
Such reactions begin with the formation of a free radical which substitutes a group or atom
present in the reactant molecule. The reaction of methane with chlorine in the presence of light is
a free radical substitution reaction.
CH4 + Cl2 CH3Cl + HCl
The first step is initiation i.e free radical formation by homolytic cleavage of bonds. Second step
is chain propagation and finally chain termination step
Initiation
Initiation step may happen spontaneously or may be induced by heat or light depending on the
type of bond.
Cl2 2Cl
Chain Propagation
In this step, a molecule reacts with a free radical to generate a new radical.
CH4 + Cl CH3 + HCl
CH3 + Cl2 CH3Cl + Cl
Chain termination
This step is also known as destruction of free radicals. This step involves a combination of two
like or unlike radicals to form a new bond.
2Cl Cl2
CH3 +
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