# What is Free Radical Substitution?

### Substitution reactions

These are reactions in which one atom in a molecule is replaced by another atom or group of atoms. Free radical substitution  often involves breaking a carbon-hydrogen bond in alkanes such as

 methane CH4 ethane CH3CH3 propane CH3CH2CH3

A new bond is then formed to something else. It also happens in alkyl groups like methyl, ethyl (and so on) wherever these appear in more complicated molecules.

 methyl CH3 ethyl CH3CH2

For example, ethanoic acid is CH3COOH and contains a methyl group. The carbon-hydrogen bonds in the methyl group behave just like those in methane, and can be broken and replaced by something else in the same way. A simple example of substitution is the reaction between methane and chlorine in the presence of UV light (or sunlight).

$CH_4 + Cl_2\rightarrow CH_3Cl + HCl$

Notice that one of the hydrogen atoms in the methane has been replaced by a chlorine atom. That's substitution.