3.10 aromatics

• What are aliphatic molecules

  Straight or branched chained organic substances
• What are aromatic or arene molecules

  One or more rings of 6 carbon atoms with delocalised electrons.
• What is the structure and bonding of benzene

  It is the simplest arene and as the molecular formula C6H6 Its basic structure is six C atoms in a hexagonal ring, with one H atom bonded to each C atom.

  
  

  Each C atom is bonded to two other C atoms and one H atom by single covalent σ-bonds. This leaves one unused electron on each C atom in a p orbital, perpendicular to the plane of the ring.

  
  

  Benzene is a planar molecule as the evidence suggests all the C-C bonds are the same and have a length and bond energy between a C-C single and C=C double bond.

  
  

  The six p electrons are delocalised in a ring structure above and below the plane of carbon atoms.
• Explain the thermodynamic stability of benzene using enthalpies of hydrogenation.

  Benzene is more stable than the theoretical alternative cyclohexane-1,3,5-triene proposed by kekule.

  
  

  We can prove this through comparing the enthalpy change from the hydrogenation reactions of them.

  
  

  If we hydrogenate cyclohex-1-ene an enthalpy change of -120kjmol-1 occurs. Therefore, theoretically the enthalpy change of hydrogenation of cyclohexane-1,3,5-triene should be 360kjmol-1 as there are three double bonds. When hydrogenating benzene the enthalpy change is lower than this at -208kjmol-1 .

  
  

  This suggests more energy is required to break the bonds as the same energy is produced when the bonds are formed, and this is due to the delocalised electron structure.
• What reactions do benzene molecules undergo?

  Electrophillic substitution
• What is the importance of the nitration of benzene?

  Nitration of benzene and other arenes is an important step in synthesising useful compounds e.g. explosive manufacture (like TNT) and formation of amines from which dyes are manufactured.
• What is the importance of friedel crafts acylation

  They introduce a reactive functional group to the benzene ring.
• Why do we not use ethanoyl chloride in industrial synthesis

  produces HCl fumes which are toxic
• The enthalpy of hydrogenation of cyclohexa-1,3-diene is not exactly double that of cyclohexene.

  Suggest a value for the enthalpy of hydrogenation of cyclohexa-1,3-diene and justify your value.

  200 kjmol-1

  because alternating double and single bonds allows overlap of p orbitals and therefore partial delocalisation