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# Free Radical Substitution in the Reaction of Methane and Bromine

## 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. If you want the mechanism explained to you in detail, there is a link at the bottom of the page.

### The facts

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.

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{Br}_2 \longrightarrow CH_3\text{Br} + H\text{Br}

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One of the hydrogen atoms in the methane has been replaced by a bromine atom, so this is a substitution reaction.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. Multiple substitution is dealt with on a separate page, and you will find a link to that at the bottom of this page.

Warning! Check your syllabus at this point. If your syllabus wants you to know about the free radical substitution reaction between methane and chlorine as well as this one, don't waste time trying to learn both mechanisms.

The two mechanisms are identical. You just need to learn one of them. If you are asked for the other one, all you need to do is to write bromine, say, instead of chlorine.

In writing the bromine mechanisms on these pages, that's exactly what I've done! If you read both chlorine and bromine versions, you'll find them boringly repetitive!

### 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.

#### Chain initiation

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

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

#### Chain propagation reactions

These are the reactions which keep the chain going.

\begin{aligned} CH_4 + \text{Br}{\bullet} &\longrightarrow {\bullet}CH_3 + H\text{Br} \\ \\ {\bullet}CH_3 + \text{Br}_2 &\longrightarrow CH_3\text{Br} + \text{Br}{\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\text{Br}{\bullet} &\longrightarrow \text{Br}_2 \\ \\ {\bullet}CH_3 + \text{Br}{\bullet} &\longrightarrow CH_3\text{Br} \\ \\ {\bullet}CH_3 + {\bullet}CH_3 &\longrightarrow CH_3CH_3 \end{aligned}