elimination

Meaning: An elimination reaction removes H and a leaving group from adjacent carbons, forming a C=C bond. In E2, a base removes the β-hydrogen as the leaving group departs in one concerted step, with H and the leaving group anti-periplanar. In E1, the leaving group leaves first, giving a carbocation, and then a weak base removes a β-hydrogen. By Zaitsev's rule the more substituted alkene is usually the major product; a bulky base such as tert-butoxide often favors the less substituted one.

Equation: Rate(E2) = k[RX][B]; Rate(E1) = k[RX]. RX is the substrate and B is the base, both in mol/L; k is the rate constant (L mol⁻¹ s⁻¹ for E2, s⁻¹ for E1).

On the MCAT: You are given a substrate, base, solvent and temperature and asked for the major product or mechanism. A strong base favors E2. A tertiary substrate with a weak base in a polar protic solvent gives E1 along with SN1. Heat favors elimination over substitution because elimination increases the number of molecules (ΔS > 0).

Common confusion: Students mix up elimination and SN2 because one reagent can do either. Tertiary carbons block backside attack, so they cannot undergo SN2, and a strong base then gives E2. Doubling the base doubles the E2 rate but not the E1 rate.