A-Level Chemistry 9701 / Topic 14 / AS

A chain reaction in sunlight.

Alkanes ignore most reagents, but shine ultraviolet light on methane and chlorine and they react explosively. One photon starts a chain of radical reactions that can run thousands of times before it stops.

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The key idea

Alkanes react with chlorine or bromine in ultraviolet light by free radical substitution. Initiation: UV light breaks Cl–Cl by homolytic fission into two chlorine radicals. Propagation: a radical reacts with a molecule to give a product and a new radical, so the chain continues. Termination: two radicals combine and the chain stops. Because radicals are so reactive, further substitution occurs and a mixture of products forms (CH₃Cl, CH₂Cl₂, CHCl₃, CCl₄, and some ethane).

Fig. 1: the three stages of the free radical substitution of methane by chlorine

Section 01

Initiation, propagation, termination.

Shine UV light on the methane and chlorine mixture in the simulator and follow the radicals step by step. Count how many propagation steps occur before termination.

Initiation: Cl₂ → 2Cl•. UV supplies the energy to break the weakest bond present, Cl–Cl.
Propagation: Cl• + CH₄ → •CH₃ + HCl, then •CH₃ + Cl₂ → CH₃Cl + Cl•. The Cl• is regenerated.
Termination: Cl• + Cl• → Cl₂; •CH₃ + Cl• → CH₃Cl; •CH₃ + •CH₃ → C₂H₆.
Overall: CH₄ + Cl₂ → CH₃Cl + HCl.

Stage 1 · Learn

Check what the sim just showed you

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What condition is needed for methane to react with chlorine?

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Which is an initiation step?

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Which is a termination step?

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Why does the reaction of methane with chlorine produce a mixture of products?

Section 02

A mixture of products.

Radicals attack any C–H bond they meet, including those in molecules that have already been substituted.

Further substitution: Cl• + CH₃Cl → •CH₂Cl + HCl, then •CH₂Cl + Cl₂ → CH₂Cl₂ + Cl•, and so on to CCl₄.
Excess methane favours CH₃Cl; excess chlorine favours CCl₄.
Larger alkanes give isomers: propane gives both 1-chloropropane and 2-chloropropane.
Ethane with chlorine: termination of two ethyl radicals gives butane, C₄H₁₀.

Examiner trap

Radicals form by homolytic fission and carry an unpaired electron, shown by a dot. Every propagation step has one radical on each side; every termination step has two radicals in and none out. A step that makes HCl and a methyl radical is propagation, not termination.

Stage 2 · Exam

Exam-style questions

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In the chlorination of ethane, which termination product contains four carbon atoms?

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How many different monochloro products form when propane reacts with chlorine in UV light?

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Which propagation step forms dichloromethane?

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Why does the reaction occur in UV light but not in the dark?

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Free radical substitution, mastered.

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Six original Cambridge-style questions

The reaction of alkanes with halogens in UV light, the three stages of the mechanism, further substitution and product mixtures.