elimination reactions

Meaning: A leaving group departs from one carbon and a β hydrogen is lost from the neighboring carbon, leaving a double bond. In E2, a strong base removes the β hydrogen as the leaving group leaves, all in one step. The H and leaving group must be anti-periplanar, so on a cyclohexane both must be axial. In E1, the leaving group first departs to give a carbocation, then a weak base removes a β hydrogen. E1 favors tertiary substrates, polar protic solvents and heat. Acid-catalyzed dehydration of an alcohol is typically E1.

Equation: E2 rate = k[R–X][B]; E1 rate = k[R–X]; k is the rate constant (M⁻¹s⁻¹ for E2, s⁻¹ for E1), and [R–X] and [B] are the substrate and base concentrations in mol/L.

On the MCAT: Passages ask you to choose between substitution and elimination, or to name the major alkene. Elimination is favored by strong or bulky bases, heat (the products have more molecules, so ΔS is positive) and hindered substrates. Zaitsev's rule predicts the more substituted alkene, but a bulky base such as tert-butoxide often gives the less substituted (Hofmann) product. Rate questions ask what happens when concentrations are doubled.

Common confusion: Students link E1 with strong bases. A strong base favors E2, while E1 occurs with weak bases and competes with SN1 through the same carbocation. Because E1 has a carbocation, rearrangements can occur. E2 does not form one.