alkenes

• alkenes = homologous series of unsaturated hydrocarbons
• heterolytic fission = splitting of a covalent bond where one atom takes both electrons, forming 2 ions
• Markownikoff's rule

  • When an asymmetrical alkene reacts with a hydrogen halide or water molecule there are two possible isomers formed
  • rule states that the hydrogen being added will bond to the carbon with the most hydrogens
• alkenes react by electrophillic addition
• carbocation / carbonium ion = positive carbon ion
• electrophillic addition:

  1. The π-bond gives alkenes a region of high electron density above and below the molecule - this induces a dipole on non polar molecules
  2. The π-bond electron pair are attracted to the atom with a positive dipole to form a covalent bond, and the small molecule is broken by heterolytic fission - the result is the formation of a positive carbocation and a negative ion
  3. A pair of electrons from the negative ion are donated to the positive carbon in the carbocation - forms a covalent bond.
• reactions of alkenes (hydrogentation)

  • reagent = hydrogen
  • conditions = 150 degrees celcius and nickel catalyst
  • product = alkane
• reaction of alkenes (bromination)

  • reagent = bromine
  • conditions = mix at RTP
  • product = dibromoalkane
• reaction of alkenes (hydrogen halides)

  • reagent = hydrogen bromide
  • conditions = mix at RTP
  • product = bromoalkane
• reaction of alkenes (hydration)

  • reagent = water (steam)
  • conditions = 300 °C and 6 Mpa, H3PO4 catalyst
  • product = alcohol
• bromine water

  1. Add five drops of bromine water to a test tube.
  2. Add five drops of oil to the bromine water and shake.
  3. Check if the bromine water decolourises.
  4. Continue adding bromine water until it no longer decolourises.
  5. Record the number of bromine water drops added.
  6. Repeat steps 1-5 with a different oil.
• Alkenes undergo addition polymerisation by linking together

  to form a long chain saturated molecules (polymers)
• disposal of waste polymers - recycling

  • conserving finite fossil fuels
  • sorted by type
  • polymers are chopped into flakes, washed, drained and melted
  • recycling polymers are cut into pellets to be used
• disposal of waste polymers - PVC recycling

  • old method was hazardous due to high chlorine content
  • solvents used to dissolve polymer
  • high grade PVC is obtained via precipitation from the solvent and solvent is reused
• disposal of waste polymers - fuel

  • polymers can be incinerated to produce heat, generating steam to drive a turbine
  • used to generate electricity for the national grid
• disposal of waste polymers - feed stock

  • chemical and thermal processes used to obtain monomers, gases or oil
  • able to handle unsorted and unwashed polymers
  • can be used as raw material to produce new polymers
• bonding

  • no free rotation on c=c bond
  • pi bond and sigma bond
  • trigonal planar - 120
• properties

  • insoluble in water - only dissolve in non polar solvents
  • boiling points increase with chain length
  • lower boiling points than equivalent alkanes
• Cahn Ingold Prelog (CIP) rules for assigning E/Z isomerism:

  • use the atomic number of the atoms direcly bonded to the atom directly bonded to the C=C bond
  • highest atomic numbers on the same side = Z isomer
  • highest atomic numbers diagonally opposite = E isomer
  • if atomic numbers are equal = the longest alkyl chain takes priority