3‑Chloropyridine is treated with excess NaNH₂ in liquid ammonia at 298 K. Which mechanism dominates the substitution and what is the major organic product? Briefly outline the steps of the favoured mechanism.
Answer
SNAr mechanism; 3‑aminopyridine
The correct answer is SNAr mechanism; 3‑aminopyridine — Step 5: The benzyne pathway is disfavoured because it requires adjacent leaving groups and high energy intermediates, which are not present here. Hence, the SNAr pathway dominates.
Step-by-step solution
- Step 1: Pyridine is an electron‑deficient aromatic ring, favouring nucleophilic aromatic substitution (SNAr).
- Step 2: NaNH₂ provides the strong nucleophile NH₂⁻ which attacks the carbon bearing the chlorine at position 3.
- Step 3: This forms a Meisenheimer complex (σ‑complex) where the ring is temporarily non‑aromatic and bears a negative charge on the ring nitrogen and the adjacent carbon.
- Step 4: The complex then eliminates Cl⁻, restoring aromaticity and giving 3‑aminopyridine as the major product.
- Step 5: The benzyne pathway is disfavoured because it requires adjacent leaving groups and high energy intermediates, which are not present here. Hence, the SNAr pathway dominates.
Common mistake: Assuming the reaction proceeds via a benzyne intermediate
Concept tested: SNAr mechanism for heteroaromatic substitution
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