3. Alkenes

• Alkenes are unsaturated hydrocarbons meaning they contain a carbon-carbon double bond
• The C-C double bond is an area of high electron density making it susceptible to attack from electrophiles
• Alkenes undergo electrophilic addition about the double bond
• Common electrophiles are:

  HBr

  Br2

  H2SO4
• Electrophiles are electron acceptors and are attracted to areas of high electron density
• Electrophilic addition shows how electrophiles attack the double bond in alkenes
• When the double bond is broken, a carbocation forms

  • A carbon atom with only three bonds, meaning it has a positive charge
• Carbocations can have varying stability - tertiary being most stable and primary the least
• In an addition reaction, multiple products can form but the major product will always be the most stable possible
• Addition polymers are produced from alkenes where the double bond is broken to form a repeating unit
• Why is there no rotation about the C-C double bond?

  Due to the pi orbital - electron density above and below the single bond which holds the carbon atoms in place
• > Reaction conditions

  The reaction conditions used in the production of these polymer chains can be altered to give the plastics produced different properties

  > High pressures and temperatures produce branched chain polymers with weak intermolecular forces

  > Lower pressures and temperatures produce straight chain polymers with strong intermolecular forces
• A compound is the major product due to more stable carbocation which is (primary/secondary/tertiary)
• There is still concern about ozone depletion because:

  • CFCs take many years to reach the ozone layer OR long residence time
  • CFCs are still being used
  • There are other ozone depleting substances
• Poly(propene) is unreactive because there are no polar bonds in chain
• purpose of fractional distillation:

  separates crude oil into mixtures of compounds with similar boiling points
• the purpose of thermal cracking is to make alkenes/shorter molecules
• the purpose of addition polymerisation is for molecules to join together or to produce long chain molecule
• It is beneficial for some polymers to be biodegradable to prevent build up of waste in landfill OR so that they can be broken down by natural processes
• How stable a carbocation is due to the positive inductive effect of the alkyl groups
• Addition polymers are unreactive

  • This is because if the monomers were reactive, we'd get lots of side reactions during polymerisation.
  • Addition polymers are NOT very reactive because the carbon chain is saturated and the side chains are usually non-polar.
• We can make straight-chain polymers more flexible using plasticisers.

  • Plasticisers are molecules that stick between the chains and keep the polymers further apart from each other.
  • This weakens the van der Waals forces and lets the chains slide more easily. This means that the polymers are more flexible.
• Poly(chloroethene) aka PVC

  • PVC is used for rigid plastics like pipes. This is because it has strong dipole-dipole forces due to the presence of the chlorine atom.
  • If you add a plasticiser, you can make more flexible PVC materials, which can be used in flooring tiles.