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Chlorination of Methane

This page gives you the facts and a simple, uncluttered mechanism for the free radical substitution reaction between methane and chlorine.

The facts

If a mixture of methane and chlorine is exposed to a flame, it explodes - producing carbon and hydrogen chloride. This is not 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.

\[ CH_4  +  Cl_2 \rightarrow CH_3Cl  +  HCl\]

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 One of the hydrogen atoms in the methane has been replaced by a chlorine atom, so this is a substitution reaction.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. The over-all process is known as free radical substitution, or as a free radical chain reaction.

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

Cl2 \(\rightarrow\) 2Cl

  • Chain propagation reactions : These are the reactions which keep the chain going.

CH4  +  Cl\(\rightarrow\)CH3  +  HCl

CH3  +  Cl2\(\rightarrow\)CH3Cl  +  Cl

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

2Cl\(\rightarrow\)Cl2

CH3  +   Cl \(\rightarrow\) CH3C l

CH3  +  CH3\(\rightarrow\)CH3CH3

Contributors

Jim Clark (Chemguide.co.uk)