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Bromination of Methane

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 One of the hydrogen atoms in the methane has been replaced by a bromine atom, so this is a substitution reaction.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

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.

Br22Br

  • 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.

2BrBr2

CH3  +  BrCH3Br

CH3  +  CH3CH3CH3

Contributors

Jim Clark (Chemguide.co.uk)