Alkenes

• Alkenes


  Unsaturated hydrocarbons
• General formula of alkenes

  CnH2n
• Alkenes


  • Contain a carbon-carbon double bond somewhere in their structure
• Alkenes


  • Ethene
  • Propene
• Arrangement of bonds around >C=C<

  • Planar with bond angle 120o
• Numbers need to be added to the name when positional isomers can occur
• C=C double covalent bond


  Consists of one sigma (σ) bond and one pi (π) bond
• π bonds


  • Exposed and have high electron density
  • Vulnerable to attack by electrophiles
• Stereoisomers


  Have the same structural formulae but have a different spatial arrangement of atoms
• 1. Z stereoisomerism in alkenes

  • Exists due to restricted rotation about the C=C bond
  • Requires two different groups/atoms attached to both ends of the double bond
• 1. Z stereoisomers


  • Z-but-2-ene
  • E-but-2-ene
• Naming E-Z stereoisomers


  • Priority group: The atom with the bigger atomic number is classed as the priority atom
  • If the priority atom is on the same side of the double bond it is labelled Z
  • If the priority atom is on the opposite side of the double bond it is labelled E
• But-1-ene is a structural isomer of But-2-ene but does not show E-Z isomerism
• Electrophile


  An electron pair acceptor
• Electrophilic addition reactions of alkenes

  1. Alkene reacts with electrophile

  2. Carbocation intermediate formed

  3. Nucleophile attacks carbocation
• Bromine addition to alkenes

  Bromine is added to the carbon with the fewest hydrogens attached
• Bromine water decolourises in the presence of a double bond
• Direct industrial hydration of alkenes

  1. Alkene reacts with water in the presence of an acid catalyst

  2. High temperature 300 to 600°C

  3. High pressure 70 atm

  4. Catalyst: concentrated H3PO4
• Hydrolysis


  A reaction where the molecule is split by the addition of water
• Addition polymers


  Formed from alkenes
• Addition polymers


  • Polyethene
  • Polypropene
• Poly(alkenes)


  • Unreactive due to strong C-C and C-H bonds
• Addition polymerisation


  The process of forming addition polymers from alkenes
• Poly(chloroethene)


  • Water proof
  • Electrical insulator
  • Doesn't react with acids
  • Rigid plastic in pure form
  • Flexible when plasticiser added