Topic overview

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SN2 proceeds in one concerted step: backside nucleophilic attack and leaving-group departure occur together, producing inversion of configuration. Its rate depends on both substrate and nucleophile concentrations, and steric hindrance at the reacting carbon slows the reaction. SN1 proceeds in two steps, first forming a carbocation and then allowing nucleophilic attack; its rate depends on substrate concentration alone. The planar carbocation can lead to racemization and may rearrange before product formation. Substrate structure, nucleophile strength, leaving-group ability, and solvent help distinguish the pathways. SN2 is generally favored by polar aprotic solvents, while polar protic solvents support SN1. Elimination can compete, particularly when conditions favor E1 or E2 reactions.