What is Free Radical Substitution?

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Notice that one of the hydrogen atoms in the methane has been replaced by a chlorine atom.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

Free radical reactions

Free radicals are atoms or groups of atoms which have a single unpaired electron. A free radical substitution reaction is one involving these radicals. Free radicals are formed if a bond splits evenly - each atom getting one of the two electrons. The name given to this is homolytic fission.

To show that a species (either an atom or a group of atoms) is a free radical, the symbol is written with a dot attached to show the unpaired electron. For example:

a chlorine radical	Cl
a methyl radical	CH₃

Contributors

Jim Clark (Chemguide.co.uk)