Propagation is the second step of the reaction and begins shortly after the
initiation step. Very few free radicals are needed for the propagation step to occur as
once initiated the reaction forms a self sustained cycle or CHAIN REACTION. Any
reaction that involves a free radical reacting with another non radical falls into this
category and help to keep the reaction going by producing a continued stream of free
radicals. Although there are a large number of possible reactions that can occur,
often only the favoured reactions are noted in the end equation. In the case of
chlorine with methane, the reaction that produces HCl is favoured as a H-Cl bond is
more exothermic than that of C-Cl and so has an increased tendency to occur.

The possible propagation steps that may occour are listed below:

The above reactions show the two main products that are formed, there are however
other side products that can be formed such as: Dichloroethane (CH₂Cl₂),
tetrachloroethane (CCl₄) and trichloroethane (CHCl₃) better known as chloroform. As the
carbon chain of the initial alkane used increases the number of products that can
possibly be formed during the reaction increases and a more diverse product range is
created. In some cases, the above examples must react with one of the products of the
over-all equation in order to form some of the less common products. This type of
reaction dipleats the final concentration of products and known as a retardation step.

Although in the examples I have given only one chain carrier, the intermediate
responsible for continuing the chain reaction, is produced by each propagation step, it
is possible to have a branching step also associated with propagation. Branching is the
production of more than one chain carrier through propagation. An example of this is
given below but can be looked at in more detail under the section "Explosions".