alkenes

• Stereoisomers

  Compounds with the same structural formula but with a different arrangement of the atoms in space.
• Unsaturated hydrocarbon


  Contains at least a single double or triple bond
• Polymer


  A large molecule formed from many thousands of repeat units of smaller molecules known as monomers.
• Monomer


  A small molecule that combines with many other monomers to form a polymer
• Addition polymerisation

  Formation of a very long molecular chain (addition polymers) by repeated addition reactions of many unsaturated alkene molecules (monomers)
• Alkenes

  The hydrocarbon homologous series with at least one double carbon-to-carbon bond
• Cis-trans isomerism

  A special type of E/Z isomerism in which there are two non-hydrogen groups and two hydrogen atoms around the C-C double bond - the cis isomer (Z isomer) has hydrogen atoms on each carbon on the same side whilst the trans isomer (E isomer) has hydrogen atoms on each carbon on different sides
• EZ isomerism

  A type of stereoisomerism in which different groups attached to each carbon of a C-C double bond may be arranged differently in space because of the restricted rotation of the C-C bond
• reactivity of alkenes

  • more reactive than alkanes
  • its double bond is made up of a sigma and pi bond
  • pi bond has a higher electron density
• pi electrons are more exposed than electrons in sigma bonds so alkenes pi bond readily breaks and undergo addition reactions easily
• addition reactions of alkenes

  • hydrogen in the presence of a Ni catalyst
  • halogens
  • hydrogen halides
  • steam in the presence of an acid catalyst
• hydrogenation of alkenes

  • mixed with hydrogen
  • passed over a Ni catalyst at 423K
  • forms an alkane
• hydration of alkenes

  • react with steam in the presence of phosphoric acid catalyst
  • used widely to produce ethanol from ethene
• an example of electrophilic addition is an alkene plus a hydrogen halide
• an electrophile is an atom attracted to an electron-rich centre and accepts an electron pair
• electrophilic addition in alkenes

  1. double bond breaks because it is attracted to partially positive H
  2. C-H bond formed
  3. H-Br breaks by heterolytic fission, electron pair goes to Br
  4. Bromide ion and carbocation is formed and they react to form the final product
• tertiary carbocations are the most stable
• primary carbocations are least stable
• Markownikoff's rule

  Hydrogen will attach itself to the carbon having a greater number of hydrogen atoms and smaller number of carbon atoms
• in general equations for addition polymerisation

  • the repeat unit is always in square brackets
  • after the bracket place a letter n to show that there is a large number of repeats
• industrial polymerisation is carried out at high temperature and high pressure using catalyst
• unsaturated alkene molecules undergo addition polymerisation to produce long saturated chains containing no double bonds
• benefits of polymers

  • readily available
  • cheap
  • lack of reactivity
• environmental concerns of polymers

  • alkene-based polymers are non-biodegradable
• Lack of reactivity→ challenge for disposal since most are non-biodegradable
• Recycling: conserves fossil fuels + decreases waste going in landfill. Have to be sorted by type, chopped into flakes, washed, dried, melted then cut into pellets + used to make new products
• Some are difficult to recycle, but have a high stored energy value so can be incinerated to produce heat, generating steam to turn turbine to produce electricity
• N shown before reactant & after square brackets at bottom for product
• test for alkenes

  • bromine water goes from orange to colourless