Alkanes react with halogens such as chlorine in the presence of UV light. A hydrogen atom is replaced by a halogen atom by free-radical substitution. For example: CH4 + Cl2 → CH3Cl + HClInitiation: UV light breaks the Cl-Cl bond by homolytic fission to form two chlorine free radicals. Cl2 → 2Cl•Propagation: the radicals react and regenerate new radicals in a chain reaction. CH4 + Cl• → •CH3 + HCl, then •CH3 + Cl2 → CH3Cl + Cl•Termination: two radicals join to form a molecule, which removes radicals and ends the chain. For example Cl• + Cl• → Cl2, •CH3 + Cl• → CH3Cl and •CH3 + •CH3 → C2H6
Free-Radical Substitution
Read the text
Read the text and highlight anything you think is important. When you go on, the text is hidden and you answer from memory.
Specification: AQA Chemistry (7404), AS-level, 3.3.2.4 Chlorination of alkanes. Topic: Alkanes.
Key terms in this lesson
- homolytic fission
- The breaking of a covalent bond in which each atom keeps one electron from the shared pair, forming two free radicals.
- free radical
- A species with an unpaired electron, often formed when a covalent bond breaks by homolytic fission.
More in Alkanes
- Combustion
- Polluting the Atmosphere
- Flue Gas Desulfurisation
- Catalytic Converters
- Greenhouse Effect
- Carbon Neutral Activities
- Further Substitution
- Ozone Layer
All 20 lessons in Alkanes · All AQA AS-level Chemistry topics