Mechanism
E2: base pulls off a β-hydrogen at the same time the leaving group leaves. One step, second-order kinetics: rate = k[substrate][base]. E1: leaving group leaves first to give a carbocation; base takes a β-hydrogen in step two. Two steps, first-order: rate = k[substrate].
Stereochemistry — the anti-periplanar trick
E2 requires the β-H and the leaving group to be exactly 180° apart — anti-periplanar. In cyclohexanes, both must be axial; if neither is axial, no E2. This is why some cis/trans isomers of substituted cyclohexyl halides react slowly or not at all under E2 conditions.
Regiochemistry — Zaitsev vs Hofmann
Most E1 and E2 reactions give the Zaitsev product: the more substituted, more stable alkene. The exception is E2 with a bulky base (like tert-butoxide) — the base is too big to grab the more hindered hydrogen, so it picks the less hindered β-H and gives the less-substituted Hofmann product instead.
How to spot which one ran
Tertiary substrate + heat + weak/no base + protic solvent → E1 (often racemic SN1 product alongside). Tertiary or secondary substrate + strong base (NaOH, EtO⁻, t-BuO⁻) → E2. If a chiral substrate gives a single alkene geometry, E2.
Draw this on the whiteboard
Open the OChem Board whiteboard — benzene rings, wedge/dash bonds, and a clickable periodic table built in. No account needed.