AQA Chemistry A-level: Alkenes (3.3.4)

Structure and Reactivity of Alkenes

• Definition: Alkenes are unsaturated hydrocarbons with a carbon-carbon double bond.
• Characteristics:
  • High electron density at the double bond, making them susceptible to electrophilic attack.
  • Composed of a normal covalent bond and a π bond.
• Identification: Bromine water test - turns from orange-brown to colorless in the presence of a double bond.

Electrophilic Addition

• Process: Alkenes undergo electrophilic addition reactions at their double bonds.
• Electrophiles: Electron acceptors attracted to high electron density areas.
  • Common electrophiles include: HBr, Br₂, H₂SO₄.
• Reaction Mechanism:
  • Electrophiles attack the double bond, breaking it and forming a carbocation (positively charged carbon with three bonds).
  • Stability of carbocations:
    • Tertiary > Secondary > Primary.
    • Major product is the most stable carbocation.
• Examples of Mechanisms:
  • Halogenoalkanes: π bond induces a temporary dipole in a bromine molecule, facilitating electron transfer.
  • Sulphuric Acid: Alkenes react with sulphuric acid to produce ethyl hydrogensulphate.

Addition Polymers

• Formation: Alkenes form addition polymers by breaking the double bond to create repeating units.
• Representation: Repeating unit shown with extended bonds through brackets.
• Reaction Conditions:
  • High pressure and temperature: Produces branched polymers with weak intermolecular forces.
  • Low pressure and temperature: Produces straight chain polymers with strong intermolecular forces.
• Uses and Properties:
  • Polymers are strong, non-reactive hydrocarbon chains.
  • Common in plastic manufacturing (e.g., poly(ethene) for shopping bags).
  • Non-biodegradable due to strong, non-polar covalent bonds.
• PVC:
  • Poly(chloroethene), known as PVC, is waterproof.
  • Properties enhanced by adding plasticisers during the reaction.