Write the free-radical substitution mechanism
Track initiation, propagation and termination in ethane halogenation with single-electron movement.
AS Level; Cambridge International Chemistry 9701, 2025–2027 syllabus. Apply the stated reagents and conditions, balance equations, and keep mechanistic claims within the named examples. Written practice is self-checked
Before you begin
- Use displayed and structural formulas, covalent bond polarity, balanced equations, and the organic reaction terms from Topic 13.
Work on paper. Try each question before revealing help, and explain why your method works.
Quick readiness check
What is formed when Cl₂ undergoes homolytic fission?
What you will learn
- Write homolytic initiation
- Identify both propagation steps
- Recognise radical termination
Initiation makes radicals
Ultraviolet light causes homolytic fission of a halogen molecule: Cl₂ → 2Cl•. Each atom receives one electron from the shared pair and becomes a radical.
Single-headed (fish-hook) arrows show movement of one electron. Do not use a full curly arrow for one-electron radical movement.
Propagation carries the chain
For ethane chlorination, Cl• + C₂H₆ → HCl + C₂H₅•, then C₂H₅• + Cl₂ → C₂H₅Cl + Cl•. Each propagation step consumes one radical and produces another, allowing the chain to continue.
Termination occurs when two radicals combine, for example Cl• + Cl• → Cl₂ or C₂H₅• + Cl• → C₂H₅Cl. A termination step removes radical carriers rather than regenerating one.
Pause and explain
What makes a step a propagation step?
Worked example
Classify each step in ethane chlorination: Cl₂ → 2Cl•; Cl• + C₂H₆ → HCl + C₂H₅•; C₂H₅• + Cl₂ → C₂H₅Cl + Cl•; Cl• + Cl• → Cl₂.
Show the worked solution
- Homolytic halogen splitting forms radicals: initiation.
- A radical reacts and another radical forms: propagation.
- The second chain step regenerates Cl•: propagation.
- Two radicals combine with none left: termination.
Answer The steps are initiation, propagation, propagation and termination.
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 the two propagation equations for ethane chlorination.
Give me a hint
First abstract H; then transfer the radical to chlorine.
Compare my reasoning
- Cl• removes H from C₂H₆, forming HCl and C₂H₅•.
- C₂H₅• reacts with Cl₂.
- The products are C₂H₅Cl and Cl•.
Cl• + C₂H₆ → HCl + C₂H₅•; C₂H₅• + Cl₂ → C₂H₅Cl + Cl•.
Look for these in your work
- First equation balanced
- Second equation balanced
- Chain radical regenerated
Classify C₂H₅• + C₂H₅• → C₄H₁₀.
Give me a hint
Count radical species before and after the step.
Compare my reasoning
- Two ethyl radicals react.
- The product has no unpaired electron.
- No radical remains, so this is termination.
Termination: two ethyl radicals combine to form butane.
Look for these in your work
- Radicals counted
- Product described
- Stage classified
A mechanism uses a full curly arrow from Cl–Cl and labels it one-electron fission. Correct the notation.
Give me a hint
Match arrow type to the number of electrons moving.
Compare my reasoning
- Homolysis gives one electron to each chlorine atom.
- Each movement is a single electron.
- Use two single-headed arrows, one from the bond toward each atom.
Use two single-headed (fish-hook) arrows to show the two bonding electrons separating one to each Cl atom.
Look for these in your work
- Electron split described
- Correct arrow type named
- Both directions shown
Common mistakes
- Calling the first radical-producing step propagation
- Calling a step termination when a radical is regenerated
- Using two-electron curly arrows for single-electron movement
What can you explain now?
What is formed when Cl₂ undergoes homolytic fission?
Compare with the explanation
Two chlorine radicals
Each chlorine atom takes one electron from the shared pair.
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
How can you tell whether a radical-chain step is propagation or termination?
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.