Aromatic

• Benzene has a relative atomic mass of 78.0.
• The empirical formula of benzene is CH.
• The molecular formula of benzene is C6H6.
• Kekule's stucture of benzene was cyclohexa-1,3,5-triene. The evidence against this was:

  • Benzene does not decolourise bromine water but the double bonds should turn it from orange to colourless.
  • All the C-C bonds in benzene are the same length, between the length of a short double bond and a long single bond.
  • Thermochemical data for the hydrogenation of benzene suggest it is more stable than the Kekule structure.
• Structure of benzene:

  • Planar structure, with each atom having 3 covalent bonds.
  • All bond angles are 120.
  • C-C bonds are intermediate in length between a single C bond and a double.
  • Each C atom has one spare electron in a p-orbital
  • Overlap of adjacent p-orbitals forms a pi bond above and below the ring (essentially an electron cloud). The electrons in the pi-bond are delocalised.
  • Delocalised structure has extra stability so the pi bonds are harder to break than a normal C-C bond.
• Electrons become delocalised if they are spread out over more than one atom or bond. Cyclohexa-1,3,diene has a less exothermic enthalpy of hydrogenation than cyclohexa-1,4-diene because the p-orbitals ae only separated by 1 bond so they can overlap. This leads to delocalisation which provides stability. The more stable the structure, the less exothermic the enthalpy of hydrogenation.
• Benzene in name:

  • chlorobenzene
  • nitrobenzene
  • methylbenzene
  • benzoic acid
• Phenyl in name:

  • phenol
  • phenyl amine
  • phenyl ethene
  • phenyl ethanoate
• The pi-bond above and below the plane of the ring is an area of high electron density that attracts electrophiles. Benzene undergoes substitution rather than addition to preserve the stability of the delocalised benzene ring.
• Nitration (adding an NO2 group):

  • Reagents: conc. HNO3, conc H2SO4 (catalyst)
  • Conditions: 50oC 9to prevent formation of other nitro compounds)
  • Generation of electrophile: H2SO4 + HNO3 -> NO2+ + HSO4- + H2O
  • Regeneration of catalyst: HSO4- + H+ -> H2SO4
• methylbenzene is also known as 'toluene'.
• Acylation:

  • Reagents: benzene, RCOCl, AlCl3 anhydrous catalyst (halogen carrier).
  • Conditions: heat
  • Generation of the electrophile: AlCl3 + RCOCl -> RCO+ + AlCl4-
  • Regeneration of catalyst: AlCl4- + H+ -> AlCl3 + HCl
• Alkylation:

  • General equation: benzene + CH3Cl -> methylbenzene + HCl
  • Generation of electrophile: CH3Cl + AlCl3 -> CH3+ + AlCl4-
  • Regeneration of catalyst: H+ + AlCl4- -> HCl + AlCl3
• Halogenation:

  • General equation: benzene + X2 -> X-benzene + HCl
  • Generation of electrophile: X2 + AlCl3 -> X+ + AlCl4-
  • Regeneration of catalyst: H+ + AlCl4- -> AlCl3 + HCl
• Reactions in cyclohexene and benzene are different because in benzene the pi electrons are delocalised so the pi electron density is lower than in cyclohexene and electrophiles are attracted/polarised less.
• The methyl group of methylbenzene releases electrons (positive inductive effect). The pi electron density is higher than in benzene so electrophiles are attracted/polarised more.
• Benzene-NO2 can be used for explosives.
• The nitro group in a nitrobenzene can be reduced to form an amine using Sn and conc. HCl.

  Equation: Benzene-NO2 + 6[H] -> benzene-NH2 + 2H2O

  6[H] and 2H2O for each NO2 group.

  Uses: dyes and pharmaceuticals