Mechanism of Free Radical Halogenation of Alkanes

It is possible to halogenate an alkane to produce an alkane derivative in which one or more halogen atoms have been swapped for hydrogen atoms in the hydrocarbon molecule.

For their non-polarity and the lack of functional groups at which reactions might take place, alkanes are well-known for being chemically inert.

 In this way, free radical halogenation can be used to functionalize alkanes in a simple and straightforward manner.

A significant constraint of radical halogenation, on the other hand, is the large number of identical C-H bonds that are present in all but the simplest alkanes, making it difficult to accomplish selective reactions in most cases.

General Reactions of Alkanes

Among the types of reactions that occur in organic chemistry, one that is very prevalent is the substitution reaction of alkanes, which can be seen as an example of a substitution reaction.

 In a substitution reaction, a tiny reacting molecule replaces an atom or group of atoms on a hydrocarbon or hydrocarbon derivative with a portion of the small reacting molecule.

Characteristics of Halogenation of Alkanes

For an alkane, the symbol R-H represents a general formula. 

The letter R in this example denotes an alkyl group.

 Adding a hydrogen atom to an alkyl group causes the production of the alkyl group’s parent hydrocarbon, which is the product of the addition.

When you look at the product side, the notation R-X indicates that it is the generic formula for a halogenated alkane. 

The general symbol for a halogen atom is represented by the letter X.

In the equation, the conditions of a reaction are documented by placing these conditions on an arrow that divides the reactants from the products.

The presence of heat or light is required for the halogenation of an alkane.

Mechanism of Halogenation  of Alkanes

• First step: When exposed to ultraviolet light, the elemental chlorine’s Cl-Cl bond undergoes hemolysis, which is the first step in the reaction. 

This reaction results in the formation of two chlorine atoms, which are also known as chlorine radicals.

• Second Step: One of the chlorine radicals steals a hydrogen atom from methane, resulting in the formation of the methyl radical. 

The second step of the propagation process also involves the regeneration of a chlorine atom, and these processes are repeated multiple times until the process comes to an end.

• Third Step: It is then necessary to make chloromethane, which is formed when the methyl radical removes one of its chlorine atoms from one of the chlorine molecules. 

The termination step occurs when a chlorine atom either interacts with another chlorine atom to form Cl₂, or when a chlorine atom reacts with a methyl radical to make chloromethane, which provides a minor pathway by which the product is produced. 

In addition, two methyl radicals can combine to generate ethane, which is a small by-product of this process and is not harmful.

This reaction does not come to a halt at this point. 

While this product can be permitted to react with more chlorine to generate polychlorinated compounds, it should not be allowed to react with other chlorinated goods. 

 It may be feasible to favour the creation of one or another probable chlorinated methane product by manipulating the reaction circumstances, including the ratio of chlorine to methane, in order to achieve a desired result.

Free Radical Halogenation

Free-radical halogenation is a form of halogenation that can occur in organic chemistry.

 Under the influence of ultraviolet light, this chemical reaction is characteristic of alkanes and alkyl-substituted aromatics. 

The technique is used in the industrial synthesis of chloroform (CHCl₃), dichloromethane (CH₂Cl₂), and hexachlorobutadiene. It is accomplished by the use of a free radical chain mechanism.

An anti-Markovnikov halogenation is the addition of hydrogen bromide to an alkene through the use of free radicals. 

In the Markovnikov addition of HBr to propene, the H atom joins with the C atom, which already contains a higher concentration of H atoms. 

2-Bromopropane is the chemical compound in question. In the presence of peroxides, the H atom attaches itself to the C atom that possesses less H atoms than the others.

Control of Halogenation 

In many cases, halogenation does not stop at monosubstitution. 

The chlorination of methane results in the formation of dichloromethane, chloroform, and carbon tetrachloride, depending on the reaction conditions.

 In most hydrocarbons, more than one potential result occurs depending on which hydrogen is substituted for the original. 

When butane (CH₃CHCl₂) is chlorinated at the “1” position, it produces 1-chlorobutane (CH₃CHCl₂), and when chlorinated at the “2” position, it produces 2-chlorobutane (CH₃CHCl₃).

The product distribution is influenced by the respective reaction rates: in this case, the “2” position of butane reacts more quickly, thus 2-chlorobutane is the predominant product produced.

In general, chlorination is less selective than bromination in terms of toxicity. 

Fluorination is not only even less selective than chlorination, but it is also very exothermic, necessitating extreme caution in order to avoid an explosion or a runaway reaction from occurring.

 In many cases, this relationship is cited as an example of the reactivity–selectivity principle, and it may be discussed in detail using the Hammond postulate.

 As a result of the low reactivity of the bromine radical, the transition state for hydrogen abstraction has a strong radical character and is reached late in the process.

 The reactive chlorine radical creates a transition state that is chemically similar to the reactant but lacks the radical nature of the reactant. 

When the alkyl radical is fully created in the transition state, it can take full advantage of any resonance stabilisation that may be present, resulting in the highest possible selectivity.

Conclusion

An alkane is halogenated to form a hydrocarbon derivative, in which one or more halogen atoms have been substituted for the hydrogen atoms in the alkane.

Alkanes are typically considered to be unreactive compounds due to the fact that they are nonpolar and lack functional groups, which allow for reactions to take place. 

As a result, free-radical halogenation is a method for functionalizing alkanes that is both simple and effective. 

However, a significant restriction of radical halogenation is the number of identical C-H bonds found in all but the simplest alkanes, which makes it difficult to accomplish selective reactions in these compounds.