In nucleophilic aromatic substitution () via the addition-elimination mechanism, which of the following halogens acts as the best leaving group when attached to a benzene ring bearing a strong electro

Question

In nucleophilic aromatic substitution ($S_NAr$) via the addition-elimination mechanism, which of the following halogens acts as the best leaving group when attached to a benzene ring bearing a strong electron-withdrawing group at the para position?

Prashant Kotian · Accepted Answer

Correct answer: D. In , the rate-determining step is the initial nucleophilic attack that forms the carbanion (Meisenheimer complex), not the cleavage of the carbon-halogen bond. The highly electronegative fluorine atom strongly polarizes the bond, accelerating the nucleophilic addition and stabilizing the resulting carbanion. Thus, the reactivity order is , which is the opposite of aliphatic reactions.

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