Addition Reactions of Alkenes (AQA GCSE Chemistry)

• The chemistry of the alkenes is determined by the C=C functional group
• Since all members of the alkene homologous series contain the same functional group then they all react similarly
• Alkenes mainly undergo addition reactions in which atoms of a simple molecule add across the C=C double bond
• The carbon-carbon double bond opens up, forming a single bond between the carbons allowing for two more atoms to bond, one on each carbon

Diagram showing the general equation for the addition reaction of alkenes

• Hydrogen, water and the halogens can take part in these reactions

Hydrogenation

• Alkenes undergo addition reactions with hydrogen in which an alkane is formed
• These are hydrogenation reactions and occur at 150ºC using a nickel catalyst
• Hydrogenation reactions are used to manufacture margarine from vegetable oils
  • Vegetable oils are polyunsaturated molecules which are partially hydrogenated to increase the Mr and turn the oils into solid fats

Hydrogen atoms add across the C=C in the hydrogenation of ethene to produce ethane

• Alkenes also undergo addition reactions with steam in which an alcohol is formed. Since water is being added to the molecule it is also called a hydration reaction
• The reaction is very important industrially for the production of alcohols and it occurs using the following conditions:
  • Temperature of around 330ºC
  • Pressure of 60 – 70 atm
  • Concentrated phosphoric acid catalyst

• When the reaction is complete, the reaction chamber holds unreacted ethene, ethanol and water
• The contents are transferred to a condenser where ethene is separated easily as it has a much lower boiling point than ethanol and water:
  • Ethanol: 78^oC
  • Ethene: -103^oC
  • Water: 100^oC

• The ethanol and water are separated afterwards by fractional distillation

A water molecule adds across the C=C in the hydration of ethene to produce ethanol

• The halogens also participate in addition reactions with alkenes
• The same process works for any halogen and any alkene in which the halogen atoms always add to the carbon atoms involved in the C=C double bond
• The reaction occurs readily at room temperature

Chlorine atoms add across the C=C in the addition reaction of ethene and chlorine