Free Radical Substitution in the Reaction of Methane and Chlorine

A Free Radical Substitution Reaction

This page gives you the facts and a simple, uncluttered mechanism for the free radical substitution reaction between methane and chlorine. If you want the mechanism explained to you in detail, there is a link at the bottom of the page.

The facts

If a mixture of methane and chlorine is exposed to a flame, it explodes – producing carbon and hydrogen chloride. This isn't a very useful reaction!

The reaction we are going to explore is a more gentle one between methane and chlorine in the presence of ultraviolet light – typically sunlight. This is a good example of a photochemical reaction – a reaction brought about by light.

Note: These reactions are sometimes described as examples of photocatalysis – reactions catalysed by light. It is better to use the term "photochemical" and keep the word "catalysis" for reactions speeded up by actual substances rather than light.

CH_4 \text{Cl}_2 \longrightarrow CH_3\text{Cl} + H\text{Cl}

The organic product is chloromethane.

One of the hydrogen atoms in the methane has been replaced by a chlorine atom, so this is a substitution reaction. However, the reaction doesn't stop there, and all the hydrogens in the methane can in turn be replaced by chlorine atoms. Multiple substitution is dealt with on a separate page, and you will find a link to that at the bottom of this page.

The Mechanism

The mechanism involves a chain reaction. During a chain reaction, for every reactive species you start off with, a new one is generated at the end – and this keeps the process going.

Species: a useful word which is used in chemistry to mean any sort of particle you want it to mean. It covers molecules, ions, atoms, or (in this case) free radicals.

The over-all process is known as free radical substitution, or as a free radical chain reaction.

Note: If you aren't sure about the words free radical or substitution, read the page What is free radical substitution?.

Chain initiation

The chain is initiated (started) by UV light breaking a chlorine molecule into free radicals.

\text{Cl}_2 \longrightarrow 2\text{Cl}{\bullet}

Chain propagation reactions

These are the reactions which keep the chain going.

\begin{aligned} CH_4 + \text{Cl}{\bullet} &\longrightarrow {\bullet}CH_3 + H\text{Cl} \\ \\ {\bullet}CH_3 + \text{Cl}_2 &\longrightarrow CH_3\text{Cl} + \text{Cl}{\bullet} \end{aligned}

Chain termination reactions

These are reactions which remove free radicals from the system without replacing them by new ones.

\begin{aligned} 2{Cl}{\bullet} &\longrightarrow \text{Cl}_2 \\ \\ {\bullet}CH_3 + \text{Cl}{\bullet} &\longrightarrow CH_3\text{Cl} \\ \\ {\bullet}CH_3 + {\bullet}CH_3 &\longrightarrow CH_3CH_3 \end{aligned}

Where would you like to go now?

Help! Talk me through this mechanism Look at multiple substitution in this reaction Look at why side reactions happen in this reaction To menu of free radical reactions To menu of other types of mechanism To Main Menu