SN2 Reaction Mechanism

What is SN2 Reaction Mechanism?

The SN2 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

SN2 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 SN2 reaction is a good example of stereospecific reaction, one in which different stereoisomers react to give different stereoisomers of the product. Also, SN2 reaction is the most common example of Walden inversion where an asymmetric carbon atom undergoes inversion of configuration.

What is an SN2 Reaction?

The SN2 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 ‘SN2’ stands for – Substitution Nucleophilic Bimolecular. This type of reaction is also referred to as bimolecular nucleophilic substitution, associative substitution, and interchange mechanism.

SN2 Reaction

Some examples of SN2 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 SN2 reactions.

Reaction Kinetics: Since an SN2 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”.

SN2 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 180o 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 SN2 reaction mechanism for the nucleophilic substitution of chloroethane with bromine acting as the nucleophile is illustrated below.

SN2 Reaction Mechanism

Stereochemistry of SN2 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 SN2 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 SN2 reactions whereas tertiary substrates can not. To learn more about this topic and other related topics, such as the mechanism of SN1 reactions, register with BYJU’S and download the mobile application on your smartphone.

Frequently Asked Questions – FAQs


What does SN2 stand for?

SN2 stands for Nucleophilic Substitution, Second Order (organic chemistry).


What is the difference between SN1 and SN2?

The phase deciding the rate is unimolecular for SN1 reactions, whereas it is bimolecular for an SN2 reaction. SN1 is a two-stage system, while SN2 is a one-stage process. The carbocation can form as an intermediate during SN1 reactions, while it is not formed during SN2 reactions.


What determines SN1 or SN2?

In the rate of reaction, SN1 reactions are unimolecular and have a step-wise mechanism. Next, this process involves LG’s bond cleavage to produce an intermediate carbocation. The carbocation formation stability will decide whether reactions to Sn1 or SN2 occur.


What is the mechanism of SN2?

The SN2 reaction — A nucleophilic substitution in which 2 components are included in the rate-determining stage. -SN2 reactions are bimolecular with bond and bond-breaking steps simultaneously.


Do SN2 reactions change stereochemistry?

When a front-side attack occurs, the product’s stereochemistry remains the same; that is, the structure is maintained. Backside Attack: The nucleophile targets the electrophilic core on the opposite side of the left party in a backside attack.

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