Propagation
A concise revision guide to the propagation stage of free-radical substitution, including hydrogen abstraction, alkyl radical formation, halogenoalkane production and regeneration of the halogen radical.
- 6B.16ii-c
What these spec points say
- 6B.16ii-c understand the propagation 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
Before You Start
This page builds on initiation and on what you know about covalent bonds from GCSE.
- A covalent bond is a shared pair of electrons.
- Breaking a bond takes in energy; making a bond releases energy.
- Initiation uses UV light to split Cl2 into two chlorine radicals, Cl•.
- A free radical has an unpaired electron, shown by a dot.
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 free-radical substitution. It uses a free radical formed in initiation and produces another free 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 halogenoalkane product.
Key idea: In propagation, a free radical is present in both the reactants and the products. This is why the mechanism continues as a chain reaction.
Propagation keeps the number of radicals the same: one in, one out.
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 free radical removes a hydrogen atom from the alkane. This forms a hydrogen halide and an alkyl free 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: Halogenoalkane Formation
In the second propagation step, the alkyl radical reacts with a halogen molecule. This forms the main substitution product, a halogenoalkane, and regenerates another halogen free 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.
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 free-radical substitution, the regenerated species is the halogen free radical.
The two propagation steps can repeat many times before radicals are removed during termination.
| 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 free radical can remove a hydrogen atom from the middle carbon. This forms hydrogen bromide and a secondary propyl free 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 halogenoalkane 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 free-radical substitution.
What is propagation in free-radical substitution?
Propagation is the chain-carrying stage of the mechanism. A free radical reacts to form a product and another free 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 free radical.
What happens in the second propagation step?
The alkyl radical reacts with a halogen molecule. This forms a halogenoalkane and regenerates a halogen free 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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