The key difference between Saytzeff and Hofmann rule is that Saytzeff rule indicates that the most substituted product is the most stable product, whereas Hofmann rule indicates that the least substituted product is the most stable product.
Saytzeff rule and Hofmann rule are very important in predicting the end product of an organic elimination reaction. These rules can indicate the nature of the final product of a particular organic chemical reaction, based on the substitution of the final product, e.g. most substituted or least substituted product.
CONTENTS
1. Overview and Key Difference
2. What is Saytzeff Rule
3. What is Hofmann Rule
4. Side by Side Comparison – Saytzeff vs Hofmann Rule in Tabular Form
5. Summary
What is Saytzeff Rule?
Saytzeff rule is an empirical rule that determines the final product of a particular reaction as the most substituted product. It is named mostly as Zaitsev’s rule. This rule is important in predicting the substitution of the final alkene product obtained from an elimination reaction. According to the carbon atom where the double bond forms in the final alkene product, we can define Saytzeff rule as “the alkene that forms via elimination of hydrogen from the beta-carbon having the least hydrogen substituents”. Therefore, the most substituted final product is the major product of the chemical reaction, which also turns out to be the most stable product.
For example, 2-iodobutane contains an iodide group at the second carbon of the carbon chain. When this compound is treated with a strong acid such as potassium hydroxide, we can get 2-butene as the major product and the minor product is 1-butene. Here, the carbon atom that has the fewest hydrogen substituents is the third carbon atom; therefore, the elimination of a hydrogen atom occurs from this carbon which gives 2-butene. The first carbon atom has the most hydrogen substituents; thus, elimination of hydrogen from the first carbon atom forms the minor product 1-butene. The reaction is as follows:
Figure 01: An Elimination Reaction that Occurs According to Saytzeff Rule
What is Hofmann Rule?
Hofmann rule is an empirical rule that determines the final product of a particular reaction as the least substituted product. That means; a chemical reaction takes place according to this rule if the majority of the final product is the least substituted olefin (alkene product). According to the carbon atom where the double bond forms in the final alkene product, we can define Hofmann rule as “the alkene that forms via elimination of hydrogen from the alpha-carbon having the most hydrogen substituents”.
Let’s look at an example:
Figure 02: A Reaction that Occurs According to Hofmann Rule
During the creation of quaternary ammonium and alkene via the reaction between a tertiary amine and excess methyl iodide, the least substituted product of alkene forms as the major product.
What is the Difference Between Saytzeff and Hofmann Rule?
The key difference between Saytzeff and Hofmann rule is that Saytzeff rule indicates that the most substituted product is the most stable product, whereas Hofmann rule indicates that the least substituted product is the most stable product. According to Saytzeff rule, the majority is the most substituted product, while according to Hofmann rule, the majority is the least substituted product.
The following infographic summarizes the difference between Saytzeff and Hofmann Rule.
Summary – Saytzeff vs Hofmann Rule
Saytzeff rule and Hofmann rule are very important in predicting the end product of an organic elimination reaction. The key difference between Saytzeff and Hofmann rule is that Saytzeff rule indicates that the most substituted product is the most stable product, whereas Hofmann rule indicates that the least substituted product is the most stable product.
Reference:
1. “Hofmann’s Rule and Zaitsev’s Rule.” Chemistry LibreTexts, Libretexts, 5 June 2019, Available here.
2. “Hofmann Elimination.” Wikipedia, Wikimedia Foundation, 1 Feb. 2020, Available here.
3. “Hofmann Rule.” OChemPal, Available here.
Image Courtesy:
1. “General Demonstration of Zaitsev’s Rule” By Ckalnmals – Own work (CC BY-SA 3.0) via Commons Wikimedia
2. “HofmannElimination” By V8rik at the English Wikipedia (CC BY-SA 3.0) via Commons Wikimedia