Zaitsev's rule

In chemistry, Zaitsev's rule, Saytzeff's rule or Saytsev's rule named after Alexander Mikhailovich Zaitsev (number of different spellings due to the name being transliterated from Russian) is a rule that states that if more than one alkene can be formed by an elimination reaction, the more stable alkene is the major product. In general, the compound that has a more highly substituted C=C double bond is more stable due to the electron donating properties of the alkyl group.

Alexander M. Zaitsev was a professor at the University of Kazan (Russia) in the late 1800s. In 1875 he put forth a generalization about the regioselectivity of β elimination reactions from alcohols.

He said, "The alkene formed in greatest amount is the one that corresponds to removal of the hydrogen from the β-carbon having the fewest hydrogen substituents."

Another way to state it is as follows: in elimination reactions, the major reaction product is the alkene with the more highly substituted (more stable) double bond. This most-substituted alkene is also the most stable. Under thermodynamic conditions β elimination occurs to form the most stable alkene. This principle is similar to Markovnikov's rule which applies to the reverse process, addition reactions.

This rule is correct only when there are no other substituents beside carbon and hydrogen. Once other atoms are added; electronegativity, resonance, and other factors complicate the situation and invalidate the rule.

Steric Hindrance
During the elimination reaction, the base that causes the double bond to form has to be sterically unhindered for the reaction to follow Zaitsev. If the base, for example, is (CH3)3CONa, the bulkiness prohibits the base from pulling the leaving group off of the most substituted carbon. Another carbon atom is chosen and the Hofmann Product forms.