C-Cl bond acquires slight partial double bond character. Consequently C-Cl bond in chlorobenzene is very strong and cannot be easily broken.


The presence of NO2 group at  para position withdraws electrons from the benzene ring and thus facilitate the attack of the nucleophile on para chlorobenzene. The carbanion thus formed is stabilised by the resonance as follows:


Thus para nitrochlorobenzene is more reactive towards nucleophillic substitution  reactions as compared to chlorobenzene.