SN2 Reaction Mechanism

What is S_N2 Reaction Mechanism?

The S_N2 reaction mechanism involves the nucleophilic substitution reaction of the leaving group (which generally consists of halide groups or other electron-withdrawing groups) with a nucleophile in a given organic compound.

The rate-determining step of this reaction depends on the interaction between the two species, namely the nucleophile and the organic compound.

Table of Contents

• What is an SN2 Reaction?
• SN2 Reaction Mechanism
• Stereochemistry of SN2 Reactions
• Frequently Asked Questions – FAQs

S_N2 reaction mechanism requires the attack of nucleophile from the back side of the carbon atom. So the product assumes a stereochemical position opposite to the leaving group originally occupied. This is called inversion of configuration. The S_N2 reaction is a good example of stereospecific reaction, one in which different stereoisomers react to give different stereoisomers of the product. Also, S_N2 reaction is the most common example of Walden inversion where an asymmetric carbon atom undergoes inversion of configuration.

What is an S_N2 Reaction?

The S_N2 reaction is a nucleophilic substitution reaction where a bond is broken and another is formed synchronously. Two reacting species are involved in the rate determining step of the reaction. The term ‘S_N2’ stands for – Substitution Nucleophilic Bimolecular. This type of reaction is also referred to as bimolecular nucleophilic substitution, associative substitution, and interchange mechanism.

Some examples of S_N2 reactions are illustrated above. The rate of this type of reaction is affected by the following factors:

• Unhindered back of the substrate makes the formation of carbon-nucleophile bond easy. Therefore, methyl and primary substrates undergo nucleophilic substitution easily.
• Strong anionic nucleophiles speed up the rate of the reaction. Nucleophilicity increases with a more negative charge, and a strong nucleophile can easily form the carbon-nucleophile bond.
• Polar aprotic solvents do not hinder the nucleophile, but polar solvents form hydrogen bonds with the nucleophile. A good solvent for this reaction is acetone.
• Stability of the anion of the leaving group and the weak bond strength of the leaving groups bond with carbon help increase the rate of S_N2 reactions.

“Reaction Kinetics: Since an S_N2 Reaction is a second-order reaction, the rate-determining step is dependant on the concentration of nucleophile as well as the concentration of the substrate”.

S_N2 Reaction Mechanism

This reaction proceeds through a backside attack by the nucleophile on the substrate. The nucleophile approaches the given substrate at an angle of 180^o to the carbon-leaving group bond. The carbon-nucleophile bond forms and carbon-leaving group bond breaks simultaneously through a transition state.

Now, the leaving group is pushed out of the transition state on the opposite side of the carbon-nucleophile bond, forming the required product. It is important to note that the product is formed with an inversion of the tetrahedral geometry at the atom in the centre.

The S_N2 reaction mechanism for the nucleophilic substitution of chloroethane with bromine acting as the nucleophile is illustrated below.

Stereochemistry of S_N2 Reactions

There are two ways in which the nucleophile can attack the stereocenter of the substrate:

1. A frontside attack where the nucleophile attacks from the same side where the leaving group is present, resulting in the retention of stereochemical configuration in the product.
2. A backside attack where the nucleophile attacks the stereocenter from the opposite side of the carbon-leaving group bond, resulting in inversion of stereochemical configuration in the product.

Since purely S_N2 reactions show 100% inversion in stereochemical configuration, it is clear that these Reactions occur through a backside attack.

Thus, the nucleophile displaces the leaving group in the given substrates. It can be noted that primary and secondary substrates can take part in S_N2 reactions whereas tertiary substrates can not. To learn more about this topic and other related topics, such as the mechanism of S_N1 reactions, register with BYJU’S and download the mobile application on your smartphone.