Reactions of alkenes

• • alkenes are much more reactive than alkanes bc of the presence of the Pi bond
  • C=C is made up of a sigma bond and a Pi bond
  • the Pi electron density is concentrated above and below the plane of the sigma bond
  • being on the outside of the double bond, the Pi electrons are more exposed than electrons in the sigma bond
  • a Pi bond readily breaks and alkenes undergo addition reactions relatively easily
• by considering the exposed nature of the Pi-electrons and the bond enthalpy data - easy to see why Pi bond breaks and sigma bond remains intact when alkenes react:

  C-C single bond (sigma) in ethene - bond enthalpy 347 KJ mol-1

  C=C double bond (Pi and sigma) - bond enthalpy 612KJ mol-1

  bond enthalpy of Pi bond in alkene can be calculated as:

  612 - 347 = 265 KJ mol-1

  Pi bond is weaker than sigma bond and is therefore broken more readily
• Addition reaction of the alkenes:

  • undergo many addition reactions, for example with:
  • hydrogen in the presence of a nickel catalyst
  • halogens
  • hydrogen halides
  • steam in the presence of an acid catalyst
  • all of these reactions involves the addition of a small molecule across the double bond, causing the Pi bond to break and for new bonds to form
• Hydrogenation of alkenes:

  • when an alkene is mixed with hydrogen and passed over a nickel catalyst at 423K
  • addition reaction takes place
  • form an alkane
  • this addition reaction, in which hydrogen is added across a double bond = hydrogenation
• All C=C bonds react with hydrogen in this way.

  e.g. buta-1,3-diene - both double bonds are hydrogenated

  requires 2 molecules of hydrogen per molecule of buta-1,3-diene
• Vegetable oils contain molecules with long unsaturated hydrocarbon chains with the C=C bonds in the cis orientation. Hydrogen gas is bubbled through the oil in the presence of a catalyst (usually nickel)

  many of the unsaturated double bonds are hydrogenated to form saturated carbon chains

  the hydrogenated products have lower melting points - more solid

  the more complete the hydrogenation process, the firmer the finished margarine product
• • the hydrogenation process does actually also form trans double bonds
  • as the catalyst works, the Pi bond is the first broken but it can then reform in the trans orientation
  • as with saturated fats, trans fats have a lower mp than cis fats
  • there are health concerns arising from trans fats and manufacturers are trying to reduce their formation
  • labels on margarines sometimes state 'low in trans fats'
• • olive oil is based on a fatty acid - oleic acid
  • 18 carbon atoms
  • systematic name is E-octadec-9-enoic acid
  • in the name, the carbon tom in the COOH carboxylic acid group is carbon1
  • the z stereoisomer of oleic acid is called elaidic acid and its hydrogenation product is stearic acid
• Halogenation of alkenes:

  • alkenes undergo a rapid addition reaction with the halogens chlorine or bromine at RT
• testing for unsaturation:

  • reaction of alkenes with bromine can be used to identify if there is a C=C bond present and the organic compound is unsaturated
  • when bromine water (orange solution) is added dropwise to a sample of an alkene and test tube shaken, bromine adds across the double bond
  • the orange colour disappears, indicating the presence of a C=C bond
  • if same test carried out with a saturated compound - no addition reaction and no change
  • any compound containing a C=C bond will decolourise bromine water
• Addition reactions of alkenes with hydrogen halides:

  • alkenes react with gaseous hydrogen halides at RT to form haloalkanes
  • if the alkene is a gas then the reaction takes place when the 2 gases are mixed
  • if the alkene is a liquid, the hydrogen halide is bubbled through it
  • alkenes also react with conc. HCl or conc hydrobromic acid - solutions of the hydrogen halides in water
• Hydration reactions of alkenes:

  • alcohols are formed when alkenes react with steam, H2O (g) in the presence of a phosphoric acid catalyst, H3PO4
  • steam adds across the double bond
  • this addition reaction is used widely in industry to produce ethanol from ethene
  • 2 possible products
• conc. sulfuric acid can also be used as the catalyst