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 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 center. The nucleophile replaces the functional group resulting in the formation of a new product. 

What do nucleophiles 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 centre.

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 Nucleophilic Substitution Reaction

The nucleophilic substitution reaction is used to form alkane using alkyl halide. There are two types of Nucleophilic Substitution Reaction. They are: 

• SN1 Reaction: The SN1 reaction is a unimolecular nucleophilic substitution reaction. This reaction occurs in two steps. Tertiary alkyl halides undergo the SN1 mechanism, and 30 alkanes are formed. 

In the first step, the carbon halogen bond breaks heterolytically, and carbocation is formed as an intermediate. 

In the second step, the nucleophile attacks the carbocation, and an alkane is formed as a product. 

For example: 

(CH3)3Br 🡪 (CH3)3+ + Br–

(CH3)3+ + Nu–🡪 (CH3)3Nu

• SN2 Reaction: SN2 reaction is a bimolecular nucleophilic substitution reaction. This reaction takes place fastest in methyl 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. 

Example: 

CH2CH2Cl+ Nu–🡪 CH2CH2Nu + Cl–

Key Differences Between the SN1 and SN2 Reactions

Addition Reaction

This reaction basically occurs on the double and triple bonds. The species approaches the double and triple bonds. This species breaks into two parts, one positive and the other negative. The bonds are broken, and the positive and negative positions are formed. 

Now, the positive intermediate is added to the negative position, and the negative intermediate is added to the positive position. Thus, the new product is formed.  

Nucleophilic Addition Reaction

These reactions are very difficult to carry out. This is because the nucleophiles are electron-rich species, and the aromatic compounds are too electron-rich. So, there is repulsion between the two. Therefore, these reactions are carried out in harsh conditions.

Conclusion

This article focuses on the nucleophilic substitution reaction. It also explains the mechanism of the reaction in detail. By going through this article, one will be able to understand the whole mechanism. We hope this article proves to be useful.