p‑Chloronitrobenzene ($\mathrm{Cl}{\mathrm{C}}_6{\mathrm{H}}_4\mathrm{N}{\mathrm{O}}_2$, chlorine para to the nitro group) is treated with excess NaNH₂ in liquid ammonia at 298 K.

Step-by-step solution

1. The nitro group is a strong –I and –M deactivator; it makes the ring less susceptible to the direct addition‑elimination (SNAr) pathway, which requires an electron‑withdrawing group ortho or para to the leaving halogen.
2. Because the nitro group is para to chlorine, the ortho positions (2 and 6) are activated for deprotonation by the very strong base NaNH₂, favouring benzyne formation.
3. Thus the benzyne mechanism is the preferred pathway.
4. (b) Attack of NaNH₂ on the benzyne intermediate mainly occurs at the carbon meta to the nitro group, giving 3‑amino‑4‑nitrobenzene as the major product.
5. (c) Mechanistic steps:
6. 1. NaNH₂ abstracts a proton from the carbon ortho to chlorine (position 2), generating a carbanion.
7. 2. The carbanion expels the chloride ion, producing a highly reactive benzyne (a triple bond between C1 and C2).
8. 3. The amide ion (NH₂⁻) adds to the benzyne at either carbon of the triple bond; addition at C2 (the carbon meta to the nitro group) is favoured because the nitro group withdraws electron density from C1, making C2 more electrophilic.
9. 4. The resulting anionic intermediate is protonated by solvent (NH₃) to give the aromatic amine product, 3‑amino‑4‑nitrobenzene.
10. The strongly basic medium (NaNH₂) is essential for both deprotonation (step 1) and nucleophilic attack (step 3).

Concept tested: Benzyne mechanism in nucleophilic aromatic substitution

Published 16 September 2026 · How this solution was prepared and checked