AROMATIC

• Aromatic compound


  Consisting of a ring of six carbon atoms with six hydrogen atoms and a ring of delocalised electrons
• Benzene


  • Each bond in the benzene ring has an intermediate length in between that of a double and single bond
  • The outer electron from the p-orbital of each carbon atom is delocalised to form the central ring
  • The ring structure makes benzene very stable compared to other molecules of a similar size
• Cyclohexatriene vs. Benzene


  When benzene was first discovered, it was predicted to have a structure similar to cyclohexatriene, with three double bonds and three single bonds
• The enthalpy change of hydrogenation for benzene was predicted to be -360kJmol-1, three times the enthalpy change of cyclohexene
• It was later discovered that the enthalpy change of hydrogenation of benzene was actually -208kJmol-1, leading to the conclusion of its different, unusual structure
• Arenes


  Compounds that contain benzene as part of their structure
• Arenes


  • They have high melting points due to the high stability of the delocalised ring, but low boiling points as they are non-polar molecules and often cannot be dissolved in water
• Electrophilic substitution


  The delocalised ring in benzene is an area of high electron density making it susceptible to attack from electrophiles. When these species attack the electron ring, it is partially destroyed then restored in the process of electrophilic substitution.
• Electrophilic substitution mechanism


  1. Electrophile attacks

  2. Partially destroys ring

  3. Restores ring
• Nitrobenzene


  The electrophile is the NO2+ ion, a reactive intermediate produced in the reaction of concentrated sulfuric acid (H2SO4) with concentrated nitric acid (HNO3)
• Nitrobenzene formation mechanism


  1. Electrophile (NO2+) attacks benzene ring

  2. Removes hydrogen ion
• Mono-substitution of a single NO2+ electrophile occurs at 55°C, higher temperatures can lead to multiple substitutions</b>
• Friedel-Crafts acylation


  The delocalised electron ring in benzene can act as a nucleophile, leading to the attack on acyl chlorides. This requires a reactive intermediate produced from the acyl chloride and an aluminium chloride catalyst.
• Friedel-Crafts acylation mechanism


  1. Acyl chloride and AlCl3 catalyst form reactive intermediate

  2. Reactive intermediate attacked by benzene ring
• The H+ ion removed from the ring reacts with the AlCl4- ion to reform the aluminium chloride, showing it to be a catalyst
• Phenylketone


  The product of Friedel-Crafts acylation, where the benzene group is called a phenyl group
• Phenylketones are commonly used in the industrial production of dyes, pharmaceuticals and even explosives