Free Radical Substitution Mechanism
- Alkanes can undergo free-radical substitution in which a hydrogen atom gets substituted by a halogen (chlorine/bromine)
- Ultraviolet light (sunlight) is needed for this substitution reaction to occur
Reacting an alkane with bromine
The fact that the bromine colour has disappeared only when mixed with an alkane and placed in sunlight suggests that the ultraviolet light is essential for the free radical substitution reaction to take place
- The free-radical substitution reaction consists of three steps:
Initiation step
- In the initiation step, the Cl-Cl or Br-Br is broken by energy from the UV light
- This produces two radicals in a homolytic fission reaction
Free radical initiation
The first step of the free-radical substitution reaction is the initiation step in which two free radicals are formed by sunlight
Propagation step
- The propagation step refers to the progression (growing) of the substitution reaction in a chain type reaction
- Free radicals are very reactive and will attack the unreactive alkanes
- A C-H bond breaks homolytically (each atom gets an electron from the covalent bond)
- An alkyl free radical is produced
- This can attack another chlorine/bromine molecule to form the halogenoalkane and regenerate the chlorine/bromine free radical
- This free radical can then repeat the cycle
Free radical propagation
The second step of the free-radical substitution reaction is the propagation step in which the reaction grows in a chain type reaction
-
- This reaction is not very suitable for preparing specific halogenoalkanes as a mixture of substitution products are formed
- If there is enough chlorine/bromine present, all the hydrogens in the alkane will eventually get substituted (eg. ethane will become C2Cl6/C2Br6)
Free radical further substitution
The free-radical substitution reaction gives a variety of products and not a pure halogenoalkane
Termination step
- The termination step is when the chain reaction terminates (stops) due to two free radicals reacting together and forming a single unreactive molecule
- Multiple products are possible
Free radical termination 
The final step in the substitution reaction is to form a single unreactive molecule
Examiner Tip
You could be asked to draw the mechanism for initiation and termination steps for free radical substitution
This mechanism will use half-headed arrows to show the movement of one electron (double-headed arrows show the movement of a pair of electrons)
A half-headed arrow is known as a ‘fish hook’ arrow.
Initiation:
Termination:
The key is the use of the ‘fish hook’ arrow to show the homolytic fission of the bond in initiation and the formation of the bond in termination.
