Free Radical Substitution (Cambridge (CIE) A Level Chemistry) : Revision Note

Free Radical Substitution Mechanism

• Alkanes can undergo free-radical substitution in which a hydrogen atom gets substituted by a halogen (chlorine/bromine)

  • Ultraviolet light (sunlight) is needed for this substitution reaction to occur

Reacting an alkane with bromine

The fact that the bromine colour has disappeared only when mixed with an alkane and placed in sunlight suggests that the ultraviolet light is essential for the free radical substitution reaction to take place

• 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

• This reaction is 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 (eg. ethane will become C₂Cl₆/C₂Br₆)

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

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^•