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6.3.3Additional lessons

Halogenation of Alkanes — IB Chemistry SL Revision Notes & Topical Questions

5 min read 6 parts Lesson 96 of 98

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Part 1 of 6Why Alkanes Need Radicals

6.3.3 Halogenation of Alkanes

The reaction proceeds as a chain reaction in three stages.

Unreactive
  • Alkanes have strong, non-polar C–C and C–H bonds, so they do not react with polar reagents.
Reaction type
  • Alkanes react with halogens by free-radical substitution when UV light is present.
Substitution
  • A hydrogen atom is replaced by a halogen atom and a hydrogen halide is formed.
Overall equation
  • For methane and chlorineCH₄(g) + Cl₂(g) → CH₃Cl(g) + HCl(g).

Part 2 of 6The Mechanism for Methane and Chlorine

1
Initiation: UV light breaks the Cl–Cl bond homolytically, Cl₂ → 2Cl•.
2
Propagation 1: a chlorine radical removes a hydrogen atom, CH₄ + Cl• → •CH₃ + HCl.
3
Propagation 2: the methyl radical attacks a chlorine molecule, •CH₃ + Cl₂ → CH₃Cl + Cl•, regenerating Cl•.
4
Termination: two radicals combine to end the chain, for example Cl• + Cl• → Cl₂.
5
Other terminations include •CH₃ + Cl• → CH₃Cl and •CH₃ + •CH₃ → C₂H₆.

Part 3 of 6Further Substitution and Products

Further Substitution and Products
Multiple substitution
  • CH₃Cl can react further to give CH₂Cl₂, CHCl₃ and CCl₄.
Mixture of products
  • A mixture is formed, which must be separated by distillation.
Excess alkane
  • A large excess of alkane makes monosubstitution the most likely outcome.
By-products
  • Termination between two alkyl radicals gives longer alkanes, such as C₂H₆.
Reactivity
  • Bromine reacts more slowly than chlorine under the same conditions with UV light.

Part 4 of 6Evaluating the Reaction

Evaluating the Reaction
Selectivity
  • Free-radical substitution is not selective, so the hydrogen atoms are replaced at random.
Safety
  • Chlorine and bromine are toxic, so reactions are done in a fume cupboard.
Conditions
  • UV light or sunlight and room temperature are needed; heating alone is not used in school work.
Use of products
  • Chloroalkanes are used as solvents and intermediates, but CFCs cause ozone depletion.

Part 5 of 6Worked Example

Question Write the initiation, propagation and one termination step for the reaction of ethane with bromine in UV light.

  1. InitiationBr₂ → 2Br• (homolytic fission by UV light).
  2. PropagationC₂H₆ + Br• → •C₂H₅ + HBr, then •C₂H₅ + Br₂ → C₂H₅Br + Br•.
  3. Terminationfor example •C₂H₅ + •C₂H₅ → C₄H₁₀.

Answer Overall: C₂H₆ + Br₂ → C₂H₅Br + HBr.

Part 6 of 6Exam Checklist & Tips

6.3.3 – Key Points for the Exam

  • Free-radical substitution A reaction where a radical chain mechanism replaces an atom in a molecule.
  • Initiation The first step, forming radicals from a non-radical.
  • Propagation Steps where a radical reacts and another radical is formed.
  • Termination Steps in which two radicals combine and the chain stops.

Common Mistakes

  • Leaving out the dots on radicals in the mechanism.
  • Showing a radical reacting in termination with a molecule, rather than another radical.
  • Writing UV light as a catalyst; it supplies energy.
  • Forgetting that propagation regenerates the chlorine radical.

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