E1 reaction

Meaning: E1 stands for unimolecular elimination. In step 1 (slow, rate-determining) the leaving group departs and leaves a carbocation. In step 2 (fast) a weak base, often the solvent, removes a hydrogen from a carbon next to the cation, and the electrons form the C=C π bond. It is favored by tertiary or secondary substrates, good leaving groups, polar protic solvents, and weak bases. The carbocation can rearrange by a hydride or methyl shift, and the more substituted alkene is usually the major product (Zaitsev's rule).

Equation: rate = k[R–X], where rate is in M/s, k is the first-order rate constant in s⁻¹, and [R–X] is the substrate concentration in M.

On the MCAT: Passages give a substrate, solvent, base and temperature, and ask you to choose among SN1, SN2, E1 and E2. Heat favors elimination over substitution because elimination increases entropy (ΔG = ΔH − TΔS). Acid-catalyzed dehydration of a secondary or tertiary alcohol is a typical E1 example. Energy diagrams show two peaks with a carbocation intermediate between them, and the first peak is higher. Changing the base concentration does not change the rate.

Common confusion: Students mix up E1 and E2. E2 is a single concerted step with a strong base, its rate depends on both substrate and base, and it needs anti-periplanar geometry. E1 has a carbocation intermediate and a weak base. E1 and SN1 share that intermediate, so they usually form products together.