Aromatic Hydrocarbons

Aromatic Hydrocarbons (Chapter 32)

Aromatic Hydrocarbons Aroma Ring of sp2 carbon atoms (4n+2) π delocalized bonding electrons CH3

Benzene

Methylbenzene

Napthalene

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 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 benzene Kekule(1865): Chemist and Dreamer …… one of the snakes has seized hold of its own tail …….

H H

C C

H C C H

C C

H H

Cl

Cl

Cl Cl

Cl

Cl

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 benzene Delocalization of Bonding Electrons •3e- in sp2 hybrids form normal covalent bonds •4th e-, one from each carbon atoms are free to visit Delocalized π-bond system 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

Reactions of Benzene •No reaction with Br2/dark •No reaction with MnO4-/H+ Benzene

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 General Mechanism: •Formation of Electrophile, E+ •Attack of E+ on π electron cloud of benzene •Loss of a proton

E+

H E

r.d.s +

-H+

E

Benzenonium ion

H E

H E

+

+ OR

+

H E sp3

+

H E

Electrophilic Aromatic Substitution H +

H E

+ E+ E +H+ Reaction coordinate

Electrophilic Aromatic Substitution Reaction

Reagent

Catalyst

Product

E+ or E

Halogenation X2(Cl,Br)

FeX3

ArCl, ArBr

X+

Nitration

H2SO4

ArNO2

+

None

ArSO3H

SO3

Ar-R

R+

HNO3

Sulphonation H2SO4 Alkylation

RX(Cl,Br) AlX3

NO2

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 c.H2SO4 80oC, reflux SO3H

H2O, H2SO4 T>100oC

SO3H Benzenesulphonic acid + H2SO4

Eact ≈ Eact* H +

H SO3-

+ SO3

SO3+H+ Reaction coordinate

k-1 k2 H + ArH + SO3  Ar+ - → ArSO3 + H SO3 k1

k-1 ≈ k2

Oxidation of alkylbenzene COO-

CH3 KMnO4/OHheat

COOH H3O+

KMnO4/OH- is better than KMnO4/H+ As C6H5COO- acts as a solvent that mix C6H5CH3 and MnO4-

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. CH2CH2CH2R

CR3

COOH

No reaction (no H on C)