AQA A Level Chemistry

Revision Notes

3.7.2 Chlorination of Alkanes

Mechanism: Free Radical Substitution

Chain reactions

  • free radical is a species with one (or more than one) unpaired electrons

An Introduction to AS Level Organic Chemistry Free Radical, downloadable AS & A Level Chemistry revision notes

The diagram shows a free radical which has one unpaired electron

  • free radical reaction is a reaction involving free radicals and is a three-step reaction:
    • Initiation is the first step and involves breaking a covalent bond using energy from ultraviolet (UV) light from the sun to form two free radicals
    • The propagation step is the second step in which the free radical can attack reactant molecules to form even more radicals
      • These in turn can again attack other molecules to form more free radicals and so on
    • In the termination step, two free radicals react together to form a product molecule

An Introduction to AS Level Organic Chemistry Radical Chain Reactions, downloadable AS & A Level Chemistry revision notes

The diagram shows the different stages of a general radical chain reaction

  • In an exam, the type of reaction will be a specific alkane and halogen
  • For example, ethane and chlorine

Free Radical Substitution Reaction of Ethane and Chlorine, downloadable AS & A Level Chemistry revision notes

The Free Radical Substitution Reaction of Ethane and Chlorine

Exam Tip

Remember:

Initiation = a molecule forming two radicals

Propagation = always two steps, a molecule and a radical reacting to form a different molecule and radical combination

Termination = two radicals reacting to form a molecule

Read the question carefully – sometimes they specify which termination step you should include.

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