In cases where a mixture of alkenes arise in an elimination reaction, usually one of the possible products is formed in greater amount than the others.
Mixtures of alkenes can form from a haloalkane if there is more than one type of hydrogen on carbon atoms adjacent to the carbon bearing the leaving group.
It is possible to predict which will predominate in the reaction mixture because many experiments indicate that the relatively more stable alkene products are formed in larger amount.
In general alkenes with the fewer hydrogens on the double bond are more stable.
Alkenes with fewer hydrogens on the double bond are referred to as more substituted.
Zaitsev's (Saytzeff's) rule states that: The most substituted alkene is formed in greatest amount.
Mixtures of alkenes can form from a haloalkane if there is more than one type of hydrogen on carbon atoms adjacent to the carbon bearing the leaving group.
 |  |
| on C adjacent to C-Cl one type of H (one alkene) | on C adjacent to C-Br two types (two alkenes) |
It is possible to predict which will predominate in the reaction mixture because many experiments indicate that the relatively more stable alkene products are formed in larger amount.
In general alkenes with the fewer hydrogens on the double bond are more stable.
Alkenes with fewer hydrogens on the double bond are referred to as more substituted.
Thus when 2-bromobutane undergoes elimination, but-2-ene predominates in the product mixture.
 | KOH(alc) |  minor |
 major |
Zaitsev's (Saytzeff's) rule states that: The most substituted alkene is formed in greatest amount.