2A4 Alkenes

• Reactions of alkenes

  • alkenes are unsaturated so tend to react to become saturated by addition reactions
  • the pi bond is lost as the molecule attacking adds onto the alkene
  • single bond remains unaffected
• 4 main reaction types:

  1. hydrogenation: addition of hydrogen
  2. electrophilic addition: of hydrogen bromide and bromine
  3. hydration: addition of water
  4. polymerization: addition of the akene itself
• Hydrogenation

  • alkenes don't react directly with hydrogen gas unless the temperature is high (150C) and there is a nickel catalyst
  • if unsaturated bonds are made saturated their melting point rises and they become solid at room temperature
• Electrophilic addition

  • the molecule being added is polar so the electronegativity is unequal between the two elements forming dipoles
  • this can attack the pi bond where there is a high electron density
• Electrophilic addition with bromine

  • colorless ethene is bubbled into liquid bromine (red-brown) or a solution of bromine in water (orange) or mixed with bromine vapour (brown) and decolorizes it
  • reaction is rapid at room temperature
  • used as a test for alkenes due to colour change
• Hydration

  • alcohols made from alkenes by dissolving it in conc. sulphuric acid and adding water
  • direct hydration: ethanol made from ethene and steam under these conditions

  1. temperature of 300C
  2. pressure of 7000kPa
  3. phosphoric acid catalyst
• Polymerisation

  • involves taking a small molecule and joining many together to make a polymer
  • all alkenes turn into polymers by the application of heat, pressure and/or a catalyst depending on the alkene involved
  • addition polymersiation
• Alkenes are a homologous series of unsaturated hydrocarbons with the general formula CnH2n+2
• Formation of a double bond

  • form a trigonal planar arrangement around each carbon from the double bond
  • p-orbitals are above and below the plane of the atoms
  • the p-orbitals overlap to make a pi orbital
  • double bond is an area of high electron density which can attract positive groups and other molecules
• Unsymmetrical alkenes

  • when an unsymmetrical electrophile attaches itself onto an unsymmetrical alkene there will be two possible products
  • they aren't produces in equal quantities
  • the major product is produced by the more stable carbocation and will have a higher yield
• Hydration (alkene --> alcohol)

  • conditions: 300C, 7000kPa, phosphoric acid catalyst
• Polymerisation

  • alkenes turn into polymers by adding heat, pressure and a catalyst
  • the final product is an alkane
  • addition polymerisation
• Geometrical isomers are molecules with the same molecular formula but a different structural formula
• Stereoisomers

  • compounds have the same molecular formula and structural formula
  • differ in spatial arrangement
  • priority groups on the same side are Z isomers and opposite are E isomers