The nucleophilic aromatic substitution of chlorobenzene with sodium amide in liquid ammonia can proceed by two pathways: direct addition–elimination (SNAr) with rate constant k_{1}=1.2\times10^{-6}\ \text{s^{-1}} and the benzyne mechanism with rate constant k_{2}=3.6\times10^{-5}\ \text{s^{-1}}. Calculate the ratio $k_2/k_1$ and state which mechanism dominates.

Step-by-step solution

1. Write the ratio of the two rate constants: $\frac{k_2}{k_1}$.
2. Substitute the given values: $\frac{3.6\times 10^{-5}}{1.2\times 10^{-6}}$.
3. Perform the division: $\frac{3.6}{1.2}\times 10^{(-5)-(-6)}=3\times 10^1=30$.
4. Since the ratio is much greater than 1, the benzyne pathway is the predominant mechanism for this reaction.

Concept tested: Rate comparison of SNAr vs benzyne mechanisms

Published 4 September 2026 · How this solution was prepared and checked