Two-step mechanism: Step 1 (RDS): R-X → R+ + X- (heterolytic bond cleavage → carbocation) Step 2 (fast): R+ + Nu:- → R-Nu (nucleophilic attack on planar carbocation)

Stereochemistry: Racemization expected (planar sp2 cation attacked from both faces). In practice, slight excess inversion due to ion-pair effect (leaving group shields one face briefly).

Rearrangement: The carbocation intermediate can undergo 1,2-hydride or methyl shifts to form a more stable cation. This leads to unexpected (rearranged) products.

Best conditions: 3° substrate + weak nucleophile (H2O, ROH) + polar protic solvent (water, ethanol). Heat favors SN1/E1 over SN2/E2.