Difference Between SN2 and E2 Reactions

Key Difference – SN2 vs E2 Reactions
 

The key difference between SN2 and E2 reactions is that SN2 reactions are nucleophilic substitution reactions whereas E2 reactions are elimination reactions. These reactions are very important in organic chemistry because the formation of different organic compounds is described by these reactions.

There are two types of nucleophilic substitution reactions named as SN1 reactions and SN2 reactions that are different from each other based on the number of steps involved in each mechanism. However, both these mechanisms include the substitution of a functional group in an organic compound with a nucleophile. There are two types of elimination reactions named as E1 and E2 reactions. These reactions give the mechanism of elimination of a functional group from an organic compound.

CONTENTS

1. Overview and Key Difference
2. What are SN2 Reactions
3. What are E2 Reactions
4. Similarities Between SN2 and E2 Reactions
5. Side by Side Comparison – SN2 vs E2 Reactions in Tabular Form
6. Summary

What are SN2 Reactions?

SN2 reactions are nucleophilic substitution reactions that are bimolecular. The SN2 reactions are single-step reactions. This means bond breaking and bond formation occur in the same step. The reaction is bimolecular because there are two molecules involved in the rate-determining step of the SN2 reaction.

The SN2 reactions take place in aliphatic sp3 carbon centres with stable leaving groups that are attached to this carbon centre. These leaving groups are more electronegative than carbon. Most of the times, the leaving group is a halide atom because halides are highly electronegative and stable.

SN2 reactions take place in primary and secondary substituted carbon atoms because steric hindrance prevents tertiary structures from going through the SN2 mechanism. If there are bulky groups around the carbon centre (that causes steric hindrance), then a carbocation intermediate will be formed. This leads to the SN1 reaction rather than SN2 reaction.

The rate of the SN2 reaction depends on various factors; nucleophilic strength determines the rate of reaction because steric hindrance affects the nucleophilic strength. The solvents used in the reaction also affect the reaction rate; polar aprotic solvents are preferred for SN2 reactions. If the leaving group is very stable, it also affects the reaction rate of SN2.

What are E2 Reactions?

E2 reactions are elimination reactions in organic chemistry, which are bimolecular reactions. These reactions are known as bimolecular reactions because the rate-determining step of the reaction involves two reactant molecules. However, the E2 reactions are single-step reactions. This means bond breaking and bond formations occur in the same step. In contrast, E1 reactions are two-step reactions.

There is a single transition state in E2 reactions. In these reactions, a functional group or a substituent is removed from an organic compound while a double bond is formed. Therefore, E2 reactions cause unsaturation of saturated chemical bonds. This type of reactions is often found in alkyl halides. Basically, primary alkyl halides along with some secondary halides undergo E2 reactions.

E2 reactions occur in the presence of a strong base. Then the rate-determining step of E2 reaction includes both the substrate (starting organic compound) and the base as reactants (this makes it a bimolecular reaction).

The main factors that affect the reaction rate of E2 reactions are strength of the base (greater the strength of the base, higher the reaction rate), type of solvent (polar protic solvents are preferred), stability of the leaving group (higher the stability of leaving group, higher the reaction rate), etc.

What are the Similarities Between SN2 and E2 Reactions?

• Both SN2 and E2 Reactions are bimolecular reactions.
• Both reactions are single-step reactions.
• Both reactions are common in primary and secondary structures of organic compounds.

What is the Difference Between SN2 and E2 Reactions?

SN2 vs E2 Reactions
SN2 reactions are nucleophilic substitution reactions that are bimolecular.	E2 reactions are elimination reactions in organic chemistry that are bimolecular reactions.
Nature
SN2 reactions are substitution reactions.	E2 reactions are elimination reactions.
Nucleophile
SN2 reactions require a nucleophile.	E2 reaction does not require a nucleophile.
Base
SN2 reactions do not require a base essentially.	E2 reactions require a strong base.
Solvent Type
SN2 reactions prefer polar aprotic solvents.	E2 reactions prefer polar protic solvents.
Factors Affecting the Reaction Rate
The SN2 reaction rate is determined by the nucleophilic strength, solvent type, stability of the leaving group, etc.	The E2 reaction rate is determined by the strength of the base, solvent type, stability of the leaving group, etc.

Summary – SN2 vs E2 Reactions

SN2 reactions and E2 reactions are very common in organic chemistry. SN2 reactions are single-step, bimolecular, nucleophilic substitution reactions. E2 reactions are single-step, bimolecular, elimination reactions. The difference between SN2 and E2 reactions is that SN2 reactions are nucleophilic substitution reactions whereas E2 reactions are elimination reactions.

Reference:

1.“14.3: Elimination by the E1 and E2 Mechanisms.” Chemistry LibreTexts, Libretexts, 21 July 2016. Available here  
2.“SN2 Reaction.” Wikipedia, Wikimedia Foundation, 11 Mar. 2018. Available here
3.“Elimination Reaction.” Wikipedia, Wikimedia Foundation, 16 Mar. 2018. Available here  

Image Courtesy:

1.’SN2 reaction mechanism’By Calvero. – Selfmade with ChemDraw., (Public Domain) via Commons Wikimedia  
2.’E2-mechanism’By Matthias M. – Own work, (Public Domain) via Commons Wikimedia