14.1.3—Mechanism: free-radical substitution
- Syllabus
- 9701–2028–2029
- Objective
- 14.1.3
- Level
- AS
Free-radical substitution proceeds through initiation, propagation and termination
UV light breaks Cl₂ or Br₂ homolytically to form radicals. A radical then abstracts hydrogen and a halogen radical substitutes it in a propagation chain.
Initiation creates radicals; propagation consumes one radical and regenerates another; termination combines radicals and removes them. Curly arrows show one-electron movement.
Cl₂ → 2Cl· is initiation. Cl· + CH₄ → HCl + CH₃· and CH₃· + Cl₂ → CH₃Cl + Cl· are propagation steps.
UV light starts the chain but is not a catalyst, and propagation is not the same as electrophilic addition.
ConceptA-Level CAIE Chemistry AS