1‑Chloro‑2,4‑dinitrobenzene ($\mathrm{Cl}{\mathrm{C}}_6{\mathrm{H}}_3(\mathrm{N}{\mathrm{O}}_2{)}_2$) is reacted with excess NaNH${}_2$ in liquid ammonia at 298 K. (a) State which nucleophilic aromatic substitution pathway (direct addition‑elimination or benzyne) is favoured and justify. (b) Write the structure of the major organic product. (c) Outline the complete mechanism leading to this product, indicating the role of the strongly basic medium.

Step-by-step solution

1. Identify the substituents: the ring bears two strongly electron‑withdrawing $\mathrm{N}{\mathrm{O}}_2$ groups at positions 2 and 4, and a leaving group $\mathrm{Cl}$ at position 1.
2. For direct SNAr (addition‑elimination) a strong $-\mathrm{N}{\mathrm{O}}_2$ must be ortho or para to the leaving group; here $\mathrm{N}{\mathrm{O}}_2$ is ortho (position 2), which would favour SNAr.
3. However, the ortho position (2) already carries $\mathrm{N}{\mathrm{O}}_2$, so the only hydrogen adjacent to the leaving group is at position 6.
4. In strongly basic liquid ammonia, NaNH${}_2$ can abstract this hydrogen (pKa of an aromatic C–H ≈ 43) because $\mathrm{N}{\mathrm{H}}_2^{-}$ is a very strong base.
5. Removal of H at C‑6 together with loss of $\mathrm{C}{\mathrm{l}}^{-}$ from C‑1 generates a highly reactive benzyne intermediate (a triple bond between C‑1 and C‑6).
6. The benzyne intermediate is attacked by $\mathrm{N}{\mathrm{H}}_2^{-}$ at either carbon of the triple bond; attack at C‑6 is favored because the adjacent $\mathrm{N}{\mathrm{O}}_2$ at C‑2 stabilises the resulting carbanion.
7. Protonation of the anionic intermediate by ammonia gives the final product, 2,4‑dinitro‑aniline (amino group at C‑6).
8. Thus the benzyne pathway dominates because the strong base can generate benzyne and the $\mathrm{N}{\mathrm{O}}_2$ groups stabilise the intermediate, while the direct SNAr route is hindered by the lack of a vacant ortho position for the Meisenheimer complex.

Concept tested: Benzyne mechanism in nucleophilic aromatic substitution

Published 15 September 2026 · How this solution was prepared and checked