Addition Reactions of Alkenes

  • 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.

Alkene - General Equation Additon, IGCSE & GCSE Chemistry revision notes

Diagram showing the general equation for the addition reaction of alkenes

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


  • 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 change vegetable oils into margarine to be sold in supermarkets.

Addition- Hydrogen to Ethene, IGCSE & GCSE Chemistry revision notes

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


  • 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: 78oC
    • Ethene: -103oC
    • Water: 100oC
  • The ethanol and water are separated afterwards by fractional distillation.

Addition- Water to Ethene, IGCSE & GCSE Chemistry revision notes

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.

imageEthene - Addition of Chlorine, IGCSE & GCSE Chemistry revision notes

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

Need help?

AQA GCSE Chemistry Notes

Want to aim for a Level 9?

See if you’ve got what it takes. Test yourself with our topic questions.

Morgan Curtin Chemistry

Author: Morgan

Morgan’s passion for the Periodic Table begun on his 10th birthday when he received his first Chemistry set. After studying the subject at university he went on to become a fully fledged Chemistry teacher, and now works in an international school in Madrid! In his spare time he helps create our fantastic resources to help you ace your exams.