Students should be able to: explain this reaction as a free-radical…

The idea

The mechanism has three named stages. Initiation: ultraviolet light supplies enough energy to break Cl₂ homolytically, one electron going to each atom, giving two chlorine radicals, Cl₂ → 2Cl•. Propagation runs twice: Cl• + CH₄ → •CH₃ + HCl, then •CH₃ + Cl₂ → CH₃Cl + Cl•. A radical is used and a radical is regenerated in each step, so one initiation event can produce thousands of product molecules; that is why the two steps add to give the overall equation. Termination is any step that removes radicals by joining two of them, such as •CH₃ + Cl• → CH₃Cl, 2•CH₃ → C₂H₆ or 2Cl• → Cl₂. The ethane in the product mixture is direct evidence that termination happens.

What earns the marks

• The initiation step Cl₂ → 2Cl• together with ultraviolet light as the condition.
• Each correct propagation step, with a radical on both sides of the arrow.
• A termination step in which two radicals combine and no radical is produced.
• The overall equation CH₄ + Cl₂ → CH₃Cl + HCl obtained by adding the two propagation steps.

Common mistakes and how to put them right

• Omitting the dot on Cl• or •CH₃, so the species is read as an ion or a molecule.

• Writing initiation as Cl₂ → Cl⁺ + Cl⁻, which is heterolytic fission and gives ions, not radicals.

• Labelling a step as termination when a radical still appears on the right-hand side.

• Using double-headed curly arrows in a free-radical mechanism, which AQA penalises.

Exam tips

• Sort any step by counting radicals: in gives one and out gives one for propagation, two in and none out for termination.
• Add your two propagation steps together; if the radicals do not cancel, one of them is wrong.
• If you use half-headed arrows, use them consistently; they are not required, but incorrect ones lose credit.
• Missing radical dots are penalised once within a clip, never credited, so check every species carries its dot before you move on.