3.2.4 Chlorination of Alkanes

 Free-radical substitution of alkanes

• Alkanes can undergo free-radical substitution in which a hydrogen atom gets substituted by a halogen (chlorine/bromine)
• Since alkanes are very unreactive, ultraviolet light (sunlight) is needed for this substitution reaction to occur
• The free-radical substitution reaction consists of three steps:
  • In the initiation step, the halogen bond (Cl-Cl or Br-Br) is broken by UV energy to form two radicals
  • These radicals create further radicals in a chain reaction called the propagation step
  • The reaction is terminated when two radicals collide with each other in a termination step

• 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

• The free-radical substitution reaction consists of three steps

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

Initiation step

• The first step of the free-radical substitution reaction is the initiation step
• In the initiation step the Cl-Cl or Br-Br 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

One reactant two radicals

Cl–Cl  2Cl^• 

Br–Br  2Br^• 

Propagation step

• The second step of the free-radical substitution reaction is the propagation step 
• This refers to the progression (growing) of the substitution reaction in a chain reaction
• Free radicals are very reactive and will attack the unreactive alkanes
• A C-H bond breaks homolytically 
  • Remember: Homolytic fissions is where each atom gets one electron from the covalent bond
• An alkyl free radical is produced

CH₃CH₃ + Cl^• → ^•CH₂CH₃ + HCl

OR

CH₃CH₃ + Br^• → ^•CH₂CH₃ + HBr

• This can attack another chlorine / bromine molecule to form a halogenoalkane and regenerate the chlorine / bromine free radical

^•CH₂CH₃ + Cl₂ → CH₃CH₂Cl + Cl^• 

OR

^•CH₂CH₃ + Br₂ → CH₃CH₂Br + Br^• 

• The regenerated chlorine / bromine 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
• For example, ethane could be substituted to become chloroethane and then further substituted:
  • First substitution:

CH₃CH₃ + Cl^• → ^•CH₂CH₃ + HCl

^•CH₂CH₃ + Cl₂ → CH₃CH₂Cl + Cl^• 

• • Second substitution:

 CH₃CH₂Cl + Cl^• → ^•CH₂CH₂Cl + HCl

^•CH₂CH₂Cl + Cl₂ → CH₂ClCH₂Cl + Cl^•

• • Third substitution:

CH₂ClCH₂Cl + Cl^• → ^•CHClCH₂Cl + HCl

^•CHClCH₂Cl + Cl₂ → CHCl₂CH₂Cl + Cl^•

• This process can continue until full substitution has occured
• So, ethane would become:
  • Hexachloroethane, C₂Cl₆, with chlorine
  • Hexabromoethane, C₂Br₆, with bromine

Termination step

• The final step in the substitution reaction is the termination step
• This is when the chain reaction terminates (stops) due to two free radicals reacting together and forming a single unreactive molecule

Two radicals → one product

• Multiple products are possible, dependent on the radicals involved
• For example, in the single substitution of ethane with chlorine:

^•CH₂CH₃ + Cl^• → ClCH₂CH₃ 

^•CH₂CH₃ + ^•CH₂CH₃ → CH₃CH₂CH₂CH₃ 

Cl^• + Cl^• → Cl₂