The halogenation of alkanes takes place by a mechanism called free radical substitution and can be considered to take place in three steps:
- initiation
- propagation
- termination
Initiation involves the homolytic fission of a Cl-Cl bond by ultraviolet radiation (photodissociation). The Cl radicals formed are very reactive and remove a hydrogen when they collide with an alkane molecule. This gives an alkyl radical which breaks a Cl-Cl bond releasing a Cl radical i.e. a chain reaction. Termination occurs when two radicals react together to form a molecule, removing radicals from the chain.
