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 One of the hydrogen atoms in the methane has been replaced by a chlorine atom, so this is a substitution reaction.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 does not 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 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.
CH3 + Cl \(\rightarrow\) CH3C l
CH3 + CH3\(\rightarrow\)CH3CH3
Jim Clark (Chemguide.co.uk)