IB Chemistry › R3.4 › R3.4.1
R3.4.1 HL

Free Radical Substitution

Alkanes react with halogens under UV light via a free radical mechanism. This has three stages:

1. Initiation Cl₂ → 2Cl • (UV light breaks Cl−Cl = homolytic fission) Each atom gets one electron → free radicals 2. Propagation (chain reaction) Cl• + CH₄ → CH₃• + HCl (radical attacks methane) CH₃• + Cl₂ → CH₃Cl + Cl• (radical regenerated) One radical consumed, one produced → chain continues 3. Termination Cl• + Cl• → Cl₂ CH₃• + Cl• → CH₃Cl CH₃• + CH₃• → C₂H₆ Two radicals combine → chain ends

Key Terms

  • Homolytic fission — bond breaks equally, one electron to each atom → radicals
  • Heterolytic fission — bond breaks unequally, both electrons to one atom → ions
  • Free radical — species with an unpaired electron (shown with •), highly reactive

Think About It

Why does free radical substitution typically give a mixture of products?

Further substitution can occur: CH₃Cl → CH₂Cl₂ → CHCl₃ → CCl₄. Also, termination can produce unexpected products like ethane (C₂H₆) when two methyl radicals combine.

← Back to R3.4R3.4.2 Nucleophilic Sub. →