Reactions of Alkenes (SQA National 5 Chemistry): Revision Note

Addition reactions of alkenes

• Alkenes undergo addition reactions

  • This is where the double bond breaks open and the atoms of another molecule are "added across" it

• In an addition reaction:

  • The C=C double bond becomes a C-C single bond

  • The product is always a saturated compound

  • Only one product is formed

• There are several important types of addition reactions

• The three most common involve adding small molecules like:

  • Hydrogen, H₂

  • Halogens, e.g. Cl₂, Br₂

  • Water, H₂O

1. Addition of hydrogen

• This reaction is:

alkene + hydrogen → alkane

• The process: Adding hydrogen to an alkene is called hydrogenation

  • The hydrogen atoms "add across" the double bond

  • This turns the unsaturated alkene into a saturated alkane

• Conditions: This reaction requires a nickel catalyst and heat

Example hydrogenation reaction

ethene + hydrogen → ethane

C₂H₄ + H₂ → C₂H₆

2. Addition of halogens

• This reaction is:

alkene + halogen → dihaloalkane

• The process: Adding a halogen to an alkene is called halogenation

  • The halogen atoms "add across" the double bond

  • This turns the unsaturated alkene into a saturated dihaloalkane

    • A dihaloalkane is an alkane molecule that has two halogen atoms attached to it

• There are specific types of halogenation reaction:

  • Chlorination is the addition of chlorine, Cl₂

  • Bromination is the addition of bromine, Br₂

    • The addition of bromine to an alkene is the standard

Example halogenation reaction

ethene + bromine → 1,2-dibromoethane

C₂H₄ + Br₂ → C₂H₄Br₂

• This is the reaction that happens in the test for unsaturation, where the orange-brown bromine solution is decolourised

3. Addition of water (steam)

• This reaction is:

alkene + water (steam) → alcohol

• The process: Adding water to an alkene is called hydration

  • The water "adds across" the double bond

  • This turns the unsaturated alkene into an alcohol

• Hydration is a key industrial method for making alcohols

• Conditions: This reaction requires:

  • High temperature

  • High pressure

  • An acid catalyst, such as phosphoric acid

Example hydration reaction

ethene + water (steam) → ethanol

C₂H₄ + H₂O → C₂H₅OH

A special type of addition: Polymerisation

• Addition reactions can also happen between many alkene molecules themselves

• This process is called addition polymerisation, which is how plastics are made