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SN1 (substitution, nucleophilic, unimolecular) is a two-step mechanism. Step 1 (RDS): R-X → $R^+$ + $X^-$ (heterolytic bond cleavage → carbocation). Step 2 (fast): $R^+$ + Nu → R-Nu.

Key features: Rate = k[substrate] (first order — nucleophile not in rate law). Stereochemistry: racemization (planar sp2 carbocation attacked from both faces), with slight excess inversion due to ion-pair effect. Substrate: 3 degree > 2 degree > 1 degree > CH3 (more stable carbocation = faster ionisation). Weak nucleophiles suffice (H2O, ROH). Polar protic solvents (water, ethanol — stabilise carbocation and LG through solvation).

Critical feature: carbocation rearrangement. The intermediate can undergo 1,2-hydride or methyl shifts to form a more stable carbocation. This leads to unexpected (rearranged) products — a diagnostic feature of SN1.

Benzylic and allylic substrates are especially fast in SN1 due to resonance-stabilised carbocations. Triphenylmethyl (trityl) bromide reacts faster than tert-butyl bromide due to extensive resonance with three phenyl rings.

SN1 always competes with E1 (same first step — carbocation formation). Higher temperature favours E1 over SN1 (entropy-driven). The Lucas test for alcohols exploits SN1: tertiary react immediately, secondary in 5-10 min, primary unreactive at RT.