Chapter 12: Aromatic Chemistry

Aromatic hydrocarbon are a family of unsaturated organic molecules based on rings of carbon atoms. The most important hydrocarbon is benzene.

Bonding in the benzene molecule

The benzene ring has a planar structure. Benzene has a planar cyclic structure consisting of a ring of carbon atoms, each with a single hydrogen atom attached that sticks out into a flat plane. Benzene has a delocalized electron system, arising due to the overlap of one p-orbital from each carbon atom, above and below the plane of the ring.

Electrons are shared evenly, so all bond lengths are the same, between that of a single bond and a double bond.

Electrophilic Substitution of Benzene

  • The benzene ring is a region of high electron density, which means it attracts electrophiles
  • Nitration of benzene: C6H6 + HNO3 → C6H5NO2 + H2O
    • Generation of the electrophile:

      • HNO3 + H2SO4 → H2NO3+ +  HSO4 → H2O + NO2+ + HSO4

Halogenation of benzene ring

The reaction of benzene with chlorine requires an aluminum chloride catalyst. C6H6 + Cl2 → C6H5Cl + HCl

Stage 1: The aluminum chloride catalyst generates the electrophile from chlorine. Cl2 + AlCl3 → AlCl4 + Cl+

Stage 2: The electrophile reacts with the benzene molecule.

Stage 3: The hydrogen ion reacts with the AlCl4 , reforming the AlCl3 catalyst.

AlCl4 + H+ → AlCl3 + HCl

Substitution – nitration

 The reaction of benzene to form nitrobenzene requires a mixture of concentrated nitric acid and concentrated sulfuric acid to generate the electrophile.

Stage 1: The electrophile is generated from concentrated nitric acid and concentrated sulfuric acid. H2SO4+ HNO3 → H2O + NO2+ + HSO4

Stage 2: The NO2+ electrophile reacts with the benzene molecule in electrophilic substitution.

Stage 3: The hydrogen ion reacts with the HSO4 , reforming the H2SO4 catalyst.

 H+ + HSO4 → H2SO4

Acylation

Acylation of benzene involves the substitution of an acyl group. This reaction takes place when benzene is reacted with ethanoyl chloride in the presence of an aluminum chloride catalyst.

Stage 1: The electrophile is generated from ethanoyl chloride and aluminum chloride.

 CH3COCl + AlCl3 → AlCl4 – + CH3CO+

Stage 2: The CH3CO+ electrophile reacts with the benzene molecule.

CH3CO+ + C6H6 → C6H5COCH3 + H+

 Stage 3: The aluminum chloride catalyst is regenerated. H + + AlCl4 – → AlCl3 + HCl

 Alkylation

 Alkylation of benzene involves the substitution of an alkyl group. The reaction occurs in the same way as the acylation reaction takes place above. It requires a halogenoalkane as a reactant as they contain a polar bond between the carbon and halogen.

Stage 1: CH3Cl + AlCl3 → AlCl4 – + CH3 +

Stage 2: CH3 + + C6H6 → C6H5CH3 + H+

 Stage 3: H+ + AlCl4 – → AlCl3 + HCl

Oxidation of carbon containing side chain

If an alkyl group is attached to benzene, it can be easily oxidized to form benzoic acid. They can be oxidized using the oxidizing agent, potassium manganate (VII).