Chlorination of Alkanes

Specification Reference Organic chemistry, Alkanes 3.3.2.4

Quick Notes

Alkanes, such as methane, can react with chlorine in a free-radical substitution reaction.
The reaction mechanism involves three steps:

AQA A-Level Chemistry mechanism diagram for free-radical substitution of methane by chlorine showing initiation, propagation, and termination

Initiation – chlorine radicals are formed using UV light.
Propagation – radicals react with methane, forming new radicals.
Termination – radicals combine to form stable molecules.

This reaction requires UV light and leads to a mixture of products.

Full Notes

Free-radical substitution has been outlined in more detail here
This page is just what you need to know for AQA A-level Chemistry :)

Methane can react with chlorine in the presence of ultraviolet (UV) light, forming chloromethane.

AQA A-Level Chemistry schematic showing methane reacting with chlorine under UV light to form chloromethane

Further substitution can occur, forming CH₂Cl₂, CHCl₃, and CCl₄.

UV light is required to initiate the reaction by homolytic fission.

The reaction is an example of free-radical substitution and occurs in several steps. We can show how the reaction occurs using a mechanism.

Free-Radical Substitution Mechanism

Step 1: Initiation (Radicals Are Formed)

UV light provides energy to break the Cl–Cl bond by homolytic fission.
Each chlorine atom ends up with an unpaired electron (•), making it a radical.

Step 2: Propagation (Radicals React and Regenerate)

Radicals react to form new radicals in a chain reaction.
Chlorine radical reacts with methane, forming a methyl radical.
Methyl radical reacts with Cl₂, forming chloromethane and a new Cl• radical:

The process continues, leading to further substitutions (see below).

Step 3: Termination (Radicals Are Removed)

Radicals combine to form stable (non-radical) molecules, stopping the reaction.
There are several possible termination reactions:

Termination stops the chain reaction.

Matt’s exam tip

When writing out mechanisms for free radical substitution, make sure you show clearly which species are radicals. And remember - if you start with a single radical on one side of the equation, you must have a single radical on the other side.

Limitations of Free-Radical Substitution

Further substitution can occur, leading to a mixture of products:

AQA A-Level Chemistry diagram showing further substitution of chloromethane to dichloromethane, chloroform and carbon tetrachloride

This explains why the reaction is difficult to control, leading to multiple side reactions.

Summary Table: Free-Radical Substitution Mechanism

Step	What Happens	Representative Detail
Initiation	Cl–Cl bond breaks by homolytic fission under UV to generate radicals.	Cl₂ → 2Cl• (UV)
Propagation	Radicals react with molecules and regenerate radicals (chain process).	Cl• + CH₄ → •CH₃ + HCl •CH₃ + Cl₂ → CH₃Cl + Cl•
Termination	Radicals combine to form stable molecules, ending the chain.	Cl• + Cl• → Cl₂ •CH₃ + Cl• → CH₃Cl •CH₃ + •CH₃ → C₂H₆