Propagation
A concise revision guide to the propagation stage of radical substitution, including hydrogen abstraction, alkyl radical formation, haloalkane production and regeneration of the halogen radical.
- 4.1.2(f)-i
- 4.1.2(f)-iii
What these spec points say
- 4.1.2(f)-i the reaction of alkanes with chlorine and bromine by radical substitution using ultraviolet radiation
- 4.1.2(f)-iii the mechanism of radical substitution involving homolytic fission and radical reactions in terms of propagation (see also 4.1.1 f–g)
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
Before You Start
This page builds on initiation, where UV light splits a halogen molecule into two radicals.
- A covalent bond is a shared pair of electrons.
- A radical has an unpaired electron, shown by a dot, as in Cl•.
- Methane, CH4, has four C-H bonds.
- In a substitution reaction one atom is replaced by another.
GCSE Recap: Chlorine Atoms, Hydrogen Halides and Swapping Atoms
Check the GCSE ideas that the propagation steps build on.
What Happens During Propagation?
Propagation is the chain-carrying stage of radical substitution. It uses a radical formed in initiation and produces another radical so the reaction can continue.
In the substitution of an alkane by a halogen, propagation normally occurs in two repeated steps. First, a halogen radical removes a hydrogen atom from the alkane. Second, the alkyl radical formed reacts with a halogen molecule to make the haloalkane product.
Key idea: In propagation, a radical is present in both the reactants and the products. This is why the mechanism continues as a chain reaction.
Count the radicals on each side: 0 to 2 is initiation, 1 to 1 is propagation, 2 to 0 is termination.
Quick Check: Counting the Radicals
Decide whether this step changes the number of radicals in the mixture.
Watch: a radical pulls a hydrogen off methane
One electron at a time, a chlorine radical takes a hydrogen from methane. What is left behind?
First Propagation Step: Hydrogen Abstraction
In the first propagation step, a halogen radical removes a hydrogen atom from the alkane. This forms a hydrogen halide and an alkyl radical.
For chlorination of methane, the chlorine radical removes one hydrogen atom from methane. The products are hydrogen chloride and a methyl radical.
CH4 + Cl• → HCl + •CH3
Exam focus: The dot belongs on the radical species. The methyl radical is written as •CH3 or CH3•, not CH3+ or CH3–.
Quick Check: Find the Fault
Find the fault in a student's first propagation step.
Second Propagation Step: Haloalkane Formation
In the second propagation step, the alkyl radical reacts with a halogen molecule. This forms the main substitution product, a haloalkane, and regenerates another halogen radical.
For chlorination of methane, the methyl radical reacts with a chlorine molecule to form chloromethane and another chlorine radical.
•CH3 + Cl2 → CH3Cl + Cl•
Why this matters: The chlorine radical is regenerated, so it can remove hydrogen from another methane molecule. This keeps the chain reaction going.
The chlorine radical used in step 1 is regenerated in step 2, so the two steps repeat as a chain.
Quick Check: Both Steps for Cyclohexane
Complete both propagation steps for cyclohexane and chlorine.
Propagation as a Chain Reaction
A chain reaction continues because a reactive species consumed in one step is regenerated in a later step. In radical substitution, the regenerated species is the halogen radical.
The two propagation steps can repeat many times before radicals are removed during termination.
OCR does not examine fish-hook arrows. Write each propagation step as an equation with a dot on every radical.
| Propagation feature | What it means | Example |
|---|---|---|
| Radical in reactants | A halogen radical starts the propagation sequence by reacting with the alkane. | Cl• reacts with CH4. |
| Radical in products | A new radical is produced, so further reaction is possible. | •CH3 forms in the first propagation step. |
| Radical regenerated | The halogen radical is remade and can attack another alkane molecule. | Cl• is regenerated when •CH3 reacts with Cl2. |
Each fish-hook arrow shows one electron moving. In an exam, write the two propagation equations with the radical dots; the arrows here show why they work.
Quick Check: One Turn of the Chain
Put the events of one turn of the chain in order.
Example: Bromination of Propane
With propane, a bromine radical can remove a hydrogen atom from the middle carbon. This forms hydrogen bromide and a secondary propyl radical.
This secondary radical is more stable than a primary radical. It can then react with Br2 to form 2-bromopropane and regenerate Br•.
CH3CH2CH3 + Br• → HBr + CH3CH•CH3
CH3CH•CH3 + Br2 → CH3CHBrCH3 + Br•
Common mistake: Do not stop after the first propagation step. The second step is needed to form the haloalkane and regenerate the halogen radical.
Quick Check: Complete the Pair of Steps
Type the missing formulae to complete both steps for butane and bromine.
Quick Check: Propagation Summary
Pull the whole page together by choosing the accurate statement in each round.
Propagation FAQs
These questions summarise the key exam points for the propagation step in radical substitution.
What is propagation in radical substitution?
Propagation is the chain-carrying stage of the mechanism. A radical reacts to form a product and another radical, allowing the reaction to continue.
What happens in the first propagation step?
A halogen radical removes a hydrogen atom from the alkane. This forms a hydrogen halide and an alkyl radical.
What happens in the second propagation step?
The alkyl radical reacts with a halogen molecule. This forms a haloalkane and regenerates a halogen radical.
Why is propagation called a chain reaction?
It is called a chain reaction because the halogen radical is regenerated and can react with further alkane molecules, repeating the propagation steps many times.
What are the propagation equations for chlorination of methane?
The two propagation equations are CH4 + Cl• → HCl + •CH3 and •CH3 + Cl2 → CH3Cl + Cl•.
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.
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