Saytzeff’s Rule

Saytzeff’s rule, also known as Zaitsev’s rule, is a principle in organic chemistry that predicts the preferred regiochemistry of elimination reactions, such as dehydrohalogenation or dehydration reactions. Saytzeff’s Rule predicts that the most stable product in an elimination reaction is the one that is the most substituted.

In this article, we will cover the various concepts related to Saytzeff’s Rule, Examples of Saytzeff’s rule, and Saytzeff’s Elimination reaction along with a comparison between the Saytzeff rule and the Hoffman rule.

What is Saytzeff’s Rule?

Saytzeff’s Rule predicts the major product for elimination reactions of haloalkanes and alcohols. It is an empirical rule for predicting the favored alkene products in elimination reactions.

This rule predicts that in an elimination reaction, the most stable product is the one that is the most substituted and it is, therefore, the most favored one. Saytzeff’s Rule effectively predicts the favored product for many elimination reactions and also is subjected to various exceptions.

Elimination reactions of some alkyl halides and alcohol will result in different alkenes, and Saytzeff’s rule is used to predict the major product. The major and minor products are predicted based on the number of alkyl groups attached to the alkene.

Saytzeff Rule Definition

According to Saytzeff Rule, if more than one alkene is possible in a reaction as a product then highly substituted alkene is the major product.

• CH₂ = CHR < RCH = CHR < R₂C = CHR < R2C = CR₂
• Mono < Di < Tri < Tetra

Saytzeff Rule Reaction

Saytzeff’s rule reaction is given below:

CH₃-CHBr-CH₂-CH₃ (Alc.KOH, Δ)→ CH₃-CH=CH-CH₂ (80%) + CH₃-CH₂-CH=CH₂ (20%)

Examples of Saytzeff Rule

Some common examples of Saytzeff Rule are:

• Dehydrohalogenation of 2-Bromobutane
• Dehydration of 2,3- Dimethyl-2-Butanol
• Dehydrohalogenation of 2-bromo-2-methylpropane

Let’s discuss some of these examples in detail.

Dehydrohalogenation of 2-Bromobutane

The dehydrohalogenation of 2-bromobutane involves the removal of hydrogen and a bromine atom from adjacent carbon atoms, resulting in the formation of a carbon-carbon double bond.

The chemical reaction for the dehydrohalogenation of the 2-bromobutane is:

Dehydrohalogenation of 2-Bromobutane

According to this rule, in the dehydrohalogenation of the 2-bromobutane, 2-butene is the major product. This reaction takes place in the presence of alcoholic KOH only.

Dehydration of 2,3- Dimethyl-2-Butanol

The dehydration of 2,3- Dimethyl-2-Butanol involves the removal of hydrogen and a hydroxide atom from adjacent carbon atoms, resulting in the formation of a carbon-carbon double bond.

The chemical reaction for the Dehydration of 2,3- Dimethyl-2-Butanol is given below:

Dehydration of 2,3- Dimethyl-2-Butanol

When 2,3- Dimethyl-2-Butanol goes under dehydration reaction 2,3- Dimethyl-2-butene is a more stable product. On the other hand, 2,3- Dimethyl-1-butene is less stable.

Learn More about, Haloalkanes and Haloarenes

Mechanism of Saytzeff Elimination Reaction

The mechanism of Saytzeff Elimination Reaction can be explained by the dehydration reaction of 2, 3-dimethylpentan-3-ol. The step-by-step mechanism is given below:

Step 1: First, Sulphuric acid ionizes and gives out a proton

H₂SO₄ → H⁺ + HSO4^–

Step 2: The proton thus obtained reacts with the OH group and gives out OH₂⁺ and a tertiary carbonation is formed due to the gain of a positive charge by the carbon atom.

Step 3: In the third step, the removal of hydrogen from tertiary carbonation gives Alkene which results in the possibilities of three alkene formations, as shown below:

Saytzeff Rule Exception

These are some exceptions to Saytzeff’s rule:

• Hofmann Rule (Anti-Saytzeff Rule) favors less substituted alkene or alkyne.

• Steric Hindrance can hinder the formation of the more substituted alkene.

• The formation of conjugated systems may override the preference for more substituted alkenes.

• Resonance can influence regioselectivity.

Difference Between Saytzeff and Hoffman Rule

Following are the main differences between Saytzeff’s and Hoffman’s Rules:

Saytzeff Rule Vs Hoffman Rule
Point of Difference	Saytzeff Rule	Hoffman Rule
Regioselectivity	Prefers the formation of the more substituted alkene product.	Prefers the formation of the less substituted alkene product.
Mechanism	E1 and E2 mechanisms are common	The E1 mechanism is more often associated with the Hoffman Reaction.
Reaction	Elimination from beta carbon having most hydrogen substitutes.	Elimination from beta carbon having the least hydrogen substitutes.
Stability	More Stable	Less Stable
Example	2-iodobutane giving 2-butene	A tertiary amine gives the alkene

Read More,

• Organic Chemistry
• Purification of Organic Compounds
• Classification of Organic Compounds

Frequently‌ ‌Asked‌ ‌Questions‌ ‌on‌ Saytzeff’s Rule

What is Difference Between Saytzeff Rule and Markovnikov Rule?

The key difference between Saytzeff and Markovnikov rule is that the Saytzeff’s rule prefers the formation of most substituted alkene as its major product whereas Markovnikov rule favored the least substituted alkene.

Why is more Substituted Alkene Preferred in Elimination Reactions?

More substituted alkenes tend to be more stable due to factors like hyperconjugation and the distribution of electron density.

What is Hofmann Rule?

The Hofmann rule states that the major product in Hofmann eliminations and other similar elimination reactions is the less stable alkene.

Which is more Stable Hoffman or Saytzeff?

The Saytzeff product is the thermodynamically more stable product.

What is Zaitsev Rule with Example?

If more than one alkene is possible in a reaction as a product then highly substituted alkene is the major product.

Example: When 2-iodobutane is treated with alcoholic potassium hydroxide, it gives two products: 2-butene(major product) and 1-butene(minor product)

What is Application of Saytzeff’s Rule?

Saytzeff’s rule predicts the regioselectivity of the alkene formed by the elimination reaction of 2^o or 3^o alkyl halides.

What is Saytzeff Rule Exception?

Saytzeff Rule is not applicable to certain products, like Hofmann products. Also this rule can only be applied to the regiochemistry of the elimination reaction.