Alkanes (CIE AS Chemistry)

• Alkanes are hydrocarbons that can be produced by the addition reaction of hydrogen to an alkene or by cracking of longer alkane chains

Production of alkanes from addition reactions

• Alkenes are unsaturated organic molecules and contain C-C double bonds
• When hydrogen gas and an alkene are heated and passed over a finely divided Pt/Ni catalyst, the addition reaction produces an alkane:
  • The Pt/Ni catalyst is finely divided to increase its surface area and therefore increase the rate of reaction
  • Eg. butane from 1-butene

Hydrogen gas is added to 1-butene which are then heated and passed over a Pt/Ni catalyst to produce butane

• The addition reaction of alkenes with hydrogen is called hydrogenation
• Hydrogenation is often used in the manufacture of margarine from vegetable oil
  • Vegetable oil is an unsaturated organic molecule with many C-C double bonds
  • When these are partially hydrogenated, their hydrocarbon chains become straighter
  • This raises the melting point of the oils which is why margarine is a soft solid and vegetable oil a liquid at room temperature

Production of alkanes from cracking

• In cracking large, less useful hydrocarbon molecules found in crude oil are broken down into smaller, more useful molecules
• The large hydrocarbon molecules are fed into a steel chamber and heated to a high temperature and then passed over an aluminium oxide (Al₂O₃) catalyst
  • The chamber does not contain any oxygen to prevent combustion of the hydrocarbon to water and carbon dioxide

• When a large hydrocarbon is cracked, a smaller alkane and alkene molecules are formed
  • Eg. octane and ethene from decane

Long hydrocarbons are cracked by heating them and using aluminium oxide catalyst into smaller hydrocarbons and an alkene