Radical substitution of alkanesEdexcel A-Level Chemistry: Revision notes
Section 1
Radicals and homolytic fission
A radical is a species with an unpaired electron, shown by a single dot, e.g. Cl• or •CH₃. Radicals are very reactive because they seek to pair up the unpaired electron.
Radicals form by homolytic fission: a covalent bond breaks so that each atom takes one of the two shared electrons. Cl–Cl → 2Cl•. Energy to break the bond is supplied by ultraviolet light.
Always put the dot on the radical: Cl• not Cl. A radical has no charge, so it is not the same as Cl⁻.
Section 2
Alkanes with halogens
Alkanes react with chlorine or bromine in ultraviolet light (or sunlight) by radical substitution: a hydrogen atom is replaced by a halogen atom.
CH₄ + Cl₂ → CH₃Cl + HCl
Alkanes are otherwise unreactive. They do not react with bromine water in the dark, which is why alkane–halogen reactions need the energy from ultraviolet light to start.
Section 3
The mechanism: initiation, propagation, termination
A chain reaction has three stages. Curly half-arrows are not expected.
Initiation: Cl₂ → 2Cl• (ultraviolet light, homolytic fission).
Propagation (a radical is used and another made, so the chain goes on):
- CH₄ + Cl• → •CH₃ + HCl
- •CH₃ + Cl₂ → CH₃Cl + Cl•
Termination (two radicals combine and the chain ends):
- Cl• + Cl• → Cl₂
- Cl• + •CH₃ → CH₃Cl
- •CH₃ + •CH₃ → C₂H₆
A propagation step must start with a radical and end with a radical. If both radicals disappear it is termination.
Section 4
Writing and recognising steps
Check each step by counting radicals:
- no radicals on the left, two radicals on the right: initiation
- one radical on each side: propagation
- two radicals on the left, none on the right: termination
For ethane with bromine: C₂H₆ + Br• → C₂H₅• + HBr, then C₂H₅• + Br₂ → C₂H₅Br + Br•.
Section 5
Limitations of radical substitution in synthesis
Radical substitution is a poor way to make a pure halogenoalkane:
- Further substitution: the product still has C–H bonds, so CH₃Cl can go on to form CH₂Cl₂, CHCl₃ and CCl₄.
- A mixture of products: in longer alkanes any hydrogen can be replaced, e.g. propane gives 1-chloropropane and 2-chloropropane, and termination gives by-products such as ethane or butane.
The mixture is hard to separate and the yield of the wanted product is low. Using an excess of the alkane reduces further substitution.
That's the notes covered.
Carry on to the next subtopic.
Exam questions on Radical substitution of alkanes
- A student places a mixture of methane and chlorine in a flask. Nothing happens in the dark, but the gases react quickly when the flask is exposed to ultraviolet light, forming chloromethane and hydrogen chloride.Explain why the mixture does not react in the dark but reacts when exposed to ultraviolet light.2 marks
- A chemist exposes a mixture of ethane and bromine to ultraviolet light and obtains bromoethane, C₂H₅Br, and hydrogen bromide, HBr, by a radical chain reaction.Write the equations for the two propagation steps in the formation of bromoethane from ethane and bromine.2 marks
- A mixture of methane and chlorine, with the methane in limited supply, is kept in ultraviolet light. Analysis of the product mixture shows chloromethane, dichloromethane, trichloromethane and a trace of ethane.Explain how the trace of ethane is formed in this reaction and why its formation stops the chain reaction.3 marks
Written by the Exaim team, led by Shaun Daswani (Head of Upper Secondary, Improve ME Institute; MSc Financial Mathematics, Imperial College London; BSc, UCL) and Jason Daswani (operational lead, Improve ME Institute; LSE).