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Aromatic Hydrocarbons

Aromatic Hydrocarbons. (Chapter 32). CH 3. Napthalene. Benzene. Methylbenzene. Aromatic Hydrocarbons. Aroma Ring of sp 2 carbon atoms (4n+2)  delocalized bonding electrons. Discovery of the structure of benzene. Molecular Formula: C 6 H 6 Structural Formula: ?.

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Aromatic Hydrocarbons

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  1. Aromatic Hydrocarbons (Chapter 32)

  2. CH3 Napthalene Benzene Methylbenzene Aromatic Hydrocarbons Aroma Ring of sp2 carbon atoms (4n+2)  delocalized bonding electrons

  3. Discovery of the structure of benzene Molecular Formula: C6H6 Structural Formula: ? Possible structure: A CH2=CH-CC-CH=CH2 B CH3-CC-CC-CH3 A is ruled out because only ONE monosubstituted chlorobenzene.

  4. Discovery of the structure of benzene Evidence: There are THREE disubstituted isomers of C6H4Cl2. B (CH3-CC-CC-CH3 )is ruled out too!

  5. Cl Cl H Cl C H H Cl C C C C Cl H H C Cl H Discovery of the structure of benzene Kekule(1865): Chemist and Dreamer …… one of the snakes has seized hold of its own tail …….

  6. Discovery of the structure of benzene • Objections to Kekule Benzene: • A 4th isomer of C6H4Cl2 • Surprising stability of benzene • All the C-C bonds are the same length

  7. Delocalized -bond system Discovery of the structure of benzene • Delocalization of Bonding Electrons • 3e- in sp2 hybrids form normal covalent bonds • 4th e-, one from each carbon atoms are free to visit • all the atoms on the ring (delocalized bonding • electrons)

  8. Enthalpy kJ/mol Resonance Energy 150.4 -358.8 -208.4 Cyclohexane Reaction co-ordinate Stability of the Benzene Ring

  9. Benzene Reactions of Benzene • No reaction with Br2/dark • No reaction with MnO4-/H+ Although benzene is unsaturated, it is resistant to oxidation and addition reaction because of the energy of delocalization is lost in such reaction.

  10. Electrophilic Aromatic Substitution • General Mechanism: • Formation of Electrophile, E+ • Attack of E+ on  electron cloud of benzene • Loss of a proton H r.d.s -H+ E E+ E +

  11. + H H H + E E E H OR + E sp3 Benzenonium ion +

  12. H + E + E+ H E +H+ Reaction coordinate Electrophilic Aromatic Substitution

  13. Electrophilic Aromatic Substitution

  14. Formation of E+ • X2 + FeX3 => X+ + FeX4- • H2SO4 + HONO2 => HSO4- + H2O+NO2 => • H2O + +NO2 • 2H2SO4 => HSO4- + H3O+ + SO3 • RX + AlX3 => R+ + AlX4-

  15. Reversible nature of sulphonation SO3H c.H2SO4 80oC, reflux Benzenesulphonic acid SO3H H2O, H2SO4 T>100oC + H2SO4

  16. Eact Eact* H + SO3 + +H+ SO3- Reaction coordinate H SO3- k-1 k2 H ArH + SO3 Ar+ ArSO3- + H+ k-1  k2 SO3- k1

  17. Oxidation of alkylbenzene COO- COOH CH3 KMnO4/OH- heat H3O+ KMnO4/OH- is better than KMnO4/H+ As C6H5COO- acts as a solvent that mix C6H5CH3 and MnO4-

  18. Oxidation of alkylbenzene Oxidation takes place at first by the abstraction of a phenylmethyl hydrogen by the O.A., once take place, oxidation continues until a carboxyl group is left. COOH CH2CH2CH2R CR3 No reaction (no H on C)

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