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SN2 (substitution, nucleophilic, bimolecular) is a one-step concerted mechanism. The nucleophile attacks from 180 degrees opposite the leaving group (backside attack), forming a pentacoordinate transition state [Nu---C---LG]‡. No intermediate is formed.

Key features: Rate = k[substrate][nucleophile] (second order). Complete Walden inversion (R → S or S → R). Substrate reactivity: CH3X > 1 degree > 2 degree >> 3 degree (never). Strong nucleophiles required ($OH^-$, $CN^-$, $I^-$, $RS^-$). Polar aprotic solvents preferred (DMSO, DMF, acetone — don't solvate nucleophile). Leaving group ability: $I^-$ > $Br^-$ > $Cl^-$ > $F^-$.

Special cases: Neopentyl bromide is primary but essentially unreactive (steric bulk from adjacent tert-butyl blocks backside approach). Allyl and benzyl halides react faster than simple primary due to pi-stabilisation of the transition state. Vinyl and aryl halides are completely unreactive (sp2 carbon + pi cloud blocks approach + partial double bond character of C-X).

The Finkelstein reaction (RCl + NaI → RI + NaCl in acetone) goes to completion because NaCl precipitates from acetone. The Swarts reaction (RCl + AgF → RF + AgCl) similarly uses precipitation to drive equilibrium.