Substitution reactions

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


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.


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).

CH4  +  Cl2CH3Cl  +  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Notice that one of the hydrogen atoms in the methane has been replaced by a chlorine atom.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.

Note:  If a bond were to split unevenly (one atom getting both electrons, and the other none), ions would be formed. The atom that got both electrons would become negatively charged, while the other one would become positive. This is called heterolytic fission.

Warning!  It is important that you get these terms the right way round. "Fission" is obvious - it just means "splitting". "Homo" and "hetero" are used in the sense of "same" (homo) or "different" (hetero). This is just like their use in "homosexual" or "heterosexual".

So, homolytic fission is splitting a bond to produce two particles which are the same in the sense that they both have a single unpaired electron (both are free radicals). Heterolytic fission produces two particles which are different because one is a positive ion and the other a negative ion.

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 radicalCl
a methyl radicalCH3

Note:  If you wanted to be fussy, the dot showing the electron really ought to be written next to the carbon atom in the methyl radical, because that's the atom with the unpaired electron - in other words as CH3. This isn't normally done unless the radical gets more complicated. Examples of this will crop up when you look at the mechanisms.

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© Jim Clark 2000