Alkenes

• Alkenes

  Unsaturated hydrocarbons comprising of a sigma and a pi bond.
• C=C double bond is an area of high electron density

  • Cause of high reactivity of alkenes.
• Shape of Alkenes

  Trigonal planar (120)

  • Two single bonds, One double bond.
  • Centres of electron density repel each other to the maximum distance apart;
  • Double bond exerts slightly greater repulsion
• Dehydration of an alcohol

  Alcohol -> Alkene

  • Excess concentrated sulphuric acid
  • Heat to 170 deg. C
• Alkenes are more reactive as the pi bond has a relatively low bond enthalpy
• Hydrogenation of alkenes

  Addition reaction

  • Forms Alkanes
  • Nickel catalyst
  • Hydrogen Gas
  • 150 celcius
• Test for unsaturation

  Decolourises bromine in an organic solvent.
• Markownikoff’s rule

  The more carbons a carbon is bonded to, the more stable it is.

  • Hydrogen will bond to the most reactive carbon.
• Waste polymers can be:

  1. Combusted for energy production
  2. Used as organic feedstock for production of plastics and other organic chemicals. (cracking)
  3. Removal of HCl formed by combustion.
• Cracking

  Long chain alkane into short chain alkane and an alkene.
• Dihaloalkanes

  Substitution reaction

  • Halogens
  • Use bromine to detect the presence of a double C=C bond; unsaturation
• Electrophile 


  Electron pair acceptor
• Reduced dependency on finite resources; alleviate problems from disposal of persistent plastic waste