Radical Mechanism of Halogenation

When one or more halogens are introduced to a substance, it undergoes halogenation. Fluorine, chlorine, bromine, iodine, and astatine are among the halogens, which make up the seventh column of the periodic table. A halogenated compound is the end product of a halogenation reaction.

Because they are non-polar and lack functional groups at which reactions can occur, alkanes are notoriously unreactive molecules. As a result, free radical halogenation provides a mechanism for functionalizing alkanes.

The amount of identical C-H bonds found in all but the simplest alkanes is a serious constraint of radical halogenation, making selective reactions difficult to execute.

A reaction in which chlorine or bromine is substituted for hydrogen on an alkane is known as free radical halogenation. This is a photochemical reaction. That is, it only happens when the procedure is carried out in the presence of ultraviolet light.

In most cases, free radical reactions are divided into three stages: initiation, propagation, and termination. When describing the movement of a single electron, note the use of a single-headed arrow.

Alkane Halogenation Characteristics in General

Take note of the following characteristics of alkane halogenation.

• An alkane is represented by the symbol R-H. In this situation, R stands for an alkyl group. 

• When a hydrogen atom is added to an alkyl group, the alkyl group’s parent hydrocarbon is formed.

• On the product side, the symbol R-X denotes the generic formula for a halogenated alkane. A halogen atom is represented by the letter X.

• Placing reaction conditions on the equation arrow that separates reactants and products is an excellent way to track them. 

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

Chlorination of Methane by Substitution

• Depending on the identity of the halogen reactant, an alkane is said to undergo fluorination, chlorination, bromination, or iodination during halogenation. 

• The two most common alkane halogenation processes are chlorination and bromination. Fluorination reactions are usually too fast to be practical, while iodination reactions are too sluggish.

• In most cases, halogenations produce a mixture of products rather than a single product. Because more than one hydrogen atom on an alkane can be substituted with halogen atoms, several products occur.

Step 1: Initiation

The reaction starts with an initiation phase, which involves the addition of ultraviolet light to split the halogen (X2) into two radicals (atoms with a single unpaired electron). Because it starts the reaction, this phase is called the initiation step.

Step 2: Propagation

The initiation stage, which involves the creation of chlorine radicals, is followed by the propagation steps, which are closely engaged in product formation. In the chlorination reaction, for example, isobutane (C4H10) will be employed. The first step is to separate the hydrogen atom from the tertiary carbon (a tertiary carbon is one that is linked to three other carbon atoms). It is important to note that these are not protons (H+ ions), but genuine hydrogen atoms, because each hydrogen contains one electron. The tertiary radical is formed during the initial step of propagation.

The tertiary radical then interacts with another chlorine molecule to generate the product in the last step. Because another chlorine radical is created, this reaction can theoretically continue indefinitely as long as there are reagents. A chain reaction is what this is called.

Step 3. Termination Step

Termination occurs if a chlorine atom interacts with another chlorine atom to produce Cl2 or if a chlorine atom reacts with a methyl radical to produce chloromethane, which is a minor pathway by which the product is produced. Ethane, a small byproduct of this process, can also be produced by combining two methyl radicals.

Furthermore, because the chlorinated methane product can combine with more chlorine to generate polychlorinated compounds, the reaction does not end yet.

By adjusting the reaction mechanism and the chlorine-to-methane ratio, It is feasible to encourage the development of one of the chlorinated methane products.

Chlorination of Methane Constraints

Methane chlorination does not always come to an end after one chlorination. Monosubstituted chloromethane may be quite difficult to get. Di, tri, and even tetra-chloromethanes are generated instead. Using a significantly higher concentration of methane than chloride is one technique to avoid this problem. This minimizes the possibility of a chlorine radical colliding with chloromethane and restarting the reaction to generate dichloromethane. One can have a relative degree of control over the product using this way of managing product ratios.

Halogenation of benzene

Benzene reacts with chlorine or bromine in an electrophilic substitution reaction, but only in the presence of a catalyst. The catalyst is either aluminium chloride (or aluminium bromide if you are reacting benzene with bromine) or iron.

Strictly speaking iron isn’t a catalyst, because it gets permanently changed during the reaction. It reacts with some of the chlorine or bromine to form iron(III) chloride, FeCl₃, or iron(III) bromide, FeBr₃.

These compounds act as the catalyst and behave exactly like aluminium chloride in these reactions.

Conclusion

While there are few reactions that can be done with alkanes, many can be done with haloalkanes. The chlorination of methane will be extensively examined to understand the mechanism better. Nothing happens when methane (CH4) and chlorine (Cl2) are combined at ambient temperature in the absence of light. However, if the circumstances are adjusted so that the reaction occurs at high temperatures, denoted by or under ultraviolet irradiation, a product, chloromethane, is generated (CH3Cl).