Aromatic

• Kekulé proposed that benzene was a ring of alternating single and double carbon-carbon bonds
• Benzene has the molecular formula of C6H6 and is highly unsaturated (multiple double bonds)
• Why Kekulé was wrong about benzene

  • They do electrophilic substitution instead of addition
  • If forms one structure with chlorine rather than two isomers
  • All bonds are the same lengths rather than single bonds being longer
  • The enthalpy of hydrogenation is less exothermic showing benzene is more stable
• Benzene has a planar hexagonal structure. Each carbon has 3 bonds and the spare electrons from the p orbital delocalise and overlap
• The enthalpy of hydrogenation of cyclohexatriene is expected to be -360 kJmol, but is actually less exothermic. Shows that benzene is more stable
• Benzene is fairly unreactive, unlike other unsaturated compounds such as alkenes
• Aromatic compounds react with electrophiles as the ring is electron rich. This results in a electrophilic substitution reaction
• Electrophilic substitution conditions

  • Concentrated HNO3
  • Concentrated H2SO4
  • 50 degrees Celsius
  • HNO3 + 2H2SO4 --> NO2+ + 2HSO4- + H3O+
  • NO2+ acts as electrophile
• Naming aromatic compounds

  • It is a benzene if other functional groups are attached to the ring
  • It is a phenyl group if C6H5 is attached to a longer chain such as an alkane or ester
  • One OH group attached to benzene is called phenol
  • When a COOH group is attached to benzene it is called benzenecarboxylic acid
• Aromatic hydrocarbons are called arenes
• The nitro group in benzene can be reduced to produce aromatic amines, which are used to make synthetic dyes
• Benzene is reduced to cyclohexane in the presence of hydrogen and a nickel catalyst
• Nitro is reduced to amines in the presence of tin or iron catalyst and HCl
• Nitrile reduced to amine in the presence of hydrogen and nickel catalyst or LiAlH4 or NaBH4
• Friedel-Crafts acylation

  • Acyl chloride/acid anhydride reacts with benzene
  • AlCl3 catalyst
  • Anhydrous conditions
  • Electrophilic addition reaction
  • Catalyst reacts with reagent to form the acylium ion (RCO+)