Alkenes

• nature of double bond

  the shape of the double bond is trigonal planar around each carbon atom

  it is an area of high electron density above and below the plane of the bond
• pi and sigma bond

  $\pi$ bond - sideways overlap of P orbitals ABOVE AND BELOW PLANE OF ATOMS

  $\sigma$ bond - end-on overlap of orbitals - CAN FREELY ROTATE
• Stereoisomer

  compounds with same structural formula but different orientation in space
• characteristics of stereoisomerism

  carbon-carbon double bond that restricts rotation

  each carbon atom must be bonded to two different groups on each end of the double bond
• RULES OF E/Z ISOMERISM

  Z - higher atomic number on top (priority)

  E - higher atomic numbers OPPOSITE
• Rules of CIS AND TRANS ISOMERISM

  each carbon on either side of the double bond must be bonded to different groups/atoms

  at least on of the groups on both carbon atoms must be the same
• Electrophile


  electron pair acceptor
• Addition reaction


  adding a smaller molecule across a double bond
• Bromine reaction


  TEST FOR ALKENES

  OBSERVATION: orange to colourless

  EQUATION: CH2=CH2 + Br2 --> BrCH2CH2Br

  Reagents and conditions:

  BROMINE (aq or l)

  Room temperature
• Hydrogen bromide reaction

  Equation: CH2=CH2 + HBr --> CH3CH2Br

  Reagents and Conditions: Hydrogen Bromide (aq or g) and room temp
• Extension reaction

  sulfuric acid

  Equation: CH2=CH2 + H2SO4 --> CH3CH2OSO3H

  Reagents and conditions: concentrated sulfuric acid and cold

  Warming ethyl hydrogen sulphate in dilute sulfuric acid results in hydrolysis and production of ethanol

  CH3CH2OSO3H + H2O --> CH3CH2OH + H2SO4 (catalyst)
• Outline Electrophilic Addition in ASYMMETRICAL ALKENES

  if the alkene and electrophile are BOTH asymmetrical, two possible products can form

  MAJOR PRODUCT: forms from the most stable cation/intermediate step

  There is slight polarity in the C-H bonds, meaning alkyl groups are slightly electron repelling, REDUCING POSITIVE CHARGE - Inductive effect
• what is the INDUCTIVE EFFECT

  there is slight polarity in C-H bonds, making alkyl groups slightly electron repelling, reducing positive charge
• why can non-polar molecules react with alkenes?

  the high electron density in the double bonds induces the dipole of the non-polar molecule
• explain major and minor products in terms of stability

  the secondary carbocation that forms the major product is more stable than the primary carbocation that forms the minor product

  OR

  the tertiary carbocation that forms the major product is more stable that the secondary/primary carbocation that forms the minor product
• Direct hydration of ethene

  INDUSTRIAL MANUFACTURE OF ALCOHOLS

  Equation: CH2=CH2 + H20 --> CH3CH2OH

  Reagents and Conditions:

  • steam
  • 300$\degree$C and phosphoric acid catalyst

  FERMENTATION: breaking down molecules anaerobically

  BENEFITS OF DH

  • very fast
  • makes only one product - PURE and 100% atom economy
  • higher yield than fermentation

  DRAWBACKS

  • made from ethene - crude oil - non-renewable
  • industrial plant is expensive
• Hydrogenation of ALKENES

  Manufacture of margarine

  Equation: CnH2n + H2 --> CnH2n+2

  Reagents and conditions:

  • H2 gas
  • 150$\degree$C
  • nickel catalyst

  INDUSTRIAL USE: hydrogenating oils from plants to raise their melting points
• What is addition polymerisation and how does it work?

  ADDITION POLYMERISATION: joining together of many small monomers (1 unit) to form one large polymer

  • catalyst present produces a radical at the end of the chain
  • when a monomer is added, the radical is recreated so that the chain can grow
  • USUALLY, THE END GROUPS OF THE CATLAYST ARE IGNORED WHEN THE STRUCTURE OF THE POLYMER IS CONSIDERED
• USAGE AND DISPOSAL OF POLYMERS

  USAGE:

  depends on properties which in turn depend on:

  • type of monomer used
  • amount of side chain
  • length of polymer chain

  DISPOSAL

  Landfill

  • landfill accumulation (non-biodegradable)

  Incineration

  • energy released can be used
  • waste of of non-renewable resource
  • toxic flames can evolve when burnt
  • toxic chlorinated compounds can evolve (HCl, Cl, COCl2)

  Recycle-wash-remould

  Feedstock recycling

  • breaks down large polymer chains down to monomers