Addition Reactions (Cambridge (CIE) IGCSE Combined Science): Revision Note

Addition reactions

Extended tier only

• Alkenes undergo addition reactions, where atoms from a small molecule add across the C=C double bond

• In an addition reaction:

  • The C=C bond breaks

  • New single bonds form

  • Only one product is formed

Alkenes and bromine

• The reaction between an alkene and bromine is an addition reaction.

  • Bromine atoms add across the C=C double bond

  • A saturated compound is formed

• Aqueous bromine is orange and is used as a test for alkenes.

  • When bromine water is added to an alkene, it is decolourised (orange → colourless)

  • This happens because bromine reacts with the C=C bond and is no longer in solution

• Example:

Alkenes and hydrogen

• When alkenes react with hydrogen, an alkane is formed

  • This is an addition reaction

  • A nickel catalyst is required

• Examples:

  • Ethene + hydrogen → ethane

  • Propene + hydrogen → propane

 Alkenes and steam

• When alkenes react with steam, an alcohol is formed

  • This is an addition reaction

  • It is also called a hydration reaction

  • An acid catalyst is required

• Examples:

  • Ethene + steam → ethanol

  • Propene + steam → propanol