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Free Radical Substitution (CIE AS Chemistry)

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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

Hydrocarbons Colour Change of Solution, downloadable AS & A Level Chemistry revision notes

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

Hydrocarbons Initiation Step, downloadable AS & A Level Chemistry revision notes

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

Hydrocarbons Propagation Step, downloadable AS & A Level Chemistry revision notes

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

Hydrocarbons Side-Products, downloadable AS & A Level Chemistry revision notes

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 Hydrocarbons Termination Step, downloadable AS & A Level Chemistry revision notes

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:

Initiation step

Termination:

Termination step

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.  

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Philippa

Author: Philippa

Expertise: Chemistry

Philippa has worked as a GCSE and A level chemistry teacher and tutor for over thirteen years. She studied chemistry and sport science at Loughborough University graduating in 2007 having also completed her PGCE in science. Throughout her time as a teacher she was incharge of a boarding house for five years and coached many teams in a variety of sports. When not producing resources with the chemistry team, Philippa enjoys being active outside with her young family and is a very keen gardener.