This kind of reactions involve the formation of free radical intermediates. Following are the steps of mechanism :-
Step 1 :- Chain initiation
In this step the bond in halide molecule breaks by a homolytic cleavage . And this leads to the formation of free radicals.
Step 2 :- Chain propagation
In this step the free radicals attack on the alkane and remove the hydrogen. This leads to the formation of carbon free radical.
Step 3 :- Chain termination
In this step the free radicals formed during the reaction are consumed in the various possible combinations as shown below.