1. Free Radical Substitution
TYPICAL PROPERTIES
• reactive species (atoms or groups) which possess an unpaired electron
• their reactivity is due to them wanting to pair up the single electron

2. CHLORINATION OF METHANE
All Combined
Initiation Cl2 ——> 2Cl• radicals created
Propagation Cl• CH ——> CH3• HCl radicals used and
Cl CH3• ——> CH3Cl Cl• then re-generated
Termination Cl• + Cl• ——> Cl2 radicals removed
Cl• + CH3• ——> CH3Cl
CH3• + CH3• ——> C2H6

3. CHLORINATION OF METHANE The single dots represent UNPAIRED ELECTRONS
Initiation Cl2 ——> 2Cl• RADICALS CREATED
The single dots represent UNPAIRED ELECTRONS
During initiation, the WEAKEST BOND IS BROKEN as it requires less energy.
There are three possible bonds in a mixture of alkanes and chlorine.
Average bond enthalpy kJ mol-1
The Cl-Cl bond is broken in preference to the others as it is the weakest and requires less energy to separate the atoms.

4. Free Radical Substitution
Moving a single electron, use a half-headed curved arrow, sometimes called a fishhook.
A full headed curved arrow shows the movement of an electron pair.
A reactive intermediate with a single unpaired electron is called a radical.

5. The Cl-Cl bond breaks. One electron goes to each atom
Chain initiation The chain is initiated (started) by UV light breaking a chlorine molecule into free radicals.
unpaired electron
Cl2                 2Cl  
chlorine molecule
chlorine radicals
The Cl-Cl bond breaks. One electron goes to each atom
The curly half-arrows are used to show the movement of one electron.
Cl Cl Cl Cl
This process is known as homolytic fission.

6. CHLORINATION OF METHANE
Propagation Cl• + CH4 ——> CH3• HCl RADICALS USED and
Cl2 + CH3• ——> CH3Cl Cl• then RE-GENERATED
A chlorine radical is produced and the whole process can start over again

7. CH4 + Cl CH3 + HCl CH3 + Cl2 CH3Cl + Cl CH3Cl + Cl CH2Cl + HCl
Chain propagation reactions These are the reactions which keep the chain going.
CH4  +  Cl                  CH3    +  HCl
chlorine radical
methyl radical
CH3    +  Cl2                CH3Cl  +  Cl  
chlorine radical
The new chlorine radical can then react with methane or chloromethane molecules.
CH3Cl    +  Cl                CH2Cl  +  HCl  
CH2Cl    +  Cl2                CH2Cl2  +  Cl  
And so on until CCl4 is formed

8. H3C H + Cl CH3 + H-Cl CH3 + Cl Cl CH3 Cl + Cl Homolytic fission
Chain propagation reactions - using curly arrows
H3C H     Cl                  CH3    +  H-Cl
Homolytic fission
New HCl bond formed from electron from chlorine radical and one of the electrons in the C-H bond
Draw the curly arrows for the next step which is the methyl radical reacting with a chlorine molecule
CH3    +   Cl Cl                CH3 Cl  +  Cl  

9. CHLORINATION OF METHANE
Termination Cl• + Cl• ——> Cl2 RADICALS REMOVED
Cl• + CH3• ——> CH3Cl
CH3• + CH3• ——> C2H6
Removing the reactive free radicals brings an end to the reaction.
This is not very likely at the start of the reaction because of their low concentration.

10. 2Cl Cl2 CH3 + Cl CH3Cl CH3 + CH3 CH3CH3
Chain termination reactions These are reactions which remove free radicals from the system without replacing them by new ones.
2Cl                  Cl2
CH3    +  Cl                  CH3Cl
CH3    +  CH3                  CH3CH3

11. CHLORINATION OF METHANE
OVERVIEW
Initiation Cl2 ——> 2Cl• radicals created
Propagation Cl• CH ——> CH3• HCl radicals used and
Cl CH3• ——> CH3Cl Cl• then re-generated
Termination Cl• + Cl• ——> Cl2 radicals removed
Cl• + CH3• ——> CH3Cl
CH3• + CH3• ——> C2H6
Summary
Due to lack of reactivity, alkanes need a very reactive species to persuade them to react
Free radicals need to be formed by homolytic fission of covalent bonds
You only need one chlorine radical to start things off
With excess chlorine you get further substitution and a mixture of chlorinated products

12. CHLORINATION OF METHANE
Further
propagation If excess chlorine is present, further substitution takes place
The equations show the propagation steps for the formation of...
dichloromethane Cl• + CH3Cl ——> CH2Cl• HCl
Cl2 + CH2Cl• ——> CH2Cl Cl•
trichloromethane Cl• + CH2Cl2 ——> CHCl2• + HCl
Cl CHCl2• ——> CHCl Cl•
tetrachloromethane Cl• + CHCl3 ——> CCl3• + HCl
Cl CCl3• ——> CCl Cl•
Mixtures Because of the many possible reactions there will be a mixture of products.
Individual haloalkanes can be separated by fractional distillation.