1. The Formation of Halogenoalkanes
Targets
To define what a radical is
To describe the reaction mechanism for free radical substitution of methane
RSC mechanisms resource:
RSC resource of misconceptions about mechanisms:
Mechanism animations
Interactive mechanisms

2. What is a halogen?
RSC mechanisms resource:
RSC resource of misconceptions about mechanisms:
Mechanism animations
Interactive mechanisms

3. ALKANES WITH HALOGENS, Cl2 or Br2
Initiated by energy from UV light
 successive substitution (replacement) of H atoms in the alkane by Cl / Br atoms.
 mixture of chloro- / bromoalkanes

4. HALOGEN-METHANE SUBSTITUTION : MECHANISM
= a FREE RADICAL SUBSTITUTION mechanism
Also a CHAIN REACTION
 a product from one stage of process is used in the next stage.
THREE stages :
1. INITIATION 
(Start)
(Products Grow )
(Finish)
2. PROPOGATION 
3. TERMINATION
CH4(g) + Cl2 (g)  CH3Cl(g) + HCl (g)
C6H14(g) + Br2 (l)  C6H13Br(l) + HBr (g)

5. Cl  + Cl Cl UV energy 1. INITIATION
Caused by energy input from UV light
 covalent bond in a few Cl2 molecules broken
 HOMOLYSIS
- one e- from bond pair to each atom
= FREE RADICALS
The dot is used to show the unpaired electron.
Free radicals are highly reactive. Cl  Cl + Cl
UV energy

6. C H C H Cl H Cl Cl• + CH4  HCl + •CH3 2. PROPOGATION STEPS
(a) Cl. free radical from initiation step collides with a molecule of CH4 and removes a H atom. C H C H Cl + H Cl +
 first substitution product (HCl)
+ a methyl free radical (H3C.).
Cl• + CH4  HCl + •CH3

7. C H C H Cl Cl Cl •CH3 + Cl2  CH3Cl + Cl•
(b) CH3. free radical from step (a) collides with a NEW molecule of Cl2 and removes a Cl atom. C H C H Cl + Cl Cl +
 second substitution product (CH3Cl)
+ a NEW chlorine free radical (Cl.).
•CH3 + Cl2  CH3Cl + Cl•

8. Propagation Step Summary
The two steps produce hydrogen chloride, chloromethane and a new Cl• free radical.
This is ready to react with more methane and repeat the two steps.
This is the chain part of the chain reaction.
The steps may take place thousands of times before the radicals are destroyed.

9. Free Radicals in Mixture Possible termination products
REMOVING THE FREE RADICALS!
3. TERMINATION STEPS
 propagation steps stopped by consumption of free radicals
 combine to form neutral molecules
Free Radicals in Mixture
Possible termination products
CH3Cl
Cl2
CH3CH3
CH3.
Cl.

10. Termination
Every case two free radicals react to form a stable compound with no unpaired electrons!
Cl• + Cl•  Cl2
•CH3 + •CH3  C2H6
Cl• + •CH3  CH3Cl

11. Abbreviated mechanism :
Initiation
Propogation 1
Propogation 2
Termination 1
Termination 2
Termination 3
By energy from uv light : Cl2  2Cl.
CH4 + Cl.  .CH3 + HCl
.CH3 + Cl2  CH3Cl + Cl.
.CH3 + Cl.  CH3Cl
.CH3 + .CH3  CH3CH3
.Cl + Cl.  Cl2
Write a similar mechanism for CH3Br  CH2Br2
Initiation
Propogation 1
Propogation 2
Termination 1
Termination 2
Termination 3
By energy from uv light : Br2  2Br.
CH3Br + Br.  .CH2Br + HBr
.CH2Br + Br2  CH2Br2 + Br.
.CH2Br + Br.  CH2Br2
.CH2Br + .CH2Br  CH2BrCH2Br
. Br + Br.  Br2

12. Other products of the chain reaction
Further reactions can occur to produce
di, tri- and tetra-haloalkanes
E.g. dichloromethane may be made at the propagation stage, if chlorine radical reacts with some chloromethane that has already formed:
CH3Cl + Cl•  •CH2Cl + HCl followed by •CH2Cl + Cl2  CH2Cl2 + Cl•

13. Other products of the chain reaction
Longer-chain alkanes will produced many isomers because the Cl• can replace any of the hydrogen atoms.
Chain reactions are not very useful because they produce such a mixture of products.
Chain reactions will occur without light at high temperatures.

14.  mono-, di-, tri- and tetrachloromethanes
ALKANES WITH HALOGENS, Cl2 or Br2  
eg Methane + Cl2
 mono-, di-, tri- and tetrachloromethanes
Excess CH4
 CH3Cl MORE likely
Excess Cl2
 CCl4 MORE likely

15. C H C H Cl U.V. + Cl2 + HCl + Cl2 C Cl C Cl H + HCl + HCl + Cl2 + Cl2
chloromethane
+ Cl2 C Cl C Cl H
+ HCl
+ HCl
tetrachloromethane
dichloromethane
+ Cl2
+ Cl2 C Cl H
+ HCl
Similarly for Br2
trichloromethane