Aromatic Chemistry

• Creation of a strong electrophile for Friedel-Crafts acylation:

  A) AlCl3

  B) AlCl4-

  
• benzene is used in pharmaceuticals and dye stuffs
• • benzene is a cyclic, planar molecule
  • one hydrogen is attached to each carbon - C6H6
• • the enthalpy of hydrogenation for cyclohexene (one double bond) is -120kJ/mol
  • if benzene had 3 double bonds, the enthalpy of hydrogenation is expected to be -360 kJ/mol (3x-120)
  • however, the hydrogenation enthalpy for benzene is -208 kJ/mol, a lot lower than expected
  • more energy is needed to break the bonds in benzene than it's theoretical structure with 3 double bonds
  • so benzene is more stable than the theoretical structure due to the delocalised electrons
• aromatics undergo electrophilic substitution mechanisms
• • benzene has a high electron density due to the delocalised ring of electrons
  • this attracts electrophiles
• there are 2 mechanisms:

  1. Friedel-Crafts Acylation
  2. nitration reaction
  3. these are both examples of electrophilic substitution
• nitrobenzenes have different uses:

  • pharmaceuticals and dye stuffs are made by reducing nitrobenzenes to aromatic amines
  • explosives are made from nitrobenzenes e.g. TNT
• nitration reaction:

  • temp - 50
  • conc HNO3 + conc H2SO4
• acylation:

  • AlCl3 catalyst
  • acyl chloride
• The product of step 1 was purified by recrystallisation:

  1. dissolved in the minimum quantity of hot water and filtered into a conical flask to remove any insoluble impurities
  2. flask left to cool to room temperature
  3. crystals formed were filtered off using a Buchner funnel and a clean cork was used to compress the crystals in the funnel
  4. cold water was poured through the crystals. After a few minutes, the crystals were removed from the funnel and weighed. A small sample was then used to find the melting point. Give reasons for each of the following practical steps.

  the minimum quantity of hot water - ensures hot solution is saturated

  flask cooled to room temperature before filtering crystals - yield lower if warm

  crystals compresses in funnel - air passes through sample

  cold water poured through crystals - wash away soluble impurities

  
• The melting point of the sample in Question 9.2 was found to be slightly lower than a data-book value. Suggest the most likely impurity to have caused this low value and an improvement to the method so that a more accurate value for the melting point would be obtained.

  impurity - water

  press sample between filter papers
• explain the bonding in and shape of a benzene molecule

  • benzene is a planar molecule
  • 6 carbons in a ring
  • each carbon has 3 covalent bonds
  • ring of delocalised electrons
  • each C-C bond is equal in length
  • spare electrons in p orbitals overlap
• compare the stability of benzene with that of the hypothetical cyclohexa-1,3,5-triene

  • the expected enthalpy of hydrogenation for cyclohexatriene is -360 kJ mol-1
  • enthalpy of hydrogenation for benzene is less exothermic by 152 kJ mol-1
  • benzene is more stable than cyclohexatriene
• the enthalpy of hydrogenation of cyclohexatriene is not exactly double that of cyclohexene, suggest a value for the ΔH of cyclohexatriene and justify (ΔH for cyclohexene is -120 kJ mol-1)

  • -239 to -121 kJ mol-1
  • double bonds separated by one single bond/alternating
  • allows some delocalisation/ overlap of p orbitals
• explain why an aqueous solution is obtained in the reduction of nitrobenzene even though phenylamine is insoluble in water

  phenylamine is present as an ionic salt