With the help of mechamism explain free radical halogenationis of allkanes.

Explanation

Methane undergoes halogenation (with `Cl _(2))` to form finally tetrachloro methane This is a substitution reaction and takes place in presence of light.
`CH _(4) + Cl _(2) underset( - HCl ) overset( hu ) to underset(" Chloromethane") ( CH _(3) Cl) overset( Cl _(2) // hv) underset(- HCl) to underset("Dichloromethane") ( CH_(2) Cl _(2)) overset( Cl _(2) // hv) underset( - HCl) to underset( " Trichloromethane") ( CH_(3) Cl _(3)) overset( - HCl ) underset( Cl _(2) //hv) to underset("Tetrachiormomethane") ( C Cl _(4))`
Mechanism :
`to` Halogenation process involves the free radical chain mechanism
`to` This mechanism involes 3 steps i) initation ii)Propagation iii) Termination
(i) Initiation : The reaction initiated by the cleavage of chlorine molexule in presence of light. Cl - Cl bond is weaker than the C-C and C-H bonds
`Cl _(2) underset("Homolysis") overset(hv) to overset(*)(Cl) + overset(*) (Cl)`
Chlorine free radicals
(ii) Propagation : Chlorine free radical attacks the `CH _(4)` molecule and generates methyl free radical.
`Cl _(2) underset("Homolysis") overset(hv) to overset(*)(Cl) + overset(*) (Cl)`
Chlorine free radicals
`CH _(4) + Cl overset( hv) to overset( *) (CH) _(3) + HCl `
Propagation takes place in several steps as follows .
`CH _(3) Cl + overset( *) (Cl ) to overset( *) (C ) H _(2) Cl + HCl`
`overset(*) (C)H_(2) Cl + Cl _(2) to CH _(2) Cl _(2) +overset(*) (C ) l`
(iii) Termination: The reaction terminated after sometime due to consumption of reactants. The following are the possible chain terminating steps
`overset(*)(C)l + overset(*) (C)l to Cl _(2)`
`overset(*) (C) H _(3) + overset(*) (C) H _(3) to CH _(3) - CH _(3)`
`overset(*) (C) H _(3) + overset(*) (C)l to CH _(3) - Cl`