Chemical Reactions Of Alkyl Halides: Nucleophilic Substitution-Stereochemical Aspect - II

An SN1 reaction is accompanied by racemisation, while an SN2 reaction proceeds with inversion of configuration.

Stereochemical aspects of an optically active alkyl halide:

General nucleophilic substitution reaction:

In this process, if the reaction conditions are favourable for first order kinetics, then it follows the mechanism of an SN1 reaction.

SN1 reaction mechanism:

This mechanism proceeds in two steps

Step-1: It is the slow step

The alkyl halide undergoes ionisation and forms a stable carbocation intermediate. This intermediate is sp2 hybridised and has a planar geometry.

Step-2: The attack of the nucleophile on the planar carbocation intermediate can take place from either face with equal ease. This result in the formation of a mixture of products in equal quantities, and this process is known as racemisation.

In SN1 mechanism 50% retention & 50% inversion products are formed. Raceimisation takes place in SN1 reaction.

If the reaction conditions are extremely favourable for second order kinetics, then the alkyl halides follow the SN2 reaction

SN2 reaction mechanism:

It's a concerted process, i.e. it proceeds in a single step.

Here, the nucleophile attacks from the direction opposite to that of the leaving group.

In this process, the displacement of the leaving group from the carbon atom of the substrate and the formation of a new bond with the nucleophile take place simultaneously.

This results in an unstable transition state with five bonds.

These bonds in transition state are distorted from the tetrahedral geometry of an alkyl halide. This state is short lived.

Then the leaving group is expelled, and results in a product with a complete inversion of configuration.

This is known as umbrella inversion.

Thus, the SN2 mechanism is accompanied by a complete inversion of configuration.

Summary

An SN1 reaction is accompanied by racemisation, while an SN2 reaction proceeds with inversion of configuration.

Stereochemical aspects of an optically active alkyl halide:

General nucleophilic substitution reaction:

In this process, if the reaction conditions are favourable for first order kinetics, then it follows the mechanism of an SN1 reaction.

SN1 reaction mechanism:

This mechanism proceeds in two steps

Step-1: It is the slow step

The alkyl halide undergoes ionisation and forms a stable carbocation intermediate. This intermediate is sp2 hybridised and has a planar geometry.

Step-2: The attack of the nucleophile on the planar carbocation intermediate can take place from either face with equal ease. This result in the formation of a mixture of products in equal quantities, and this process is known as racemisation.

In SN1 mechanism 50% retention & 50% inversion products are formed. Raceimisation takes place in SN1 reaction.

If the reaction conditions are extremely favourable for second order kinetics, then the alkyl halides follow the SN2 reaction

SN2 reaction mechanism:

It's a concerted process, i.e. it proceeds in a single step.

Here, the nucleophile attacks from the direction opposite to that of the leaving group.

In this process, the displacement of the leaving group from the carbon atom of the substrate and the formation of a new bond with the nucleophile take place simultaneously.

This results in an unstable transition state with five bonds.

These bonds in transition state are distorted from the tetrahedral geometry of an alkyl halide. This state is short lived.

Then the leaving group is expelled, and results in a product with a complete inversion of configuration.

This is known as umbrella inversion.

Thus, the SN2 mechanism is accompanied by a complete inversion of configuration.

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