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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 hydrocarbons2

CH3

Napthalene

Benzene

Methylbenzene

Aromatic Hydrocarbons

Aroma

Ring of sp2 carbon atoms

(4n+2)  delocalized bonding electrons

discovery of the structure of benzene
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.

discovery of the structure of benzene4
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!

discovery of the structure of benzene5

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 …….

discovery of the structure of benzene6
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
discovery of the structure of benzene7

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)
stability of the benzene ring

Enthalpy

kJ/mol

Resonance

Energy

150.4

-358.8

-208.4

Cyclohexane

Reaction co-ordinate

Stability of the Benzene Ring
reactions of benzene

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.

electrophilic aromatic substitution
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

+

benzenonium ion

+

H

H

H

+

E

E

E

H

OR

+

E

sp3

Benzenonium ion

+

electrophilic aromatic substitution12

H

+

E

+ E+

H

E

+H+

Reaction coordinate

Electrophilic Aromatic Substitution
formation of e
Formation of E+
  • X2 + FeX3 => X+ + FeX4-
  • H2SO4 + HONO2 => HSO4- + H2O+NO2 =>
  • H2O + +NO2
  • 2H2SO4 => HSO4- + H3O+ + SO3
  • RX + AlX3 => R+ + AlX4-
reversible nature of sulphonation
Reversible nature of sulphonation

SO3H

c.H2SO4

80oC, reflux

Benzenesulphonic acid

SO3H

H2O, H2SO4

T>100oC

+ H2SO4

slide16

Eact Eact*

H

+ SO3

+

+H+

SO3-

Reaction coordinate

H

SO3-

k-1

k2

H

ArH + SO3 Ar+ ArSO3- + H+ k-1  k2

SO3-

k1

oxidation of alkylbenzene
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-

oxidation of alkylbenzene18
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)