Combustion & Free Radical Substitution of Alkanes (CIE A Level Chemistry)

• Alkanes are combusted (burnt) on a large scale for their use as fuels
• They also react in free-radical substitution reactions to form more reactive halogenoalkanes

Complete combustion

• When alkanes are burnt in excess (plenty of) oxygen, complete combustion will take place and all carbon and hydrogen will be oxidised to carbon dioxide and water respectively
  • For example, the complete combustion of octane to carbon dioxide and water

The complete combustion of alkanes

Complete combustion involves a plentiful supply of oxygen

Incomplete combustion

• When alkanes are burnt in only a limited supply of oxygen, incomplete combustion will take place and not all the carbon is fully oxidised
• Some carbon is only partially oxidised to form carbon monoxide
  • For example, the incomplete combustion of octane to form carbon monoxide

The incomplete combustion of alkanes

Incomplete combustion involves a limited supply of oxygen

• Carbon monoxide is a toxic gas as it will bind to haemoglobin in blood which can then no longer bind to oxygen
• As no oxygen can be transported around the body, victims will feel dizzy, lose consciousness and if not removed from the carbon monoxide, they can die
• Carbon monoxide is extra dangerous as it is odourless (it doesn’t smell) and will not be noticed
• Incomplete combustion often takes place inside a car engine due to a limited amount of oxygen present

 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