Reactions of alkenes

• Alkenes on much more reactive than alkanes because of the presence of the pi Bond being on the outside of the double bond. The pie electrons are more exposed than electrons in the sigma bond. So a pi bond readily breaks and alkenes undergo addition reactions relatively easily.
• Alkenes undergo many addition reactions with:

  • hydrogen in the presence of a nickel or platinum catalyst
  • halogens
  • hydrogen halides
  • steam in the presence of an acid catalyst
• Hydrogenation of alkenes

  When an alkene is mixed with hydrogen and passed over a nickel or platinum catalyst app 423K an addition reaction takes place to form an alkane. This addition reaction, in which hydrogen is added across a double bond, is known as hydrogenation.
• Halogenation of alkenes

  Alkenes undergo a rapid addition reaction with the halogens chlorine or bromine at room temperature.
• The 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 (an orange solution) is added to a sample of an alkene, bromine adds across the double bond. The orange colour disappears indicating the presence of a C=C bond.
• Addition reactions of alkenes with hydrogen halides

  Alkenes react with gaseous hydrogen halides at room temperature to form haloalkanes. If the alkene is a gas then the reaction takes place when the two gases are mixed. If the alkene is a liquid then the hydrogen halide is bubbled through it. Alkenes also react with concentrated hydrochloric or hydrobromic acid.
• Hydration reactions of alkenes

  When alkenes react with steam in the presence of an acid catalyst like phosphoric acid an alcohol is formed. Steam adds across the double bond. There are two possible products.