E2 reaction

Meaning: E2 stands for bimolecular elimination. A base removes a β-hydrogen while the leaving group leaves the α carbon and the C=C π bond forms, all in one concerted step with no carbocation. Base and substrate are both in the rate-determining step, so the reaction is second order overall. The H and leaving group must be anti-periplanar, so on a cyclohexane both must be axial. By Zaitsev's rule the more substituted alkene is usually the major product, though a bulky base such as tert-butoxide often favors the less substituted (Hofmann) alkene.

Equation: rate = k[R–X][B], where k is the rate constant (L mol⁻¹ s⁻¹), [R–X] is the alkyl halide concentration (mol/L), and [B] is the base concentration (mol/L).

On the MCAT: Expect questions on which conditions favor E2: a strong base, heat, and a secondary or tertiary substrate. Doubling the base concentration doubles the rate. You may be asked to predict the major alkene or its stereochemistry from the anti-periplanar requirement.

Common confusion: E2 and SN2 are both second order and both use strong bases or nucleophiles. A tertiary halide cannot undergo SN2, so a strong base gives E2. E1 is the different, two-step pathway: it goes through a carbocation, its rate depends only on the substrate, and it is favored with a weak base.