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Notice that one of the hydrogen atoms in the methane has been replaced by a chlorine atom.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