Addition Reactions (Cambridge International IGCSE Science)

Spec point — State that in an addition reaction only one product is formed Describe the properties of alkenes in terms of addition reactions with: bromine or aqueous bromine hydrogen in the presence of a nickel catalyst steam in the presence of an acid catalyst and draw the structural or displayed formulae of the products

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:

Bromine atoms add across the C=C in the addition reaction of ethene and bromine

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

Hydrogen atoms add across the C=C in the addition reaction of ethene and hydrogen

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

A water molecule adds across the C=C in the addition reaction (hydration) of ethene with water to produce ethanol