Explain the mechanism of halogenations of methane.

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) overset( hv) underset(-HCl)to underset("chloromethane") (CH _(3) Cl )overset( Cl _(2) //hv) underset(- HCl)tounderset("Dichloromethane") (CH_(2) Cl _(2)) underset(-HCl ) overset(Cl_(2) //hv)to underset("Trichloromethane") (CH_(3) Cl _(3)) underset(Cl _(2)//hv) overset(-HCl) tounderset("Tetrachloromethane ") (C Cl _(4))`
`to` Halogenation process involves the free radical chain mechanism
`to ` This machanism incvolves 3 steps i) Initation ii) Propagation iii) Termination
i) Initiation : The reaction initiated by the cleavage of chlorine molecule in presence of light.
Cl -Cl bond is weaker than the C-C and C-H bonds
`Cl _(2) underset( "Homolysis")overset(hv) to ounderset("Chlorine free radicals")(overset(*)Cl+ overset(*) (Cl))`
Propagation : Chlorine free radical attacks the `CH _(4)` molecule and generates methyl free radical.
`Cl _(2) underset( "Homolysis")overset(hv) to ounderset("Chlorine free radicals")(overset(*)Cl+ overset(*) (Cl))`
The methly reaical formed atacks the `2 ^(nd) ` molecule of chlorine to form another chlorine free radical.
`overset(*) (CH_(3)) + Cl _(2) overset(hv)to CH _(3) - Cl + Cl^(+)`
Propagation takes place in several steps as follows.
`CH _(3) Cl +overset(*) (Cl) to overset(*) (CH_(2)) + HCl `
`overset(*) (C) H_(2) Cl + Cl _(2) to CH _(2) Cl _(2) + Cl`
Termination: The reacton terminated after sometime due to consumption of reactants. The following are the possible chain teminating steps
`overset(*) (Cl) + overset(*) (Cl) to Cl_(2)`
`overset(*) (CH_(3)) + overset(*) (CH _(3)) to CH_(3) - CH _(3)`
`overset(*) (CH_(3)) + overset(*) (Cl) to CH_(3) - Cl`