Saytzeff’s rule, also known as Zaitsev’s rule, is a principle in organic chemistry that helps predict the preferred regiochemistry (the position of chemical bonds in relation to atoms in a molecule) of elimination reactions, particularly in alkene formation. The rule is named after the Russian chemist Aleksandr Zaitsev (also spelled Saytzeff or Saytzev), who formulated it in the 19th century.
Saytzeff’s rule states that, in an elimination reaction (such as dehydrohalogenation), the more substituted alkene is the major product. Specifically, the rule predicts that the double bond will be formed at the carbon atom that has the greater number of alkyl substituents.
The general principle can be stated as follows:
In elimination reactions, the more substituted alkene is the major product.In elimination reactions, the more substituted alkene is the major product.
Here’s a simple example to illustrate Saytzeff’s rule:
Consider the elimination of hydrogen halide (H-X) from a halogen-substituted alkane (HX) to form an alkene:
CH3−CH2−CH2−CH2−�→CH2=CH−CH2−CH3CH3​−CH2​−CH2​−CH2​−X→CH2​=CH−CH2​−CH3​
According to Saytzeff’s rule, the major product will be the more substituted alkene, so the double bond will form between the second and third carbon atoms.
It’s important to note that while Saytzeff’s rule is often a useful guideline, there are cases where it may not be followed due to steric hindrance or other factors. Additionally, the Hofmann rule, which predicts the formation of the less substituted alkene, can be observed in some elimination reactions, particularly under specific conditions or with certain substrates. The application of these rules depends on the reaction conditions and the specific characteristics of the molecules involved