1. Reaction Mechanisms: Free Radical Substitution1

2. Monochlorination of methane
CH Cl2 → CH3Cl HCl
methane chloromethane

3. Initiation
Ultraviolet light causes splitting of chlorine molecules into chlorine atoms
Cl - Cl → Cl● + Cl●
Chlorine Chlorine atoms
molecule (free radicals)

4. Propagation CH4 + Cl● = CH3● + HCl
methane Methyl radical Hydrogen chloride
CH3● Cl = CH3Cl Cl ●
Chloromethane
Chlorine atom free to react with another methane molecule

5. Termination
Cl● Cl● = Cl2
CH3● Cl ● = CH3Cl
CH3● + CH3● = C2H6

6. Evidence for this mechanism
Use of ultraviolet light even for a very short period causes a chain reaction
Formation of trace quantities of ethane
Reaction speeded up by sources of free radicals such as tetraethyl lead

7. Monochlorination of ethane
C2H Cl2 → C2H5Cl HCl
ethane chloroethane

8. Initiation
Ultraviolet light causes splitting of chlorine molecules into chlorine atoms
Cl - Cl → Cl● + Cl●
Chlorine Chlorine atoms
molecule

9. Propagation C2H6 + Cl● = C2H5● + HCl
ethane ethyl radical hydrogen chloride
C2H5● Cl = C2H5Cl Cl ●
Chloroethane
Chlorine atom free to react with another ethane molecule

10. Termination Cl● + Cl● = Cl2 C2H5● + Cl ● = C2H5Cl
C2H5● + C2H5● = C4H10

11. Evidence for this mechanism
Use of ultraviolet light even for a very short period causes a chain reaction
Formation of trace quantities of butane
Reaction speeded up by sources of free radicals such as tetraethyl lead