?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 type reaction called the?propagation step
  • The reaction is terminated when two radicals collide with each other in a?termination step

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

• In the?initiation?step?the Cl-Cl or Br-Br is broken by energy from the UV light
• This produces two radicals in a?homolytic?fission reaction

The first step of the free-radical substitution reaction is the initiation step in which two free radicals are formed by sunlight

Propagation step

• The?propagation?step?refers to the?progression?(growing) of the substitution reaction in a?chain type reaction
  • Free radicals?are very reactive and will attack the unreactive alkanes
  • A C-H bond breaks?homolytically?(each atom gets an electron from the covalent bond)
  • An?alkyl?free radical is produced
  • This can attack another chlorine/bromine molecule to form the?halogenoalkane?and?regenerate?the chlorine/bromine free radical
  • This free radical can then?repeat?the cycle

The second step of the free-radical substitution reaction is the propagation step in which the reaction grows in a chain type reaction

• • 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)

The free-radical substitution reaction gives a variety of products and not a pure halogenoalkane

Termination step

• The?termination step?is when the chain reaction?terminates?(stops) due to two free radicals reacting together and forming a single unreactive molecule
  • Multiple products are possible

The final step in the substitution reaction to form a single unreactive molecule