Which substituent on an aryl ring activates it toward nucleophilic aromatic substitution by stabilizing the Meisenheimer complex?

• A. Nitro group.
• B. Methoxy group.
• C. Methyl group.
• D. Ethyl group.

Answer: (A)

In nucleophilic aromatic substitution via the Meisenheimer pathway, the rate is governed by how well the negative charge in the sigma-adduct can be stabilized. After the nucleophile adds to the ring, this Meisenheimer complex carries a substantial negative charge delocalized around the ring. An electron-withdrawing substituent helps stabilize that negative charge through resonance and inductive effects, making the intermediate—and thus the overall reaction—more favorable.

The nitro group is a powerful electron withdrawer both inductively and by resonance. It can delocalize and stabilize the negative charge in the Meisenheimer complex, especially when placed ortho or para to the leaving group, greatly accelerating the reaction. A methoxy group, on the other hand, donates electron density via resonance, which destabilizes the negative sigma-adduct. Methyl and ethyl groups are mainly electron-donating by hyperconjugation/induction, also destabilizing the Meisenheimer intermediate. So the nitro substituent best activates the ring toward nucleophilic aromatic substitution by stabilizing the Meisenheimer complex.