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3.2.4Organic Chemistry

Chlorination of Alkanes — Oxford AQA International A Level Chemistry Revision Notes & Topical Questions

5 min read 5 parts Lesson 10 of 55

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Part 1 of 5Conditions and Overall Reaction

3.2.4 Chlorination of Alkanes
Conditions and Overall Reaction
Reagents
  • An alkane reacts with chlorine in the presence of UV light, replacing hydrogen atoms with chlorine atoms.
Overall equation
  • CH₄ + Cl₂ → CH₃Cl + HCl is the first substitution, giving chloromethane.
Why UV
  • UV light provides the energy to break the Cl–Cl bond, which is the weakest bond present.
Reaction type
  • It is a free radical substitution, because radicals are involved and a hydrogen is replaced.

Part 2 of 5The Mechanism

1
Initiation: UV light splits Cl₂ homolytically into two chlorine radicals, Cl₂ → 2Cl•.
2
Propagation step 1: a chlorine radical removes a hydrogen atom from methane, CH₄ + Cl• → •CH₃ + HCl.
3
Propagation step 2: the methyl radical reacts with chlorine, •CH₃ + Cl₂ → CH₃Cl + Cl•, regenerating a chlorine radical.
4
Termination: two radicals combine, such as Cl• + Cl• → Cl₂ or •CH₃ + Cl• → CH₃Cl.
5
Another termination step, •CH₃ + •CH₃ → C₂H₆, forms ethane as a by-product and shows how the carbon chain can lengthen.

Part 3 of 5Further Substitution

More chlorination
  • Chloromethane can react again, giving CH₂Cl₂, CHCl₃ and CCl₄ in turn.
Product mixture
  • The reaction cannot be controlled to give one product, so mixtures are formed that must be separated.
Excess alkane
  • Using excess methane makes it more likely that Cl• meets methane than a chlorinated product, favouring monosubstitution.
Larger alkanes
  • Propane has two types of hydrogen, so 1-chloropropane and 2-chloropropane both form.
Bromine
  • Bromine reacts in the same way with UV light, but more slowly than chlorine.

Part 4 of 5Worked Example

Question Write the equation for the formation of dichloromethane from chloromethane and the two propagation steps involved.

  1. OverallCH₃Cl + Cl₂ → CH₂Cl₂ + HCl.
  2. Propagation 1: CH₃Cl + Cl• → •CH₂Cl + HCl.
  3. Propagation 2: •CH₂Cl + Cl₂ → CH₂Cl₂ + Cl•.

Answer CH₃Cl + Cl₂ → CH₂Cl₂ + HCl via •CH₂Cl and Cl• radicals.

Part 5 of 5Exam Checklist & Tips

3.2.4 – Key Points for the Exam

  • Free radical substitution A reaction in which a hydrogen is replaced via a radical chain mechanism.
  • Initiation The first step, in which radicals are made.
  • Propagation A step in which a radical reacts to give another radical.
  • Termination A step in which two radicals combine to end the chain.

Common Mistakes

  • Writing propagation steps that do not regenerate a radical.
  • Using full-headed arrows for single electron movements instead of half-headed arrows.
  • Forgetting to state UV light as the condition.
  • Missing that termination steps use up two radicals and produce none.

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