13.3 Reactions of alkenes

• Reactivity of alkenes:

  • A C=C bond is made up of a sigma bond and a pi bond
  • Alkenes are much more reactive than alkanes because of the pi bond which is weaker than the sigma bond and so is broken more easily
  • Alkenes react in addition reactions and the pi bond breaks, sigma bond stays intact
  • Each reaction involves the addition of a small molecule across the double bond
• Hydrogenation of alkenes:

  • alkenes react with hydrogen in the presence of a nickel catalyst to form alkanes
• Halogenation of alkenes:

  • alkenes react with the halogens, Br2 or Cl2, at room temperature to form dihaloalkanes
  • e.g. dibromo --- or dichloro ---
• Alkenes reacting with hydrogen halides:

  • alkenes react with gaseous hydrogen halides (HCl(g), HBr(g)) at room temperature to form haloalkenes
  • Alkenes also react with concentrated hydrochloric acid or hydrobromic acid which are solutions of hydrogen halides in water
  • If the alkene is unsymmetrical then 2 possible products are formed which are structural isomers
• Hydration of alkenes:

  • alkenes react with steam, H2O(g), in the presence of a concentrated phosphoric acid catalyst (H3PO4) to form alcohols
  • If the alkene is unsymmetrical then 2 possible products are formed that are structural isomers
• Halogenation reaction mechanism:

  • an alkene reacts with a halogen e.g. Br2 which acts as the electrophile to form a haloalkene with 2 halides on it