• 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

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

• Carbon monoxide is a?toxic?gas as it will bind to haemoglobin in blood which can then no longer bind?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