Zaitsev's rule

Meaning: In elimination, a leaving group departs from one carbon and a β-hydrogen is removed from the adjacent carbon, forming a C=C bond. When more than one β-carbon carries hydrogens, several alkenes can form. Zaitsev's rule predicts that the major product is the alkene with the most alkyl groups on the double-bond carbons. Alkyl groups stabilize an alkene by hyperconjugation and by donating electron density to the sp² carbons, and the transition state, which has partial double-bond character, is lowered in energy in the same way. Example: 2-bromobutane with ethoxide gives mainly 2-butene (trans favored over cis) rather than 1-butene.

On the MCAT: You are asked for the major product of an E1 or E2 reaction, such as acid-catalyzed dehydration of an alcohol or an alkyl halide treated with a small strong base. Questions may also ask you to rank alkene stability, where the more substituted alkene has the smaller heat of hydrogenation. A passage may switch to a bulky base such as tert-butoxide and ask how the product ratio changes. Expect the less substituted alkene to gain ground there.

Common confusion: Zaitsev's rule is about elimination, which makes alkenes. Markovnikov's rule is about addition to alkenes. It is a strong tendency, not a law. A bulky base removes the more accessible hydrogen, which favors the Hofmann product, and in E2 the required anti-periplanar geometry can override it.