Explain free-radical chain reactions
Identify initiation, propagation and termination in a free-radical substitution mechanism.
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
Which feature defines a free radical?
What you will learn
- Define a free radical
- Classify initiation, propagation and termination
- Represent free-radical substitution as a chain mechanism
A radical has an unpaired electron
A free radical is a species with an unpaired electron, commonly shown with a dot. Homolytic fission can create radicals when a covalent bond splits evenly.
In a chain reaction, initiation forms radicals, propagation uses a radical and forms another radical, and termination combines radicals so no new radical is produced in that step.
Recognise the stages in halogenoalkane formation
For methane chlorination, initiation is Cl₂ → 2Cl• under ultraviolet light. A propagation step is Cl• + CH₄ → HCl + CH₃•; another is CH₃• + Cl₂ → CH₃Cl + Cl•.
A termination step combines radicals, for example Cl• + Cl• → Cl₂. Curly half-arrows show one electron moving; chain propagation regenerates a radical carrier.
Pause and explain
What happens in a termination step?
Worked example
Classify Cl₂ → 2Cl•, Cl• + CH₄ → HCl + CH₃• and Cl• + Cl• → Cl₂.
Show the worked solution
- Bond homolysis forms chlorine radicals: initiation.
- A radical reacts and a new radical forms: propagation.
- Two radicals combine and no radical remains: termination.
Answer The equations are initiation, propagation and termination, respectively.
Try a topic test
Choose Easy, Normal or Difficult. Save your answers and open worked review after finishing.
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 an initiation equation for chlorine radical formation under UV light.
Give me a hint
Use homolytic fission of Cl₂.
Compare my reasoning
- Start with a Cl–Cl bond.
- Split the bonding pair evenly.
- Write two Cl radicals.
Cl₂ → 2Cl•.
Look for these in your work
- Correct reactant
- Homolysis stated
- Two radicals formed
Classify CH₃• + Cl₂ → CH₃Cl + Cl•.
Give me a hint
Check whether a radical is consumed and another is formed.
Compare my reasoning
- A methyl radical reacts with Cl₂.
- A chlorine radical is produced.
- The chain radical is regenerated, so this is propagation.
Propagation.
Look for these in your work
- Reacting radical identified
- New radical identified
- Stage classified
Why does Cl• + CH₃• → CH₃Cl end a chain rather than propagate it?
Give me a hint
Count radicals before and after the step.
Compare my reasoning
- Two radicals react.
- The product is a closed-shell molecule.
- No new radical is formed, so the chain carriers are removed.
It is termination because radicals are consumed and no radical is regenerated.
Look for these in your work
- Radicals counted
- Product type identified
- Chain effect stated
Common mistakes
- Calling every radical step propagation
- Forgetting that a propagation step makes a new radical
- Drawing pair arrows for one-electron radical movement
What can you explain now?
Which feature defines a free radical?
Compare with the explanation
An unpaired electron
The unpaired electron is shown as a dot.
After trying it yourself, choose your next review. This is your self-assessment.
Your review choice appears on Today. Sign in to sync it across devices.
Make it stick
What must a propagation step do to the radical carrier?
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.