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Nucleophilic substitution in haloarenes
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Which of the following compounds undergoes nucleophilic substitution reaction most easily?Aryl halides are normally unreactive towards nucleophilic substitution because the C–Cl bond has partial double bond character due to resonance.
The reaction proceeds through a negatively charged intermediate (carbanion) formed by attack of the nucleophile on the ring carbon bearing chlorine.
An electron-withdrawing group such as $-NO_2$ at the ortho or para position stabilises this carbanion by delocalising the negative charge onto itself (−R and −I effects), so it makes the substitution much easier.
Electron-releasing groups such as $-OCH_3$ (+R) and $-CH_3$ (+I, hyperconjugation) increase the electron density of the ring, destabilise the carbanion and make the substitution even more difficult than in chlorobenzene.
Hence 4-nitrochlorobenzene undergoes nucleophilic substitution most easily.