Free Radical Substitution and Its Mechanism for JEE

The substitution reaction is defined as a reaction in which one chemical substance's functional group is replaced by another group, or a reaction in which one atom or molecule of a compound is replaced by another atom or molecule.

$A-B+X \rightarrow A-X+B$

These reactions can be divided into three categories:

1.  Free Radicals Substitution

2.  Electrophilic Substitution 

3. Nucleophilic Substitution

We will discuss here about the free radical substitution and its reactions and mechanism as well as the substitution of alkanes and the free radical halogenation process.

Free Radical Substitution 

A free radical-substitution is a substitution reaction in which the reactive intermediate is free radicals.

At least two phases, and perhaps a third, are always involved in the reaction.

The interaction of methane and chlorine in the presence of UV light (or sunlight) is a simple example of free radical substitution. One of the hydrogen atoms has been replaced with a chlorine atom in the methane.

Another example is free radical substitution of Cl₂ on benzene to form chlorobenzene.

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Free Radical Substitution Reactions

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Free Radical Substitution Mechanism

In the mechanism of free radical substitution, three stages are involved:

1. Initiation

2. Propagation

3. Termination

1. Initiation: The initiation phase is the first step in the creation of a radical species. Because of the enormous energy barriers involved, this is usually a homolytic cleavage event that occurs only infrequently. To overcome the energy barrier, heat, UV light, or a metal-containing catalyst are frequently used.

2. Propagation: The 'chain' aspect of chain reactions is described by the propagation phase. Once a reactive free radical has been produced, it can combine with stable molecules to produce more reactive free radicals. These new free radicals produce even more free radicals, and so on. Hydrogen abstraction or radical addition to double bonds are frequently used in propagation phases.

3. Termination:When two free radical species react to generate a stable, non-radical adduct, chain termination occurs. Because of the low concentration of radical species and the minimal chance of two radicals interacting, this is a highly rare thermodynamically downward occurrence.

Mechanism of Free Radical Substitution Reaction

1. Chain initiation- In the presence of heat or light, the chlorine molecule undergoes homolytic cleavage, releasing a chlorine free radical, which commences the reaction. 

 As a result, the C-C and C-H bonds are easier to break.

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2. Chain Propagation- The chlorine free radical attacks the methane molecule and drives the reaction ahead by breaking the C-H bond, resulting in the creation of methyl free radical and H-Cl.

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With the liberation of another chlorine free radical through homolytic fission of chlorine molecules, the methyl radical attacks the second molecule of chlorine to create CH₃-Cl.

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A series of reactions was started by the chlorine and methyl free radicals produced in the reaction. The propagation steps (a) and (b) directly give primary products, although there are numerous other propagation phases that can and will occur. The creation of more halogenated compounds from the principal product, chloromethane, can be explained in two ways.

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3. Chain Termination- The reaction comes to a halt after a period of time due to the consumption of reactants and/or the occurrence of the following side reactions:

The following are examples of probable chain-ending steps:

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Substitution Reaction of Alkanes

In the presence of light, alkanes undergo a substitution reaction with halogens.

Methane, for example, interacts with halogen molecules like chlorine and bromine when exposed to ultraviolet light.

Consider the following scenario:

Methylbromine + hydrogen bromide $\rightarrow$ methane + bromine

CH₃Br + HBr $\rightarrow$ CH₄ + Br₂

Because one of the hydrogen atoms in methane is replaced by a bromine atom, this is a substitution reaction.

Free Radical Halogenation

Free-radical halogenation is such that under UV light, this chemical reaction is typical of alkanes and alkyl-substituted aromatics. Chloroform (CHCl₃), dichloromethane (CH₂Cl₂), and hexachlorobutadiene are all manufactured via this procedure. A free-radical chain mechanism is used to carry it out.

General mechanism

Using the chlorination of methane as an example, the chain mechanism is as follows:

1. Initiation: Ultraviolet radiation causes a chlorine molecule to split or homolyze into two chlorine atoms. A chlorine atom is a free radical because it has an unpaired electron.

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2. Propagation: A hydrogen atom is extracted from methane, leaving a primary methyl radical in the process of chain propagation (two processes). After that, the methyl radical draws a Cl• from Cl₂.

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This produces the required product as well as a second chlorine radical. This radical will then participate in a second propagation reaction, resulting in a chain reaction. Other products, such as CH₂Cl₂, may occur if there is enough chlorine.

3. Termination:The reaction comes to a halt after a period of time due to the consumption of reactants-

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The final option in the termination step will produce an impurity in the final combination; specifically, an organic molecule with a longer carbon chain than the reactants.

The overall reaction is as follows:

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Conclusion

Atoms or groups of atoms with a single unpaired electron are known as free radicals. These radicals are involved in a free radical substitution reaction. When a bond breaks equally, each atom receives one of the two electrons, resulting in the formation of free radicals. Homolytic fission is the name given to this process.

There are 3 steps which are involved in the mechanism of free radical substitution namely- chain initiation, chain propagation and chain termination. A reaction undergoes these three steps in mechanism to produce the desired compound. Alkene also undergoes free radical substitution in the presence of UV-light.