A free radical is a molecule which has an extra electron/open shell configuration. It does not satisfy the octet rule and is usually highly reactive.

• Reaction : CH4 + Cl2 --> CH3Cl + HCl
• The reaction involves a chain reaction consisting of 3 steps: initiation, propagation, termination
Step 1: Initiation
Formation of the Cl· free radical via homolytic fission of the Cl-Cl bond:
${\displaystyle Cl-Cl\to Cl\bullet +Cl\bullet }$
Step 2: Propagation
Generation of more free radicals via the chain reaction:
(a) Highly reactive chlorine radicals collide with CH4 molecules and abstract one of its H atoms, forming HCl and a methyl radical
${\displaystyle Cl\bullet +CH_{4}\to CH_{3}\bullet +HCl}$
(b) Methyl radical produced then abstracts a Cl atom from a Cl2 molecule, chloromethane and Cl· radical
${\displaystyle CH_{3}\bullet +Cl_{2}\to CH_{3}Cl+Cl\bullet }$
Then (a), (b), (a), (b)...
The chlorine free radical produced can attack another CH4 molecule and hence the process continues - a chain reaction
Step 3: Termination
Termination of the chain reaction occurs upon the collision of two free radicals with the formation of a single covalent bond:
${\displaystyle 2Cl\bullet \to Cl_{2}}$
${\displaystyle CH_{3}\bullet +Cl\bullet \to CH_{3}Cl}$
${\displaystyle 2CH_{3}\bullet \to CH_{3}CH_{3}}$ (trace amount)

Overall: ${\displaystyle CH_{4}+Cl_{2}\to CH_{3}Cl+HCl}$
• The reaction does not stop at this stage. Further substitution may arise, resulting in a mixture of multi-substituted products: