Chlorination of Alkanes (Oxford AQA International A Level Chemistry)

Free Radical Substitution of Methane

• Alkanes can undergo free radical substitution under suitable conditions

  • This is where a hydrogen atom is substituted by a halogen

• The free radical substitution of methane involves the standard steps:

  • Initiation

  • Propagation

  • Termination

Initiation step

• The covalent Cl-Cl bond is broken by energy from the UV light

• Each atom takes one electron from the covalent bond

• This produces two radicals in a homolytic fission reaction

Cl–Cl 2Cl^•

Propagation step

• The halogen free radicals are very reactive and will attack the unreactive alkanes

• One of the methane C-H bond breaks homolytically to produce an alkyl radical

CH₄ + Cl^• → ^•CH₃ + HCl

• The alkyl radical can attack another chlorine molecule to form a halogenoalkane

• This also regenerates the chlorine free radical

^•CH₃ + Cl₂ → CH₃Cl + Cl^• 

• The regenerated chlorine free radical can then repeat the cycle

Termination step

• Multiple products are possible, dependent on the radicals involved

• For example, in the single substitution of methane with chlorine:

^•CH₃ + Cl^• → CH₃Cl 

^•CH₃ + ^•CH₃ → CH₃CH₃ 

Cl^• + Cl^• → Cl₂

• Free radical substitution using bromine instead of chlorine is possible and follows similar initiation, propagation and termination steps

Further substitution

• Often, free radical reactions are not very suitable for preparing specific halogenoalkanes as a mixture of substitution products are formed

• If there is enough chlorine / bromine present, all the hydrogens in the alkane will eventually get substituted

• For example, methane could be substituted to become chloromethane and then further substituted

• Single substitution:

CH₄ + Cl^• → ^•CH₃ + HCl

^•CH₃ + Cl₂ → CH₃Cl + Cl^• 

• Second substitution:

CH₃Cl + Cl^• → ^•CH₂Cl + HCl

^•CH₂Cl + Cl₂ → CH₂Cl₂ + Cl^•

• Third substitution:

CH₂Cl₂ + Cl^• → ^•CHCl₂ + HCl

^•CHCl₂ + Cl₂ → CHCl₃ + Cl^•

• Complete substitution:

CHCl₃ + Cl^• → ^•CCl₃ + HCl

^•CCl₃ + Cl₂ → CCl₄ + Cl^•