S_N2 also called Substitution Nucleophilic Bimolecular reaction mechanism is an essential process in organic chemistry. It involves a nucleophile attacking the central atom while a leaving group is simultaneously displaced. The “S_N” in S_N2 stands for “substitution nucleophilic,” and the “2” indicates that the rate-determining step is bimolecular.

In this article, we will look into the S_N2 reaction mechanism, its examples, energy diagrams, applications, etc.

What are S_N2 Reactions?

In an S_N2 reaction, a strong nucleophile attacks the carbon atom to which the leaving group is attached, forming a new bond to the carbon via a backside attack. In contrast, the leaving group detaches from the reaction center in a concerted fashion. This reaction is characterized by its bimolecular nature, as both the nucleophile and the substrate are involved in the rate-determining step.

Example of S_N2 Reaction

An example of an S_N2 reaction is the saponification of a triglyceride, which forms soaps. In this reaction, a triglyceride reacts with an alkali base (like sodium or potassium hydroxide) to produce glycerol. The reaction can be represented as follows:

Triglycerides + Alkali Base —–> Glycerol + Soap

Some examples of S_N2 reaction are added in the image below,

Examples of SN2 Reactions

S_N2 Reaction Mechanism

S_N2 reaction mechanism proceeds through a concerted backside attack of a nucleophile upon an alkyl halide. The critical steps of the S_N2 reaction mechanism can be summarized as follows:

• Nucleophile Approach: Nucleophile approaches the electrophilic carbon from the back side, opposite to the leaving group.

• Simultaneous Bond Formation and Cleavage: Nucleophile attacks the carbon while the leaving group starts to leave. It forms a new bond with the carbon via a backside attack, and the leaving group detaches from the reaction center in a concerted fashion.

• Inversion of Configuration: The reaction leads to the inversion of configuration at the reaction center, especially in the case of a molecule with a chiral center.

Example of S_N2 Reaction Mechanism

An example of an S_N2 reaction mechanism is the reaction between methyl bromide and hydroxide ion. The reaction can be represented as follows:

CH₃Br + OH^– ——-> CH₃OH + Br^–

Mechanism of this reaction can be broken down into the following steps:

Nucleophile Approach: The hydroxide ion approaches the carbon atom of the methyl bromide from the back side, opposite to the leaving group (bromine atom). The reaction can be represented as follows:

CH₃Br + OH^– ——-> [CH₃Br-OH]^–

Simultaneous Bond Formation and Cleavage: The hydroxide ion attacks the carbon atom while the bromine atom starts to leave. This results in forming a new bond with the carbon via a backside attack, and the bromine atom detaches from the reaction center in a concerted fashion. The reaction can be represented as follows:

[CH₃Br-OH]^– ——-> CH₃OH + Br^–

Inversion of Configuration: The reaction leads to the inversion of configuration at the reaction center, as the hydroxide ion replaces the bromine atom on the opposite side of the carbon atom.

Energy Diagram of S_N2 Reaction Mechanism

Energy diagram of an S_N2 reaction mechanism shows a single curve since it is a single-step reaction. The products, CH₃OH and Br^–, are in a lower energy state compared to the reactants, CH₃Br and OH^–. The top of the curve represents the transition state, the highest-energy structure involved in the reaction.

Energy Diagram of SN2 Reaction Mechanism

Transition state involves partial, partially formed, and partially broken bonds and is very unstable with no appreciable lifetime. The transition state structure is usually shown in a square bracket with a double dagger.

Reaction process goes through the transition state, and the positions of the three hydrogens around carbon are all pushed to the other side for the product. The energy diagram indicates that the overall reaction is exothermic, and the products are more stable.

Factors Affecting S_N2 Reaction Mechanism

Factors affecting S_N2 reactions are as follows:

• Strength of Nucleophile: A strong nucleophile is required for an S_N2 reaction. The reaction is faster with a strong nucleophile.

• Stability of Leaving Group: The more stable the leaving group, the lower the transition state energy, leading to a faster reaction rate.

• Structure of Alkyl Halide: The reaction is faster with a less hindered alkyl halide. The S_N2 transition state is very crowded, with five groups around the electrophilic center. If the three substituents in this transition state are small, there is slight steric repulsion, leading to a faster reaction.

• Type of Solvent: Using polar, aprotic solvents can increase the reaction rate. The rate of an S_N2 reaction is significantly influenced by the solvent in which the reaction takes place. Using protic solvents decreases the power of the nucleophile through strong solvation, leading to a slower reaction.

Properties of S_N2 Reaction Mechanism

Pproperties of S_N2 reaction mechanism are:

• It is a type of nucleophilic substitution reaction that involves breaking and forming a bond in a single step.

• Term S_N2 stands for “Substitution Nucleophilic Bimolecular,” indicating that two reacting species are involved in the rate-determining step of the reaction.

• S_N2 reaction proceeds through a concerted backside attack of the nucleophile upon the alkyl halide, resulting in an inversion of configuration.

• Energy diagram of an S_N2 reaction is a single-step reaction, showing a single curve. The transition state is the highest-energy structure involved in the reaction and involves partial, partially formed, and partially broken bonds. The reaction is exothermic, and the products are more stable.

• Due to extreme steric factors, vinyl and aryl halides are unreactive toward S_N2 displacement.

• S_N2 reaction occurs 500 times faster in acetone than in methanol.

Stereochemistry of S_N2 Reactions

The seterocenter of the subtrate is attacked by the nucleophile in two ways that are,

• Frontside Attack
• Backside Attack

Frontside Attack: In frontside attack the nuclephile attacks from the same side where the leaving group is present, in this case the stereochemical configuration of the product is retained.

Backside Attack: In backside attack the nuclephile attacks from the opposite side of the carbon-leaving group bound, in this case the stereochemical configuration of the product is inversed.

Difference Between S_N1 and S_N2 Reaction Mechanism

The difference between S_N1 and S_N2 reaction Mechanism are added in the table below,