Alkenes

• Alkenes

  Unsaturated hydrocarbons with the general formula of CnH2n
• Alkenes


  • Attracted by electrophiles due to their double bond
  • Double bond has a high density of electrons and is attacked by electrophiles
• Electrophile


  Electron pair acceptor, deficient in electrons and attracted to the double bond
• Electrophiles


  • Charged ions NO2+, H+
  • Polar molecules e.g H2SO4, H-Br
• Addition of hydrogen halides

  1. Electron pair on a double bond is attracted to a delta positive H+ ion breaking the H-Br bond

  2. Hydrogen joins the molecule and a carbocation intermediate is formed

  3. The -Br is attracted to the positive carbon
• Reacting hydrogen halides with unsymmetrical alkenes

  • Produce 2 different products
  • The amount of these 2 products are determined by the stability of the carbocation intermediate
• Carbocation intermediate stability

  • More Alkyl groups bonded to the carbocation the more stable the intermediate is
  • Alkyl groups push electrons towards the positive carbocation stabilising it
  • Primary to secondary to tertiary carbocations the stability increases
• Carbocation intermediate stability

  The more stable a carbocation intermediate is the more likely the products will form
• Bromine water test


  Adding Br water to an alkene causes a colour change from brown and orange to colourless
• Making alcohol from an alkene

  1. H2SO4 is used as a catalyst

  2. Step 1: Making an alkyl hydrogen sulphate

  3. Step 2: Adding cold H2O forms the alcohol and reforms sulfuric acid
• Alkenes


  • Monomers that join to form additional polymers
  • Natural polymers (protein and natural rubber)
  • Synthetic polymers (poly(ethane) and poly(propene))
• Polyalkene properties

  • Non polar, only van der waals bonds
  • Longer chains = higher van der waals = higher melting point
  • Short and branched polymers are flexible and weaker
  • Longer and unbranched are rigid and stronger
  • Some have halogens e.g PVC so they form stronger permanent dipole-dipole
• Plasticisers


  • Made for polymers to become more flexible
  • The slide between polymer chains pulls them apart making it easier to bend
• Plasticisers


  • Used in PVC to increase flexibility from long chained and closely packed polymer = hard but brittle to more flexible for clothes and electrical cables