There are different types of reactions that take place in the nature around us. Every reaction has its own mechanism. There is no loss of energy. The substrates break, and products are formed. In between, the intermediates are formed. The mechanism of the bimolecular nucleophilic substitution reaction is described in detail below.

Substitution Reaction

The reaction in which a functional group of a compound substitutes another functional group. In other words, a substitution reaction is one in which a functional group of a compound is removed, and another functional group is replaced in its place.

Substitution reactions are widely seen in organic compounds. There are mainly two types of reactions, namely. 

• Electrophilic substitution reaction
• Nucleophilic substitution reaction

What do you mean by Nucleophilic Substitution Reaction?

The reaction which involves the nucleophile is known as the nucleophilic substitution reaction. In this process, the nucleophile approaches the electron-deficient centre. The nucleophile replaces the functional group resulting in the formation of a new product. 

What do you understand by nucleophiles?

Nucleophiles are chemical species that are electron-rich. They are known as Lewis bases because they contain the free electrons ready for donation. The nucleophile donates the pair of electrons to the electrophile and forms the chemical bond. 

An atom or a compound having a lone pair of electrons acts as a nucleophile. The pi bond of the atoms also acts as a source of electron-rich centres.

Some types and their examples of Nucleophiles

• Negatively charged nucleophile: The nucleophile that carries a negative charge is known as a negatively charged nucleophile.

• H–, OH–, CN–

• All Lewis bases

• LiAlH4, RMgX, CH3CN

Mechanism of SN2 Nucleophilic Substitution Reaction

SN2 Reaction:

• The SN2 reaction is a bimolecular nucleophilic substitution reaction. 
• It is a second-order reaction. 
• The rate of the reaction depends on the concentration of the substrate as well as the nucleophile. 
• This reaction takes place fastest in methyl alkyl halide. The rate of the SN2 reaction follows the order of 30 > 20 > 10 alkyl halide.
• SN2 reaction occurs in only one step. 
• No intermediates are formed. Only the transition states are formed. 
• The bond is weakened from one side, and the nucleophile makes the bond from the other side. 

A few examples of SN2 reaction are given below: 

• CH3CH2Cl + Nu–🡪  [Nu— CH3CH2—Cl]  🡪 CH3CH2Nu + Cl–
• CH2CH2Br + CN–🡪 [CN—CH2CH2—Br] 🡪 CH2CH2CN + Br–

Factors Affecting the Rate of Reaction

• Substrate: It is the most important factor that affects the rate of the reaction. This is so because the nucleophile attacks the substrate from the backside, the carbon halogen bond is broken, and the carbon nucleophile bond is formed. Thus, the substrate should be as less hindered as possible. 

• Nucleophile: The steric hindrance of the nucleophile should also be as less as possible. 

• Solvent: The role of solvent plays a major role in the formation of the carbon nucleophile bond. It depends on the solvent and how much hindrance it offers to the flow of nucleophiles to reach the carbon. 

• Leaving group: The strength of the leaving group decides the rate of the reaction. Therefore, the weaker the leaving group, the more is the rate of the reaction. 

Key Differences Between the SN1 and SN2 Reactions

Conclusion

This article focuses on the SN2 nucleophilic substitution reaction. It also explains the mechanism of the reaction in detail. The key differences between the SN1 and SN2 reactions have been listed. By going through this article, one will be able to understand the whole mechanism. We hope this article proves to be useful.