4.2.3Core Organic Chemistry
Free Radical Substitution of Alkanes — OCR A Level Chemistry A H432 Revision Notes & Topical Questions
5 min read 5 parts Lesson 9 of 29
Section 1 of 5The Overall Reaction
Reagents
- Methane reacts with chlorine in the presence of ultraviolet light to give chloromethane and hydrogen chloride.
Equation
- CH₄ + Cl₂ → CH₃Cl + HCl, in which one hydrogen atom is substituted by a chlorine atom.
UV light
- Ultraviolet radiation supplies enough energy to break the Cl–Cl bond by homolytic fission.
Radical mechanism
- The alkane has no polar bonds, so the reaction proceeds through free radicals rather than ions.
Section 2 of 5The Three Stages
1
Initiation: UV light splits chlorine into radicals by homolytic fission, Cl₂ → 2Cl•.
2
Propagation step 1: a chlorine radical removes a hydrogen atom from methane, Cl• + CH₄ → •CH₃ + HCl.
3
Propagation step 2: the methyl radical reacts with a chlorine molecule, •CH₃ + Cl₂ → CH₃Cl + Cl•, making a new chlorine radical.
4
The regenerated chlorine radical goes on to attack another methane molecule, so a chain reaction continues.
5
Termination: two radicals combine to remove radicals, for example Cl• + Cl• → Cl₂ or •CH₃ + Cl• → CH₃Cl.
Section 3 of 5Termination Products
Termination Products
Chlorine regenerated
- Two chlorine radicals combine to re-form Cl₂, which stops two chains.
Chloromethane
- A methyl radical and a chlorine radical join to give the desired product.
Ethane
- Two methyl radicals combine to form C₂H₆, a by-product that is evidence for the radical mechanism.
Chain ending
- In each case two radicals are used and none is made, so termination removes radicals from the mixture.
Section 4 of 5Further Substitution and Limitations
Multiple substitution
- CH₃Cl can react again with Cl•, forming CH₂Cl₂, CHCl₃ and finally CCl₄.
Excess alkane
- Using an excess of methane makes it more likely that chlorine radicals collide with methane rather than with CH₃Cl.
Mixture
- The reaction is unselective, so a mixture of products is formed that must be separated, usually by distillation.
Longer alkanes
- In propane or longer alkanes, substitution can occur at different carbons and form isomeric products.
Bromine
- Bromine undergoes the same type of reaction, but its radicals are less reactive.
| Stage | Radicals | Net change |
|---|---|---|
| Initiation | Two made from none | Increase |
| Propagation | One used, one made | Unchanged |
| Termination | Two used, none made | Decrease |
Section 5 of 5Exam Checklist & Tips
4.2.3 – Key Points for the Exam
- Free radical substitution A reaction in which a hydrogen atom is replaced through a mechanism involving radicals.
- Initiation The first step, in which radicals are formed from a molecule by homolytic fission.
- Propagation Steps in which a radical reacts and another radical is formed, continuing the chain.
- Termination A step in which two radicals combine and no radical is regenerated.
Common Mistakes
- Using curly arrows with full heads in a radical mechanism; use single-headed (fish-hook) arrows.
- Forgetting the dot to show the unpaired electron on a radical.
- Writing a propagation step that uses two radicals; propagation uses one radical and creates one.
- Leaving out UV light as a condition for initiation.
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