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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 Hydrocarbons

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Aromatic hydrocarbons l.jpg

Aromatic Hydrocarbons

(Chapter 32)


Aromatic hydrocarbons2 l.jpg

CH3

Napthalene

Benzene

Methylbenzene

Aromatic Hydrocarbons

Aroma

Ring of sp2 carbon atoms

(4n+2)  delocalized bonding electrons


Discovery of the structure of benzene l.jpg

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.


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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!


Discovery of the structure of benzene5 l.jpg

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 …….


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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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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 l.jpg

Enthalpy

kJ/mol

Resonance

Energy

150.4

-358.8

-208.4

Cyclohexane

Reaction co-ordinate

Stability of the Benzene Ring


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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.


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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

+


Benzenonium ion l.jpg

+

H

H

H

+

E

E

E

H

OR

+

E

sp3

Benzenonium ion

+


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H

+

E

+ E+

H

E

+H+

Reaction coordinate

Electrophilic Aromatic Substitution


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Electrophilic Aromatic Substitution


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Formation of E+

  • X2 + FeX3 => X+ + FeX4-

  • H2SO4 + HONO2 => HSO4- + H2O+NO2 =>

  • H2O + +NO2

  • 2H2SO4 => HSO4- + H3O+ + SO3

  • RX + AlX3 => R+ + AlX4-


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Reversible nature of sulphonation

SO3H

c.H2SO4

80oC, reflux

Benzenesulphonic acid

SO3H

H2O, H2SO4

T>100oC

+ H2SO4


Slide16 l.jpg

Eact Eact*

H

+ SO3

+

+H+

SO3-

Reaction coordinate

H

SO3-

k-1

k2

H

ArH + SO3 Ar+ ArSO3- + H+ k-1  k2

SO3-

k1


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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-


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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)


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