Which of the following compounds is least reactive in the nucleophilic aromatic substitution reactio — Reaction Mechanism Chemistry Question

💡 Solution & Explanation

In the $S_NAr$ mechanism, the rate-determining step is the initial attack of the nucleophile on the aromatic ring. This attack is highly facilitated by strong inductive electron-withdrawing groups ($-I$ effect) attached to the ring. Because Iodine provides the weakest $-I$ effect among the halogens, p-nitroiodobenzene stabilizes the anionic transition state the least and is therefore the least reactive.