 Nucleophilic substitution

A nucleophilic substitution replaces a leaving group by a nucleophile. In a concerted SN2 reaction, bond formation and departure occur in the same elementary step, commonly with inversion at the attacked stereocenter.

An SN1 reaction instead forms an intermediate before capture, often a carbocation, so rearrangement and a different stereochemical outcome may be possible. Substrate structure, solvent, leaving-group ability, and nucleophile properties influence which pathway is available. The substitution product alone does not uniquely identify the mechanism.