Saytzeff's Rule | Zaitsev's Rule
Saytzeff's rule, also known as Zaitsev's rule, predicts the most favored alkene product in an elimination reaction. It states that in an elimination reaction, the most alkyl groups attached to the doubly bonded carbon atoms (highly substututed alkene) is the major product. Saytzeff's rule only describes the regiochemistry of the elimination reaction, not the stereochemistry of the product.
Mechanism of Saytzeff Elimination Reaction
Mechanism can be explain by taking an example of dehydration of alcohol in acidic medium. When 2-butanol is dehydrated, it produces two products: 1-butene and 2-butene. 2-butene is the major product because it is more stable and more easily replaceable.
First Step: Proton from acid abstract the -OH group of alcohol as water and forms a carbocation.
Second Step: Hydrogen from adjacent carbon depats to form alkene.
Exceptiosn of Saytzeff's Elimination Reaction
These are some exceptions to Saytzeff's rule:
1. Anti-Saytzeff Rule (Hofmann's Rule) favors less substituted alkene or alkyne.
Second product is not possible because base (NaOCMe3) is bulky so, cannot abstract a proton from the ring.
2. No Antiperiplanar beta Hydrogen on Cyclohexane.
The mechanism of an E2 elimination reaction is only possible when the leaving group and a beta hydrogen are antiperiplanar. For a cycloalkane rotation around the C-C bonds is restricted limiting when a beta hydrogen may be antiperiplanar to the leaving group.
3. Steric Hindrance can hinder the formation of the more substituted alkene.
The alkene obtained through path(b) is minor due to steric hinderance.
4. Resonance can influence regioselectivity.
First product is the major product due to conjugation.
Examples of Saytzeff's Rule
