6.1

• Benzene is simplest member of ARENE homologous series С6H6 empirical : CH • liquid at room temperature • increases efficiency of car engine in gasoline
• Kekulé model of benzene

  • German scientist suggested it was six- membered ring with alternating single + double bonds between carbon atoms

  • discovered that when one group was added to benzene, only one isomer was made but when two was added there were 3 structural isomers → used this experimental evidence about types of isomers produced when benzene reacted, to support his theory
• Problems with Kekulé Model although the model is used to draw it is incorrect

  1. unlike alkenes, benzene is resistant to addition
  2. Enthalpy of hydrogenation of benzene is much more stable than predicted
  3. All 6 carbon bonds in benzene are same length
• Resistance to reaction using kekulé model, you would expect benzene to undergo similar reactions as ethene but benzene will readily undergo substitution of H atom rather than addition

  • Kekulé tried to explain this by saying double + single bonds changed position in a very fast equilibrium
• Hydrogenation

  addition of H to an unsaturated chemical + using bond enthalpy we can calculate enthalpy for complete hydrogenation of cyclohexene + Cyclo-1,3,5-hexatriene (Kekulé’s model)

  • however enthalpy change for hydrogenation of benzene -208 kJmol -1 which shows it is 152 kJmol -1 more energetically stable than predicted
• Bond lengths

  X-ray diffraction techniques have shown that all six carbon bonds in benzene are 0.140nm, which is between a C-C single bonds changed position at 0.147nm and a C=C double bond at 0.135nm

  • suggests there should be 3 short C=C bonds + 3 longer C-C bonds , disproving Kekulé model as they are all same length
• Delocalisation structure of benzene

  delocalised model can explain all three pieces of experimental evidence that do not support Kekulé’s structure

  • Each one of the c atoms donate one electron from p-orbital + then combine to form a ring above and below plane of molecule
  • all rings are identical , unlike Kekulé’s
  • because so much energy is needed to overcome , benzene is very stable + resistant to addition reactions
• Kekule benzene