In nucleophilic aromatic substitution () via the addition-elimination mechanism, which of the follow — Aromatic Hydrocarbons Chemistry Question
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?
Answer: D
💡 Solution & Explanation
In $S_NAr$, 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 $C-F$ bond, accelerating the nucleophilic addition and stabilizing the resulting carbanion. Thus, the reactivity order is $F > Cl > Br > I$, which is the opposite of aliphatic $S_N1/S_N2$ reactions.
💬Ask on WhatsApp →
Still have doubts about this question?
Send it to our AI chemistry tutor on WhatsApp — gets answered in minutes