Aromatic Hydrocarbons

    Cards (7)

    • Draw electrophilic substitution mechanisms 

      .
    • Generating the electrophile +NO2:
      • Reagents: concentrated HNO3 and concentrated H2SO4
      • Equation: HNO3 + 2H2SO4 → NO2+ + H3O+ + 2HSO4-
      • HNO3 + H+ → H2NO3 → NO2+ + H3O+ (nitric acid acts as a base)
      • Conditions: lower temperatures to prevent further nitration occurring
    • Friedel-Crafts acylation, electrophilic substitution:
      C6H6 + RCOCl → C6H5COR + HCl
      C6H6 + RCO+ → C6H5COR + H+
      Generating the electrophile:
      • Reagents: acyl chloride, AlCl3 acts as a catalyst
      • RCOCl + AlCl3 → RCO+ + AlCl4-
      • The carbon has the positive charge for the electrophile
      The product of acylating benzene is a phenylketone.
    • Benzene is cyclic, planar 120 bond angle, molecule with the formula c6h6 (structure)
      Carbon has 4 outer electrons, each carbon is bonded to 2 other carbons and 1 hydrogen atom. The final lone electron is in the p-orbital which sticks out above and below the planar ring (2 regions of high electron density)
      Bonding: (3 bonds, 1 delocalised electrons per C)
      P-orbitals overlap to form a delocalised ring of electrons (pi orbital)
      Due to delocalised electron structure all the C-C bonds are the same (same bond length and strength)
    • Why the Kekule model was wrong
      1. C-C bonds are not the same length
      2. Kekule should be able to undergo electrophilic addition
      3. the enthalpy change of hydrogenation expected to be 360kj mol -1 ( if the Kekule structure was correct, benzene should have 3x the enthalpy of hydrogenation as cyclohexene since it has 3x double bonds. Its actual enthalpy of hydrogenation is much less exothermic, suggesting that the structure is more stable than the structure proposed by Kekule.)
      (difference) 152 (more stable)
      less energy released
      more energy to break reactant bonds
    • Benzene is stable due to the delocalised ring of electrons so is unreactive
      The increase in stability connected to delocalisation is called the delocalisation energy
    • Explain the structure and bonding of benzene
      every C has 3 bonds, spare electrons in p orbitals overlap ( to form a pi cloud), c
      planar, 120 degree bond angle, 6 carbon ring, C-C bond equal in length
      expected delta H hydrogenation of cyclohexatriene = -360
      delta hydrogenation difference in 152
      benzene is lower in energy than cylcohexatriene/ more stable
    See similar decks