Producing Alkanes (CIE A Level 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
  • E.g. butane from 1-butene

The hydrogenation reaction

Hydrogen gas is added to 1-butene which is 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, 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
  • E.g. octane and ethene from decane

An example of cracking

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