The SN2 is a nucleophilic substitution reaction. In this reaction, ‘S’ stands for substitution. ‘N’ stands for nucleophilic and ‘2’ stands for bimolecular. It takes place in one step. However, it does not involve the formation of an intermediate. The rate-determining step involves the two species. Therefore, it is a bimolecular reaction. This reaction shows 100% inversion of configuration.

Mechanism of an SN2 reaction

In this mechanism, the nucleophile attacks the substrate from the side directly opposite the leaving group. As the reaction proceeds,  the bond between substrate and nucleophile becomes strong. In comparison, the bond between the substrate and leaving group weakens.

“The SN2 reaction progress in one step with an unstable arrangement of the atom is known as a transition state.”

In the transition state, both leaving group and nucleophiles partially bonded to the substrate. The transition state exists for a very short time. For example, it lasts only for 10-12 s. The hybridization of carbon changes from sp3 in the substrate to sp2 in the transition state. However, it again returns to sp3 in the product.

Kinetic Evidence of sn2 reaction

The rate-determining step of the SN2 reactions involves both substrate and nucleophile. Therefore, the rate of reaction depends on the concentration of substrate, as well as the concentration of nucleophiles. Hence, it follows second-order kinetics.

However, if the solvent act as a nucleophile. The mechanism will be bimolecular but the kinetics will be first order. This is because there will be no change in solvent concentration due to its large quantity. Therefore, the rate depends only on the concentration of the substrate. Hence, such reactions are known as pseudo-first-order.

For example,  in the above reaction water act as both solvent and nucleophile. Therefore, the rate of reaction depends only on the concentration of alkyl halide.

  • k’ = k (H2O)

Stereochemical evidence of sn2 reaction

In the SN2 reactions of primary alkyl halide, no stereochemistry is involved. This is because α-carbon is not chiral. If there is any chiral center is present then the reaction proceeds through inversion of configuration. For example, hydrolysis of 2-bromooctane with NaOH gives 2-octanol with an inverted configuration.

Concepts Berg

What determines SN1 or SN2?

The structure of the substrate determines that the reaction proceeds via SN1 and SN2. If the substrate contains primary alkyl halide then the reaction will proceed through the SN2 mechanism. Whereas the substrate contains tertiary alkyl halide. It will follow the SN1 mechanism.

What is the mechanism of SN2?

The SN2 is a one-step mechanism. In which nucleophile attacks the substrate and leaving group leaves in the same step.

Do SN2 reactions change stereochemistry?

During SN2 reaction nucleophile attacks from the backside. Therefore, the stereochemistry of the carbon changes.

What are the orders of SN1 and SN2 reactions?

The SN1 is a first-order reaction.  Whereas the SN2 is a second-order reaction.

Why is alkyl iodide more reactive than alkyl chloride in Sn2 reactions?

The C-I bond is weaker than the C-Cl bond. Therefore, alkyl iodide undergoes substitution more readily as compared to alkyl chloride.

Reference Books 

  • Organic Chemistry by T.W. Graham Solomons (University of South Florida), Craig B. Fryhle (Pacific Luthern University), and Scott A. Snyder (University of Chicago)

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