Halogenation Reaction Mechanism : Definition and Examples

Group-17 elements collectively known as halogens. They are highly reactive non-metallic elements. Group 17 includes Fluorine, Chlorine, Bromine, Iodine, and Astatine. Fluorine has the most electronegativity and smallest size, so it forms maximum, 

number of interhalogen compounds. Astatine is the radioactive element.Halogens have the smallest atomic radius in their respective periods due to maximum effective nuclear charge.What is Halogenation? When a halogen reacts with a substance, the latter undergoes halogenation. A halogenated compound is the end product of a halogenation reaction.

Alkanes are unreactive since they lack non-polar and functional groups. Free radical halogenation provides a mechanism to make alkanes reactive. If you were wondering what alkanes are, they are compounds that consist entirely of carbon and hydrogen atoms with a C-C or a C-H bond. 

The number of identical C-H bonds in most alkanes prevents radical halogenation, making some reactions challenging to execute.

What is free radical halogenation?

A reaction in which chlorine or bromine is substituted for hydrogen in an alkane is free radical halogenation. This is a photochemical reaction, which means the procedure occurs in the presence of ultraviolet light.

In most cases, free radical reactions are divided into three stages: Initiation, Propagation, and Termination. 

Properties of Halogens 

At ordinary temperatures and pressures, the halogen group is the only group of elements in the periodic table to include elements in all three recognised states of matter.

1. This colourless and odiferous gas is fluorine (F2).

2. Gases of chlorine (Cl2) have a colour similar to nitrous oxide.

3. Toxic Bromine (Br2) is a reddish-orange liquid with a pungent odour.

4. Instead of a black solid, you will receive a purple vapour when you heat iodine (I2).

5. The chemical compound Astatine (At) is an opaque black solid with a crystal structure.

6. In general, the smell of all halogens is disagreeable.

7. Halogen-containing elements are especially toxic.

8. These materials are poor conductors of heat and electricity.

9. Temperatures are extremely low.

10. Halogens create negative ions and become very reactive when inhaled. 

11. When it comes to chemical reactions, fluorine is one of the most volatile elements.

12. These molecules have relatively weak intermolecular interactions.

Halogenation reaction mechanism

When an alkane halogenation takes place, an alkane is represented by the symbol R-H where the alkyl group is the letter R. When a hydrogen atom is added to an alkyl group, the alkyl group’s parent hydrocarbon is formed.

R-H + X2 → R-X + HX

On the product side, R-X symbolises the generic formula for halogenated alkane. The letter X represents the halogen atom. The inclusion of heat or light is required for the halogenation of an alkane.

Chlorination of Methane by Substitution

During halogenation, depending on the halogen, an alkane will undergo fluorination, chlorination, bromination, or iodination. 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.

CH4  +  Cl2  → CH3Cl + HCl

In most cases, halogenation gives a mixture of products. A hydrogen atom in an alkane can be substituted with a halogen atom and form several products. 

Here are the steps that occur during the chlorination of methane.

1. Initiation

During the initiation phase, ultraviolet light splits the halogen into two radicals (atoms with a single unpaired electron). 

2. Propagation

During propagation, the halogen is made to react with an alkane. The hydrogen atom is de-linked from the carbon resulting in formation of methyl radical.

The methyl radical then interacts with another chlorine molecule to generate the product in the last step. As chlorine radicals are continuously created, this reaction can theoretically continue indefinitely, like a chain reaction, as long as there are reactants. 

3. Termination 

Termination occurs when the chlorine atom interacts with another chlorine atom to produce Cl2 or when the chlorine atom reacts with a methyl radical to produce chloromethane. Ethane can also form when two methyl radicals combine. 

Furthermore, the reaction doesn’t end yet because the chlorinated methane product can combine with more chlorine to generate polychlorinated compounds. By adjusting the chlorine-methane ratio, it is feasible to control the generation of a chlorinated methane product.

Controlling chlorination of methane 

Methane chlorination doesn’t always stop at producing monosubstituted chloromethane. The process continues to produce di, tri, and even tetra-chloromethanes. Using a significantly higher concentration of methane than chloride is a technique to control the rate and period of chlorination. The chlorine radical can be stopped from colliding with more chloromethane atoms to generate dichloromethane. 

Conclusion

Haloalkanes are more reactive than alkanes. The mechanism of halogenation can be understood through the chlorination of methane. No reaction occurs when methane (CH4) and chlorine (Cl2) are combined at room temperature in the absence of light. However, in higher temperatures or under Markovnikov irradiation, chloromethane (CH3Cl) is generated.