alkenes

• Alkenes


  Unsaturated hydrocarbons
• 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


  Alkenes can exhibit this type of isomerism due to restricted rotation about the C=C bond

  
• 1. Z isomers


  • 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
• 1. Z stereoisomers


  • Z-1,2-dichloroethene
  • E-1,2-dichloroethene

  
• 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. Bromine addition

  2. Hydrogen bromide addition

  3. Sulfuric acid addition
• Bromine addition to alkenes

  Forms dihalogenoalkane
• Hydrogen bromide addition to alkenes

  Forms haloalkane
• Markownikoff's Rule


  In electrophilic addition to alkenes, the major product is formed via the more stable carbocation intermediate

  
• Bromine water decolourises in the presence of a double bond
• Direct industrial hydration of alkenes to form alcohols

  1. High temperature 300 to 600°C

  2. High pressure 70 atm

  3. 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 the strong C-C and C-H bonds
• Addition polymerisation


  The process by which addition polymers are formed
• Poly(chloroethene)


  • Water proof
  • Electrical insulator
  • Doesn't react with acids
  • Rigid plastic due to strong intermolecular bonding
  • Flexible when plasticiser added