Name: Introduction to Haloalkanes and Haloarenes
Uploaded: 2019-07-19
Duration: 4 min 42 s
Description: Haloalkanes are formed by replacing a hydrogen atom in an alkane with a halogen atom. Haloarenes are derived by replacing the hydrogen atom attached to the b...

Question

Explain

{S_{N}}^{1}

and

{S_{N}}^{2}

reactions with an example.

Accepted Answer

The ${S_{N}}^{1}$ reaction is a substitution nucleophilic unimolecular reaction. It is a two-step reaction. In the first step, The carbon-halogen bond breaks heterolytically with the halogen retaining the previously shared pair of electrons. In the second step, the nucleophile reacts rapidly with the carbocation that was formed in the first step.

This reaction is carried out in polar protic solvents such as water, alcohol etc. This reaction follows first order kinetics. Hence, this is named as substitution nucleophilic unimolecular. This reaction takes place in two steps as described below.

Step-1:

The bond between carbon and halogen breaks due to the presence of a nucleophile and formation of carbocation takes place.

It is the slowest and the reversible step as a huge amount of energy is required to break the bond.

The bond is broken by solvation of the compound in a protic solvent, thus this step is slowest of all.

The rate of reaction depends only on haloalkane, not on nucleophile.

Step-2:

The nucleophile attacks the carbocation formed in step 1 and the new compound is formed.

Since, the rate defining step of the reaction is the formation of a carbocation, hence greater the stability of formation of an intermediate carbocation, more is the ease of the compound undergoing substitution nucleophilic unimolecular or ${S_{N}}^{1}$ reaction.

In the case of alkyl halides, tertiary alkyl halides undergo ${S_{N}}^{1}$ reaction very fast because of the high stability of tertiary carbocations.

Hence, allylic and benzylic halides show high reactivity towards the ${S_{N}}^{1}$ reaction.

.

The ${S_{N}}^{2}$ reaction follows second order kinetics and the rate of reaction depends upon both haloalkane and participating nucleophile. Hence, this reaction is known as substitution nucleophilic bimolecular reaction. In this reaction, the nucleophile attacks the positively charged carbon and the halogen leaves the group.

It is a one-step reaction. Both the formation of carbocation and exiting of halogen take place simultaneously. In this process, unlike the ${S_{N}}^{1}$ mechanism, the inversion of configuration is observed. Since, this reaction requires the approach of the nucleophile to the carbon bearing the leaving group, the presence of bulky substituents on or near the carbon atom has a dramatic inhibiting effect.

So opposite to ${S_{N}}^{1}$ reaction mechanism, this is favoured mostly by primary carbon, then secondary carbon and then tertiary carbon. Nucleophilic substitution reaction depends on a number of factors. Some important factors include.

Effect of the solvent.

Effect of the structure of the substrate.

Effect of the nucleophile.

Effect of leaving-group.

Embibe Experts Solutions for Chapter: Haloalkanes and Haloarenes, Exercise 1: Telangana Board-2020

Embibe Experts Chemistry Solutions for Exercise - Embibe Experts Solutions for Chapter: Haloalkanes and Haloarenes, Exercise 1: Telangana Board-2020

Questions from Embibe Experts Solutions for Chapter: Haloalkanes and Haloarenes, Exercise 1: Telangana Board-2020 with Hints & Solutions

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