6.1 Benzene and aromatic compounds

• Benzene


  C6H6, highly unsaturated, member of cyclic molecules with similar properties called aromatic molecules | arenes
• Benzene


  • Delocalised π bond model
  • Kekule model
• Benzene bonding


  1. Each C forms 3σ bonds

  2. 6 π orbitals overlap sideways in both directions forming delocalised π bond system above and below plane of atoms

  3. C-C bond lengths intermediate between single and double
• Kekule model


  Alternating single and double C-C bonds, localised π bonds, p orbitals overlap sideways in one direction, bond lengths alternate short double and long single
• Delocalised π bond system

  Low energy, stable
• Evidence for delocalised π bond system in benzene:
• Enthalpy change of hydrogenation for cyclohexene to cyclohexane

  • 120 kJ/mol
• Predicted enthalpy change of hydrogenation for benzene

  • 360 kJ/mol
• Actual enthalpy change of hydrogenation for benzene

  • 208 kJ/mol
• Benzene unreactive due to high stability of delocalised π system and low electron density
• Electrophilic substitution in benzene

  • Only strong electrophiles react (Br+)
  • Mononitration with NO2+ from conc. HNO3 and conc. H2SO4 catalyst
  • Monohalogenation with halogen carrier catalyst like AlBr3
• Mononitration of benzene


  1. Formation of nitronium ion NO2+ from HNO3 and H2SO4

  2. Electrophilic substitution of NO2+ onto benzene ring

  3. Reformation of catalyst H+ + AlBr4- → HBr + AlBr3
• Monohalogenation of benzene


  1. Formation of electrophile Br+ from Br2 and AlBr3

  2. Electrophilic substitution of Br+ onto benzene ring

  3. Reformation of catalyst H+ + AlBr4- → HBr + AlBr3
• Catalyst must be anhydrous, as water can donate lone pair into Al orbital in AlX3 preventing catalyst
• Alkenes react rapidly with bromine, decolourising bromine water, due to high electron density of C=C bond polarising Br2 and attracting electrophilic Br
• Friedel-Crafts alkylation


  Alkyl group attached to benzene ring using haloalkane and anhydrous halogen carrier catalyst like AlBr3
• Friedel-Crafts alkylation


  1. Formation of alkyl electrophile from haloalkane and catalyst

  2. Electrophilic substitution of alkyl group onto benzene ring

  3. Reformation of catalyst H+ + AlBr4- → HBr + AlBr3
• Phenol


  • Aromatic hydroxyl -OM directly
  • Hatched to Benzene ring C6H5OH solid rtp
  • Weak acids-partially dissociate H+
• Phenol dissociation


  C6H5OH(aq) = C6H5O- + H+
• Phenoxide


  Phenol dissociation product
• Phenoxide salts


  • Form with strong aqueous bases
  • Only with reactive group one metals
  • Phenols do not react with carbonates as they are weak bases
• Phenols vs carboxylic acids

  Phenols are not strong enough acids to react with carbonates and produce CO2 bubbles
• Phenols vs alcohols


  Phenols do not readily react with carboxylic acids to form phenyl esters, acyl chlorides/acid anhydrides are used instead
• Phenol reaction with bromine

  C6H5OH(aq) + 3Br2(aq) → C6H2Br3OH + 3HBr
• Phenol-bromine reaction


  • Decolorises bromine and forms a white precipitate
  • No halogen carrier catalyst needed
• Phenol reaction with dilute nitric acid

  C6H5OH + HNO3 → 2-nitrophenol + 4-nitrophenol + H2O
• Phenol reactivity


  • More reactive than benzene
  • Lone pair of electrons from O atom donated to benzene ring
  • Occupies p-orbital that overlaps with delocalised π electrons around benzene ring
  • Increases reactivity and activates the ring
  • Electron density donated by sideways overlap of lone pair from O
  • Higher electron density π system, stronger ability to polarise Br2 forming induced dipole