Difference Between E1 and E2 Reactions

Key Difference – E1 vs E2 Reactions
 

The E1 and E2 reactions are two types of elimination reactions that differ from each other based on the mechanism of elimination; the elimination can be either a one-step or a two-step mechanism. The key difference between E1 and E2 reactions is that E1 reactions have unimolecular elimination mechanism whereas E2 reactions have bimolecular elimination mechanism.

In organic chemistry, elimination reactions are a special type of chemical reactions in which substituents are removed (eliminated) from organic compounds.

CONTENTS

1. Overview and Key Difference
2. What are E1 Reactions
3. What are E2 Reactions
4. Similarities Between E1 and E2 Reactions
5. Side by Side Comparison – E1 vs E2 Reactions in Tabular Form
6. Summary

What are E1 Reactions?

E1 reactions are a type of two-step elimination reactions found in organic chemistry. In these elimination reactions, substituents in organic compounds are eliminated or removed. The reaction mechanisms of E1 reactions are known as unimolecular eliminations.

E1 reactions are two-step reactions, which means, an E1 reaction occurs through two steps named as ionization and deprotonation. In the ionization process, a carbocation is formed due to the removal of a substituent. In the second step (deprotonation), the carbocation is stabilized by removal of a hydrogen atom as a proton.

Usually, E1 reactions take place with tertiary alkyl halides. But sometimes, secondary alkyl halide also undergoes this type of elimination reactions. There are two reasons for this; bulky alkyl halides (highly substituted) are unable to undergo E2 reactions, and highly substituted carbocations are more stable than primary or secondary carbocations. In E1 reactions, the carbocation formation is the slowest step. Therefore, it is the rate-determining step pf E1 reactions, and the reaction rate depends only on the concentration of the alkyl halide.

Figure 01: Mechanism of an E1-Reaction in Organic Chemistry

E1 reactions usually take place in the complete absence of bases or the presence of weak bases. Acidic conditions and high temperatures are preferred for successful E1 reaction. And also, E1 reactions include carbocation rearrangement steps.

What are E2 Reactions?

E2 reactions are a type of one-step elimination reactions found in organic chemistry. In these elimination reactions, substituents in organic compounds are eliminated or removed in a single step. The reaction mechanisms of E2 reactions are known as bimolecular eliminations.

The E2 reaction mechanism is a single step elimination reaction with a single transition state. Therefore, the chemical bond breakdown and formation occurs in the same step. This type of reactions is often found in primary alkyl halides. But this can also be found in some secondary alkyl halides. The reaction involves two compounds; the alkyl halide and a base. Hence it is known as a bimolecular reaction. E2 reactions occur in the presence of a strong base. The most common example for E2 reactions is dehydrohalogenation.

Figure 02: An E2 Reaction Mechanism

The factors affecting the rate of E2 reaction are the strength of the base (greater the strength of the base, higher the reaction rate), solvent type (polar protic solvents increase the reaction rate), nature of the leaving group (better the leaving group, higher the reaction rate).

What are the Similarities Between E1 and E2 Reactions?

• Both E1 and E2 Reactions are types of elimination reactions.
• Both reactions are favoured by polar protic solvents.
• Both types of reactions can be observed in secondary alkyl halides.
• The rate of both reactions is increased if there are better-leaving groups are present in the alkyl halide.

What is the Difference Between E1 and E2 Reactions?

Summary – E1 vs E2 Reactions

Elimination reactions are chemical reactions where substituent groups are eliminated from organic compounds; especially from alkyl halides. The difference between E1 and E2 reactions is that E1 reactions have unimolecular elimination mechanism whereas E2 reactions have bimolecular elimination mechanism.

Reference:

1.“Elimination Reaction.” Wikipedia, Wikimedia Foundation, 16 Mar. 2018. Available here 
2.“14.3: Elimination by the E1 and E2 Mechanisms.” Chemistry LibreTexts, Libretexts, 21 July 2016. Available here 

Image Courtesy:

1.’E1-mechanism’By Matthias M. – Own work, (Public Domain) via Commons Wikimedia  
2.’E2 Elimination Reaction’By V8rik at the English language Wikipedia, (CC BY-SA 3.0) via Commons Wikimedia 

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