benzene

• compound containing benzene ring = aromatic compounds or arenes
• molecular formula C6H6
• delocalised model of benzene

  • planar (FLAT)
  • 3 sigma bonds for each carbon to bond (2 to bond other carbons, 1 to bond to hydrogen)
  • p-orbitals overlap sideways to make a pi-bond above and below
  • creates a ring of 6 delocalised electrons providing stability
• Kekule model proposed

  • addition reactions
  • alternating, irregular bond lengths
  • enthalpy change of hydrogenation 3x that of cyclohexene
• Three pieces of evidence which appear to disprove Kekulé's model:

  1. Lack of reactivity
  2. Lengths of C-C bonds
  3. Hydrogenation enthalpies
• The lack of reactivity of benzene

  • If benzene contained the C=C bonds it should decolourise bromine in an electrophilic addition reaction
  • benzene does not undergo electrophilic addition reactions
  • benzene does not decolourise bromine under normal conditions
• Lengths of the carbon-carbon bonds in benzene

  • Found that all the bonds in benzene between the length of a single and a double bond
• Hydrogenation enthalpies

  • The actual enthalpy change of hydrogenation of benzene is only -208 kJ mol-
  • 152 kJ mol-' less energy is produced than expected
  • The actual structure of benzene is therefore more stable than the theoretical Kekulé model of benzene