Other aromatic systems

• Huckles rules states that a molecule is aromatic only if it has:

  • planar system
  • conjugation
  • contains a total of 4n+2 pi e-, where n is an integer
• Molecules that are planar, conjugated with 4n pi e- are anti-aromatic as the delocalisation of the their e- would lead to destabilisation
• Cyclopentadiene is more acidic than expected for aliphatics as the anion is aromatic so is stable
• Carbocation of cycloheptatriene is sp2 hybridised, empty pz orbital is conjugated with double bonds, molecule becomes planar
• Reactions typical of aromatic compounds:

  • heat of hydrogenation measurements - aromatic stabilisation
  • electrophilic substitution rather than addition reactions