Aromatic Hydrocarbons

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Aromatic Hydrocarbons

• (Chapter 32)

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Aromatic Hydrocarbons
Aroma Ring of sp2 carbon atoms (4n2) ?
delocalized bonding electrons
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Discovery of the structure of benzene
Molecular Formula C6H6 Structural Formula
?
Possible structure A CH2CH-C?C-CHCH2 B
CH3-C?C-C?C-CH3
A is ruled out because only ONE monosubstituted
chlorobenzene.
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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!
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Discovery of the structure of benzene
Kekule(1865) Chemist and Dreamer one of
the snakes has seized hold of its own tail .
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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

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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)

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Stability of the Benzene Ring
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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.
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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

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Benzenonium ion

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Electrophilic Aromatic Substitution
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Electrophilic Aromatic Substitution
Reaction Reagent Catalyst Product E or E
Halogenation X2(Cl,Br) FeX3 ArCl, ArBr X
Nitration HNO3 H2SO4 ArNO2 NO2
Sulphonation H2SO4 None ArSO3H SO3
Alkylation RX(Cl,Br) AlX3 Ar-R R
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Formation of E

• X2 FeX3 gt X FeX4-
• H2SO4 HONO2 gt HSO4- H2ONO2 gt
• H2O
  NO2
• 2H2SO4 gt HSO4- H3O SO3
• RX AlX3 gt R AlX4-

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Reversible nature of sulphonation
SO3H
c.H2SO4 80oC, reflux
Benzenesulphonic acid
SO3H
H2O, H2SO4 Tgt100oC
H2SO4
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Oxidation of alkylbenzene
COOH
COO-
CH3
KMnO4/OH- heat
H3O
KMnO4/OH- is better than KMnO4/H As C6H5COO-
acts as a solvent that mix C6H5CH3 and MnO4-
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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)