Vicarious nucleophilic substitution

Vicarious nucleophilic substitution replaces aromatic hydrogen using a nucleophile that carries its own leaving group. In nitroarenes, reversible carbanion addition gives a sigma-H adduct; base-induced elimination of the hydrogen and the nucleophile-borne leaving group, followed by protonation, gives substitution without expelling hydride.

A sigma-H adduct forms by nucleophilic addition at an aromatic carbon that initially bears hydrogen. Both the original hydrogen and the incoming nucleophile remain attached at that carbon before the rearomatization steps.