Aromatic Chemistry

Aromatic Chemistry

1. Structure of Benzene

   • C6H6
   • 6 electrons form a delocalised π bond above and below the plane of the atoms
   • Evidence for this can be seen in the enthalpy values for the hydration of benzene compared to the hydration of cycloalkenes

2. Physical Properties

   • Liquid at room temperature
   • Clear colourless
   • Immiscible/insoluble in water
   • Soluble in non-polar solvents
   • Very high melting point compared to C6H14
     • This is because it is a flat molecule and so when it is solid, the molecules can pack very close together, and so the intermolecular forces are much stronger
   • Very similar boiling point to C6H14

3. Reactions of Arenes

   • Combustion
     • e.g. C6H6 + 7.5O2 → 6CO2 + 3H2O
     • Unlikely to be complete combustion due to low C:H ratio
     • Lots of soot produced
   • Electrophilic Substitution
     • The delocalised ring is an area of high electron density
     • Electrophiles are attracted to the delocalised ring
   • Nitration
     • Formation of Nitronium ion electrophile:
       • H2SO4 + HNO3 → H2NO3+ + HSO4-
         • H2NO3+ → H2O + NO2+
     • H2SO4 acts as a catalyst
       • The H+ ion produced reacts with the HSO4- ion to reform H2SO4
     • Overall Reaction:
       • C6H6 + HNO3 → C6H5NO2 + H2O
   • Friedel-Crafts Acylation
     • Electrophilic Substitution
     • Requires a RCO+ ion electrophile
       • Formed by the reaction of AlCl3 with an acyl chloride
     • H+ + AlCl4- → AlCl3 + HCl
       • As the AlCl3 is regenerated, it has acted as a catalyst

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