SN2 Reaction Mechanism in Chemistry: A Detailed Study

by Yuvi K - December 17, 2023

What is SN2 Reaction Mechanism?

SN2 reaction mechanism is one of the most widely used mechanisms in chemistry, especially at the organic level. It stands for Substitution Nucleophilic Bimolecular and is a type of reaction that involves the exchange of a nucleophile with an electrophilic compound. This reaction is commonly used in molecules with tertiary and secondary carbons as well as carbonyl compounds.

It is important to understand the basics of SN2 reaction in order to understand its mechanism. In an SN2 reaction, a nucleophilic species attacks the carbon of the reactant molecule, displacing the leaving group and forming a new bond. In the process, the leaving group creates a negative charge on the carbon that it was displaced from. This negative charge is then saturated by another nucleophile which completes the SN2 reaction.

Stepwise Mechanism of SN2 Reactions

The stepwise mechanism of SN2 reactions is as follows:

Step 1: The electrophilic species attacks the nucleophilic centre of the reactant.
Step 2: The leaving group is then removed, creating an anion at the reaction centre.
Step 3: The nucleophile then attacks the anionic centre, replacing the leaving group and forming a new bond.
Step 4: The new bond forms a covalent bond between the nucleophilic species and the electrophilic species.

How Does SN2 Reactions Work?

SN2 reactions involve the exchange of a nucleophile with an electrophilic compound. This exchange is known as a “substitution reaction”. The reaction occurs when the nucleophile attacks the electrophilic centre of the reactant. This attack is followed by the displacement of the leaving group, usually by a special form of electron donation known as a back-side attack. This is followed by the nucleophile being saturated by the negative charge of the reaction centre, which forms the product.

Example of SN2 Reactions

One of the most common examples of SN2 reactions in organic chemistry is the reaction between alkyl halides and water. When an alkyl halide reacts with water, an SN2 reaction occurs with the hydroxide ion, or the OH–, as the nucleophile. The mechanism of this reaction is as follows:

Step 1: The electrophilic centre of the alkyl halide reacts with the OH–.
Step 2: The halide ion (leaving group) is displaced, creating an anion at the reaction centre.
Step 3: The OH– is then saturated by the positive charge at the reaction centre, forming a new covalent bond.
Step 4: The product is an alcohol, with a new covalent bond between the OH– and the alkyl halide.

Useful Tips for Carrying Out SN2 Reactions

The most important factor to consider when carrying out an SN2 reaction is the reaction rate. A slower reaction rate leads to more complete reaction and higher yields.
The reaction rate can be increased by increasing the concentration of the reactants, raising the temperature of the reaction, or both. However, it is important to note that these techniques can also lead to side reactions and decreased yields.
Nucleophiles should generally be selected with a high electron density so that they can attack the electrophilic centre more easily. Conversely, leaving groups should be selected with a low electron density to facilitate their removal.
A solvent can also play an important role in an SN2 reaction. Polar protic solvents, such as water or alcohols, can facilitate the reaction while solvents with a low dielectric constant, such as carbon tetrachloride, can slow down the reaction rate.

Conclusion

SN2 reactions are widely used in organic chemistry. Understanding the basics of SN2 reaction, the stepwise mechanism, and tips for carrying out the reaction can aid in the successful completion of the reaction and higher yields for the end product. Thus SN2 reaction is an important concept for chemists to understand and master.