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LearnYCH11WCH11 · Unit 1: Structure, Bonding and Introduction to Organic ChemistryIntroductory organic chemistry and alkanesAlkanes and free radicals
Introductory organic chemistry and alkanes · Subtopic 2 of 2

Alkanes and Free Radicals

Alkanes are saturated hydrocarbons, fairly unreactive because C–C and C–H bonds are strong and non-polar. They're used as fuels: complete combustion gives CO₂ and H₂O; incomplete gives CO and soot. Cracking breaks long chains into shorter alkanes and alkenes (thermal: high T and p; catalytic: zeolite, lower T). With Cl₂ or Br₂ in UV light, alkanes undergo free-radical substitution: initiation (Cl2→2Cl∙\mathrm{Cl_2\rightarrow2Cl^\bullet}), propagation (Cl∙+CH4→HCl+∙CH3\mathrm{Cl^\bullet+CH_4\rightarrow HCl+{}^\bullet CH_3}; ∙CH3+Cl2→CH3Cl+Cl∙\mathrm{{}^\bullet CH_3+Cl_2\rightarrow CH_3Cl+Cl^\bullet}), termination (two radicals join). The mixture of products is a disadvantage. Cl• radicals from CFCs destroy ozone catalytically.

Cambridge 9701 Paper 1Cambridge 9701 Paper 2Edexcel YCH11 WCH11AQA 7405 Paper 2≈ 45 minRevision 1 · updated 2026-10-30
Section 1 of 14

Learning objectives

  • Explain the unreactivity of alkanes.
  • Describe combustion, cracking and pollution.
  • Describe the free-radical substitution mechanism.
  • Explain ozone depletion by chlorine radicals.
Pause and think

A mixture of methane and chlorine is stable in the dark but reacts explosively in sunlight. Why?

Section 2 of 14

Core idea

Start hereA chain reaction

One photon of UV light can trigger thousands of reactions between methane and chlorine. Free radicals keep the chain going.

The big idea

Alkanes are unreactive except in combustion and radical reactions. Free-radical substitution runs through initiation, propagation and termination.

1. The radical chain

Free-radical substitution

Propagation repeats until radicals are removed.

Worked exampleFurther substitution

Write propagation steps forming CH₂Cl₂ from CH₃Cl.

Goal
Two equations.
Strategy
Same pattern, different alkane.
  1. Cl∙+CH3Cl→HCl+∙CH2Cl\mathrm{Cl^\bullet+CH_3Cl\rightarrow HCl+{}^\bullet CH_2Cl}
    Abstract an H.
  2. ∙CH2Cl+Cl2→CH2Cl2+Cl∙\mathrm{{}^\bullet CH_2Cl+Cl_2\rightarrow CH_2Cl_2+Cl^\bullet}
    Regenerate Cl•.
AnswerDichloromethane forms.
✓ CheckOne radical in, one out in each step ✓.
Key takeawayOzone depletion Cl• + O₃ → ClO• + O₂; ClO• + O₃ → Cl• + 2O₂. Net: 2O₃ → 3O₂, with Cl• as catalyst.
Pause and think

Why does excess methane favour chloromethane over more substituted products?

Summary

  • Combustion and cracking.
  • Initiation, propagation, termination.
  • CFCs and ozone.
Key words
Free radical
Species with an unpaired electron.
Homolytic fission
Bond breaking where each atom takes one electron.
Cracking
Breaking large alkanes into smaller alkanes and alkenes.
HistoryThe ozone hole. In 1985 British Antarctic Survey scientists discovered the ozone hole. The 1987 Montreal Protocol banned CFCs, and the ozone layer is now slowly recovering.
Fun factFiredamp. Methane in coal mines ("firedamp") caused terrible explosions; Humphry Davy's 1815 safety lamp stopped the flame igniting the gas.
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