Interpret free-radical substitution mechanisms
Use one-electron arrows and the three chain stages to interpret alkane halogenation mechanisms.
AS Level; Cambridge International Chemistry 9701, 2025–2027 syllabus. Use the published functional-group scope and nomenclature limits; draw formula and mechanism evidence with correct connectivity, electron movement and stereochemistry. Written practice is self-checked
Before you begin
- Use covalent structures, valency, molecular formulas, lone pairs, and single, double and triple bonds.
Work on paper. Try each question before revealing help, and explain why your method works.
Quick readiness check
What initiates chlorine radical formation in the model?
What you will learn
- Recognise radical substitution
- Write initiation, propagation and termination steps
- Track single-electron movement and radical regeneration
Substitution replaces a hydrogen atom
In free-radical substitution of an alkane, a halogen atom replaces a hydrogen. For methane plus chlorine, chloromethane and hydrogen chloride can form under ultraviolet light.
The reaction proceeds by a chain mechanism: initiation creates radicals, propagation steps consume and regenerate radicals, and termination removes radicals by combination.
Mechanism arrows show one electron at a time
In initiation, UV light causes homolytic fission of Cl₂. During propagation, a chlorine radical abstracts H from methane; the methyl radical then reacts with Cl₂ to make CH₃Cl and regenerate Cl•.
Single-headed arrows show movement of one electron. Write each stage and conserve atoms; do not use a full two-electron curly arrow for a radical step.
Pause and explain
Which propagation step regenerates Cl•?
Worked example
Give a propagation sequence for substitution of methane by chlorine.
Show the worked solution
- Cl• abstracts H from CH₄ to form HCl and CH₃•.
- CH₃• reacts with Cl₂ to form CH₃Cl and Cl•.
- Cl• is regenerated, allowing the chain to continue.
Answer Cl• + CH₄ → HCl + CH₃•; CH₃• + Cl₂ → CH₃Cl + Cl•.
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Test this topic →From guided practice to a new situation
Write your answer and reasoning first. Use a hint only if you are stuck. The model solution is for self-checking; your written work is not automatically marked.
Write the first propagation step for chlorination of methane.
Give me a hint
A chlorine radical abstracts hydrogen from methane.
Compare my reasoning
- Start with Cl• and CH₄.
- Form H–Cl and CH₃•.
- Check the radical has moved to the methyl group.
Cl• + CH₄ → HCl + CH₃•.
Look for these in your work
- Reactants correct
- Products correct
- Radical tracked
Explain why CH₃• + Cl₂ → CH₃Cl + Cl• is propagation.
Give me a hint
Check that a chain-carrying radical appears again.
Compare my reasoning
- The methyl radical reacts with Cl₂.
- A chlorine radical forms in the products.
- A radical is regenerated, so propagation continues.
It is propagation because one radical is consumed and another is produced.
Look for these in your work
- Radical reactant
- Radical product
- Stage justified
A student shows Cl• + Cl• → Cl₂ as propagation. Correct the classification.
Give me a hint
Determine whether any radical remains after the step.
Compare my reasoning
- Two radicals combine.
- The product Cl₂ has no unpaired electron.
- No radical is regenerated, so this is termination.
It is termination, not propagation.
Look for these in your work
- Radicals counted
- Product examined
- Stage corrected
Common mistakes
- Calling substitution an addition because chlorine is added to the system
- Omitting the radical dot
- Using two-electron arrows in a radical mechanism
What can you explain now?
What initiates chlorine radical formation in the model?
Compare with the explanation
Ultraviolet light causes Cl₂ homolysis
UV light supplies the energy for homolytic cleavage.
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Make it stick
What structural change makes free-radical substitution a substitution reaction?
If you needed the solution, close it and solve the problem again from a blank page. A correct answer is stronger when you can explain the reason for each step.