Reaction mechanisms

• 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.