Termination
A concise revision guide to the termination stage of free-radical substitution, including radical combination, chain reaction stopping, bromination of propane and common termination equations.
- 6B.17-b
- 6B.16ii-d
What these spec points say
- 6B.17-b understand limitations of radical substitution in synthesis due to formation of a mixture of products
- 6B.16ii-d understand the termination steps of the radical substitution mechanism
Radical Chain Reaction: Methane and Chlorine
Choose a step to follow every electron: UV light makes the radicals, the chain repeats, and two radicals meeting end it.
© Dr. Mohammed Al-Fatah – onlinelearningsystem.net
What Happens During Termination?
Termination is the stage of free-radical substitution where two free radicals collide and react together. This removes free radicals from the reaction mixture.
Unlike propagation, termination does not regenerate another free radical. This means the chain reaction is stopped when radicals are used up in these combination steps.
Key idea: In termination, radicals are removed from the system. A stable molecule is formed and no new free radical is produced.
Watch: two radicals about to meet
How to tell initiation, propagation and termination apart at a glance, and what happens when two radicals collide.
Before You Start
This page builds on initiation and propagation in free-radical substitution.
- A free radical has an unpaired electron, shown by a dot.
- A covalent bond forms when two atoms share a pair of electrons.
- Propagation uses one radical and makes another, so the chain carries on.
- Methane, CH4, and ethane, C2H6, are the first two alkanes.
GCSE Recap: Making Bonds and Colliding Particles
Check the GCSE ideas about making bonds and collisions that termination builds on.
Radical Combination
Termination happens when two radical species combine using their unpaired electrons to form a new covalent bond.
The product is usually a stable molecule. Because no radical remains in the product, that pathway cannot continue the chain reaction.
Termination step: A reaction in which two free radicals combine to form a stable molecule, removing radicals and stopping that part of the chain reaction.
Exam focus: A termination equation should have radicals on the reactant side, but no radical species on the product side.
Quick Check: Ending the Chain for Propane
Complete the passage about the termination steps for propane and chlorine.
Termination in Chlorination of Methane
In the chlorination of methane, several different radicals may be present in the reaction mixture. These include chlorine radicals and methyl radicals.
Any two radicals can collide and combine. The possible termination steps include formation of chloromethane, ethane or chlorine.
•CH3 + Cl• → CH3Cl
•CH3 + •CH3 → CH3CH3
Cl• + Cl• → Cl2
Why ethane can form: If two methyl radicals combine, the product is ethane. This is a minor termination product and can appear as an impurity.
Any two radicals can meet, so termination gives Cl₂, CH₃Cl or ethane.
Quick Check: Where Did the Butane Come From?
Decide what an unexpected product in the mixture tells you.
Example: Bromination of Propane
In bromination of propane, a radical can form on the middle carbon of propane. This gives the secondary propyl radical, written as CH3CH•CH3.
If this radical combines directly with a bromine radical, the product is 2-bromopropane. No new free radical is regenerated, so this is a termination step.
CH3CH•CH3 + Br• → CH3CHBrCH3
Common mistake: Do not confuse this with propagation. In propagation, Br• is regenerated. In this termination step, Br• is consumed and no radical appears in the product.
Propagation Compared with Termination
The easiest way to identify termination is to compare the radicals in the reactants and products. Propagation keeps radicals going. Termination removes them.
| Feature | Propagation | Termination |
|---|---|---|
| Radicals in reactants | At least one radical is present. | Two radicals are present. |
| Radicals in products | A new radical is formed. | No new radical is formed. |
| Effect on chain reaction | The chain reaction continues. | The chain reaction is stopped. |
| Typical example | •CH3 + Cl2 → CH3Cl + Cl• | •CH3 + •CH3 → CH3CH3 |
Counting radicals on each side names the stage: 0 → 2, 1 → 1 or 2 → 0.
Quick Check: Find the Termination Steps
Click every step that is a termination step.
How to Recognise Termination in Exam Questions
When writing or identifying a termination step, check whether two radical reactants combine to make a non-radical product.
The dot represents an unpaired electron. During termination, the two unpaired electrons form a covalent bond, so the dot disappears in the product.
| Question clue | What it means | Correct response |
|---|---|---|
| Two radicals collide | The reaction is likely to be termination. | Write a stable product with no radical dot. |
| No radical regenerated | The chain is being stopped rather than carried forward. | Describe the step as termination. |
| Ethane forms from methyl radicals | Two CH3 radicals have combined. | •CH3 + •CH3 → CH3CH3 |
Remember: Termination reduces the number of radicals in the mixture. This is why it eventually stops a free-radical chain reaction.
Quick Check: Write the Termination Step
Write each step on paper, then flip the card to check.
Quick Check: Pick the Accurate Statement
Finish the page by choosing the accurate statement about termination in each round.
Termination FAQs
Review the key points about the final stage of a free-radical substitution mechanism.
What is termination in free-radical substitution?
Termination is the stage where two free radicals collide and combine to form a stable molecule. No new radical is produced, so the chain reaction is stopped.
How can you identify a termination step?
A termination step has two radical species on the reactant side and no radical species on the product side.
Why does ethane form as an impurity in chlorination of methane?
Ethane can form when two methyl radicals combine: •CH3 + •CH3 → CH3CH3. This is a minor termination pathway.
What is the termination equation for making 2-bromopropane from a secondary propyl radical?
The equation is CH3CH•CH3 + Br• → CH3CHBrCH3. No radical is regenerated.
What is the difference between propagation and termination?
Propagation forms another radical and continues the chain reaction. Termination consumes radicals and stops the chain reaction.
Copyright notice: This OLS revision content, including the explanations, layout, diagrams, tables and embedded learning structure, is authored for Online Learning System by Dr. Mohammed Al-Fatah. It may not be copied, reproduced, redistributed or adapted without written permission.
Keep this note — free
Save your progress across every Edexcel topic. A free account remembers which topics you have covered, saves your question scores, and syncs across your phone and laptop.
- Track every topic you have finished
- Keep your practice-question scores
- No payment, no card, free forever
Already registered? Log in