Question

Among chlorobenzene and 4-nitrochlorobenzene, which is more reactive towards nucleophilic substitution reactions and why?

Answer 1

1. Electron-Withdrawing Group Effect: 4-Nitrochlorobenzene has a nitro (-NO₂) group at the para position relative to the chlorine. This nitro group is a strong electron-withdrawing group via both inductive (-I) and resonance (-M) effects, which stabilizes any negative charge formed in the intermediate during nucleophilic substitution.
2. Intermediate Stabilization: In nucleophilic aromatic substitution, the nucleophile attacks the aromatic ring, forming a negatively charged Meisenheimer complex intermediate. The nitro group in 4-nitrochlorobenzene stabilizes this intermediate, lowering the activation energy required for substitution.
3. Reactivity of Chlorobenzene: Chlorobenzene lacks such an electron-withdrawing substituent. The aromatic ring is electron-rich, and the chlorine substituent is less susceptible to nucleophilic attack because the intermediate is not stabilized, making nucleophilic substitution difficult.