In many cases elimination reactions may proceed to give alkenes that are isomeric but with one formed in excess of the other (i.e. a major product). Zaitsev's Rule, based on experimental observations of the dehydration of alcohols, expresses the preference for eliminations to give the highly substituted (more stable) alkene, which may also be described as the Zaitsev product. As sp2 hybridized carbons are electron-deficient, increased substitution allows more electron donation by inductive and hyperconjugative effects to the sp2 carbon. Examples of Zaitsev’s Rule are demonstrated below:
The rule is not always obeyed; some reactions give the anti-Zaitsev product, which is sometimes described as the Hoffman product. Care is needed with E2 eliminations of cyclic systems since the antiperiplanar alignment of the C-H and C-LG bonds can dictate that the anti-Zaitsev products dominate. In other cases, extreme steric crowding can lead to anti-Zaitsev products (e.g. using a bulky base on a tertiary halide).