Saytzeff’s Rule

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What is Saytzeff’s Rule?

Known as Zaitsev’s rule, Saytzeff’s rule is used to predict the major product for elimination reactions of haloalkanes and alcohols. It is an empirical rule for the prediction of the favoured alkene products in elimination reactions.

A Russian scientist named Alexander Zaitsev came up with the Saytzeff’s Rule by studying multiple elimination reactions and observing a general trend in the resulting alkenes. He stated that the alkene formed in the greatest amount is the one which corresponds to the removal of the hydrogen from the alpha-carbon with the fewest number of hydrogen substituents.

This rule predicts that in an elimination reaction, the most stable product is the one which is the most substituted and it is, therefore, the most favoured one. This rule makes no generalizations about the stereochemistry of the newly formed alked, however, it is applied only to the regiochemistry of the elimination reaction. Saytzeff’s rule effectively predicts the favoured product for many elimination reactions and also is subjected to various exceptions.

Elimination Reaction by Halo - Alkanes: How, What and Why?

To understand this rule in a better way, let’s also revise the elimination reaction. An elimination reaction competes with substitution reactions and in this reaction, a substrate which is usually an alkyl halide eliminates one equivalent unit of acid to form an alkene.

Mechanism of Elimination Reaction

There are two possible mechanisms, E1 and E2 mechanism for the elimination reaction. E1 and E2 reactions are different from each other as their nature of the transition states varies, which determines the regiochemistry of the product. The product forming step of an E1 reaction mechanism is highly exothermic as compared to that of an E2 reaction mechanism, and thus it has a relatively earlier transition state. On the other hand, E2 pathway involves a transition state, initiating from a starting material resulting directly to the product.

An elimination reaction can be defined as a type of reaction which is mainly taking place to transform saturated compounds i.e. organic compounds containing single carbon-carbon bonds, to unsaturated compounds i.e. compounds featuring double or triple carbon-carbon bonds.

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Elimination reaction is an important step for the preparation of alkenes. Here, a chemical reaction is taking place where several atoms (in pairs or groups) are removed from a molecule. The removal normally occurs due to the action of acids and bases or action of metals. It can also take place by heating at high temperatures.

The distinction in elimination reactions can be done by the kind of atoms/group of atoms leaving the molecule. The two major methods involved in elimination reactions are dehydration and dehydrohalogenation. In the former method, elimination of a water molecule from compounds like alcohol takes place and is also referred to as beta elimination reaction and in this, the leaving group and hydrogen are placed at neighbour carbon atoms. The latter method involves the removal of a hydrogen atom and a halogen atom.

Hofmann Elimination Mechanism

Hofmann elimination is a type of elimination reaction of an amine, where the least stable or least substituted alkene, called the Hofmann product, is formed. This tendency, known as the Hofmann alkene synthesis rule, varies from the usual elimination reactions, where Zaitsev's rule is based on the formation of the most stable alkene. The discoverer of the Hofmann elimination mechanism is, August Wilhelm von Hofmann.

In this reaction, the formation of a quaternary ammonium iodide salt takes place with the treatment of the amine with excess methyl iodide via exhaustive methylation, also followed by treatment with silver oxide and water forming a quaternary ammonium hydroxide. If this salt is decomposed by heat, it leads to the formation of the Hofmann product due to the steric bulk of the leaving group thereby causing the hydroxide to abstract the more easily accessible hydrogen.

Important Concept of Saytzeff’s Rule

If more than one elimination product is possible in the reaction, the most stable product or also called the major product is the most substituted alkene.

\[CH_{2} = CHR < RCH = CHR < R_{2}C = CHR < R_{2}C = CR_{2}\].

FAQ (Frequently Asked Questions)

1. What is an Elimination Reaction?

A. An elimination reaction is a kind of organic where two substituents are separated from a molecule in either a one-step or a two-step process. The reaction is known as dehydrohalogenation when the removal of a hydrogen atom and a halogen atom takes place. If both leaving atoms are halogens, then the reaction is known as dehalogenation. There are E1 and E2 reactions involved in the mechanism of elimination reactions.

2. How Does Stereochemistry Affect Saytzeff’s Rule?

A. Sometimes, the stereochemistry of the starting material (substrate) prevents the formation of the Zaitsev product.

For instance, when menthyl chloride reacts with sodium ethoxide, the Hofmann product is exclusively formed, but it is of meagre yield. Here, E2 eliminations require antiperiplanar geometry where the proton and leaving the group to lie on opposite sides of the carbon-carbon bond but in the same plane.