15.1.5—SN1/SN2 substitution mechanisms

SN2 substitution occurs in one step as the nucleophile attacks while the leaving group leaves. SN1 occurs in two main steps: C–X ionisation, then nucleophile attack on the carbocation.

SN2 is favoured by less hindered primary centres; SN1 is favoured when a more substituted carbocation is stabilised by alkyl inductive effects. Solvent and leaving-group ability also matter.

OH⁻ attacking bromoethane illustrates SN2. A tertiary halogenoalkane can ionise first and then react with water by SN1.

SN1 does not mean “one molecule reacts”; it names the rate-determining molecularity of the ionisation step.