: 250
E2 (bimolecular elimination): One-step concerted. Base abstracts beta-H while C-X bond breaks simultaneously. Rate = k[substrate][base]. Requires anti-periplanar geometry (H-C-C-X dihedral = 180 degrees). In cyclohexanes, this means trans-diaxial arrangement.
Zaitsev rule (non-bulky base): more substituted alkene is major product (thermodynamically more stable). Hofmann rule (bulky base like KOtBu): less substituted alkene is major (base can't reach hindered internal H).
Competition with SN2: For 1 degree and 2 degree substrates with strong base/nucleophile. Temperature increase favours E2 (entropy). Bulky base strongly favours E2.
E1 (unimolecular elimination): Two-step — same first step as SN1 (carbocation formation). Then base removes beta-H from the carbocation → alkene. Rate = k[substrate]. Always accompanies SN1. Zaitsev product always (no steric constraint on deprotonation). Rearrangement possible (carbocation intermediate).
Decision framework: 3 degree + strong base → E2 only (never SN2). 3 degree + weak Nu/protic → SN1 + E1 mixture. 1 degree/CH3 + strong Nu/aprotic → SN2. 1 degree + bulky base → E2. 2 degree → most complex; depends on base strength, size, solvent, temperature.