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 bromine. This reaction between methane and bromine happens in the presence of ultraviolet light - typically sunlight. This is a good example of a photochemical reaction - a reaction brought about by light.
\[CH_4 + Br_2 \rightarrow CH_3Br + HBr\]
The organic product is bromomethane. One of the hydrogen atoms in the methane has been replaced by a bromine atom, so this is a substitution reaction.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 bromine atoms.
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 bromine molecule into free radicals.
- Chain propagation reactions: These are the reactions which keep the chain going.
CH4 + BrCH3 + HBr
CH3 + Br2CH3Br + Br
- Chain termination reactions: These are reactions which remove free radicals from the system without replacing them by new ones.
CH3 + BrCH3Br
CH3 + CH3CH3CH3
Jim Clark (Chemguide.co.uk)