= Nucleophilic substitution
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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.
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