- Alkanes take part in radical substitution with Halogens
- eg. the reaction between methane and Bromine (UV radiation)
- CH4 + Br2 ----------> CH3Br + HBr
- Initiation: UV radiation breaks the covalent bond between the bromine molecules Br--Br, this is homolytic fission. Radicals are formed.
- Br2--------> 2Br (radicals)
- Propagation: The products of the reaction are made.
- Br (radical) + CH4-------->CH3 (radical) + HBr
- CH3 (radical) + Br2 ----------> Ch3Br + Br (radical)
In propagation, a Bromine radical is used in the first step and then a Bromine radical is regenerated in the second step. Bromine radicals catalyse the propagation stage. They are not used up in the overall reaction.
Termination: Any two radicals can combine.
CH3 (radical) + Br (radical) -------> CH3Br
Br (radical) + Br (radical) -------> Br2
CH3 (radical) + CH3 (radical) -------> C2H6
Further substitution reactions can take place as the bromoalkane product can react further with bromine radicals. A mixture of products is processed.