Zaitsev's rule
/ ZYE-tsef /
When a molecule can eliminate in more than one direction — there are beta hydrogens on two different neighbouring carbons — it usually does not split the difference. It tends to make the more substituted, more crowded-looking double bond. Zaitsev's rule, named after the Russian chemist Alexander Zaitsev, captures this: the major product of an elimination is generally the more substituted (and therefore more stable) alkene.
The reason is alkene stability. A double bond is stabilized by the alkyl groups attached to it, through hyperconjugation and the way alkyl groups donate electron density into the pi system; so a tetrasubstituted alkene is more stable than a trisubstituted one, which is more stable than a disubstituted one, and so on. In reactions where the transition state for forming the alkene resembles the alkene itself (as in E1 and many E2 cases), the lower-energy, more substituted alkene forms faster and predominates. So Zaitsev is really a statement about stability showing up in the product ratio.
Treat Zaitsev as the default, not a law. It holds well for E1 and for E2 with small bases, but it is overturned by bulky bases (which give the Hofmann product) and by certain leaving groups, and it can be complicated by which beta hydrogen is geometrically accessible for an anti-periplanar E2. Knowing when Zaitsev applies — and when Hofmann's rule takes over — is exactly the kind of judgement that distinguishes predicting a reaction from merely memorizing one.
2-bromobutane with a small base such as ethoxide gives mainly but-2-ene (disubstituted, more stable) over but-1-ene (monosubstituted), a roughly 4-to-1 preference for the Zaitsev product.
The more substituted, more stable alkene is usually the major product.
Zaitsev is about the most stable alkene, not literally the most substituted carbon; a less substituted but conjugated (and so more stable) alkene can be the Zaitsev product instead.