The reactions of alkenes

• Alkenes are much more reactive than alkanes because of the presence of the Pi bond. Being on the outside of the double bond, the Pi electrons are more exposed than the electrons in the sigma bond. A Pi bond readily breaks and alkenes undergo addition reactions relatively easily
• The Pi bond is weaker than the sigma bond and is therefore broken more readily
• 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 


  Each 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.
• When an alkene is mixed with hydrogen and passed over a nickel catalyst at 423K, an addition reaction, in which hydrogen is added across a double bond, takes place to form an alkane. This is hydrogenation.
• The halogenation of alkenes is the addition reaction of halogens with alkenes. Alkenes undergo rapid halogenation with 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 is added dropwise to a sample of an alkene, bromine adds across the double bond. The orange colour of the bromine water disappears, indicating the presence of a C=C bond.

  If the test is carried out with a saturated compound there is no addition reaction so no colour change
• Alkenes react with gaseous hydrogen halides at room temperature to form haloalkanes. Alkenes also react with concentrated hydrochloric or concentrated hydrobromic acid which are solutions of the hydrogen halides in water
• When an unsymmetrical alkene reacts with and unsymmetrical compound two products are possible
• Alcohols are formed when alkenes react with steam, H2O(g), in the presence of a phosphoric acid catalyst, $H_3PO_4$. Steam adds across the double bond. This addition reaction is used widely in industry to produce ethanol from ethene. There are two possible products