Lesson eLearning

Explain why nucleophilic substitution of chlorobenzene with NaNH₂ proceeds via the benzyne mechanism rather than a direct SNAr pathway, and outline the steps of this mechanism.

hard 5 marks Class 12 · Chemistry · Haloalkanes and Haloarenes
Answer
Because the aromatic ring is deactivated toward SNAr, the reaction proceeds by elimination of Cl⁻ to form benzyne, followed by nucleophilic attack of NH₂⁻ on the benzyne intermediate.

Step-by-step solution

  1. Recognize that chlorobenzene is an electron‑rich aromatic system; direct addition‑elimination (SNAr) requires an electron‑withdrawing group ortho/para to the leaving group, which is absent.
  2. Under strong basic conditions (NaNH₂, liquid NH₃), a proton ortho to the chlorine is abstracted, generating a carbanion.
  3. The carbanion expels the chloride ion, forming the highly reactive benzyne intermediate (a triple bond in the ring).
  4. The nucleophile NH₂⁻ then attacks the benzyne at either of the two carbon atoms of the triple bond, giving a σ‑adduct.
  5. Protonation of the σ‑adduct restores aromaticity, yielding aniline as the final product.
  6. Thus the overall pathway is elimination (Cl⁻ loss) → benzyne formation → nucleophilic addition → protonation.
Common mistake: Assuming a typical addition‑elimination SNAr mechanism without recognizing the need for benzyne formation.

Concept tested: Benzyne mechanism in nucleophilic aromatic substitution

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