Free Radical Substitution

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  • Created by: Veqz
  • Created on: 11-10-13 19:30

Free Radical Substitution

There are THREE main stages!

1 - Initiation

This is where a molecule, such as a halogen in Group 7 is exposed to UV light and its bonds break. This then forms TWO radicals (a species with unpaired electrons).


Cl  ------------> Cl  ・ + Cl  


Since as both atoms gained one electron each, this is known as 'homolytic fission'.

2 - Propagation

There are always TWO steps in the propagation stage. The chlorine radicals will react with an organic compound, to form more and more radicals...

Let's use methane as our organic compound.


CH₄ + Cl ・ -----> CH₃ ・ + HCl

This is the first step. The chlorine radical reacts with the methane to produce a methyl radical and hydrochloric acid. This is because the chlorine radical breaks one of the hydrogen bonds with the carbon atom, and forms a new bond with the hydrogen to form HCl. This means the hydrogen donates one of its electrons


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