This is the first step of the reaction and involves the production of radicals. There
are many initiators of homolytic fission, the most common of these are azo compounds
and ultra violet light. This works by breaking a relatively weak bond such as that of
chlorine or oxygen in peroxide. The radical produced can then be used to break far more
stable bonds such as the carbon-carbon bond in alkanes. Alkanes are fairly unreactive
due to the non-polar nature of the C-H bonds, so they are not open to attack by ionic
or polar reactants. Any reaction must involve breaking either C-C or C-H bond, both of
which are short strong covalent bonds which require a large endothermic input to be
broken.

To help demonstrate the radical reactions I will use a specific example, this will be
the reaction between chlorine free radicals and methane. The initiation step of this
reaction is the breaking of the chlorine-chlorine bond by irradiation with ultra violet
light. The light energy is enough to break the bond and cause the two atoms to form
their radicals. The chlorine-chlorine bond needs only 240 kj of energy to be broken
compared to over 400 kj needed to break the C-H bond. Chlorine is able to undergo this
type of reaction because it is a coloured compound and so absorbs light energy from
this end of the spectrum easily. This unsymmetrical breaking of the bond is called
HOMOLYTIC FISSION and forms the initial radicals.

The breaking of the chlorine-chlorine bond in this example is a photochemical
process. This type of reaction is very important and plays a key role in many
biological process'. A good example of photochemical reactions in biology is in the
synthesis of carbohydrates from carbon dioxide and water. Chlorophyll absorbs both red
and blue light and uses the subsequent energy to bring about the synthesis.