Chemistry, B.Sc. Sem-Ist Session-2 Chapter-4 Aromatic Hydrocarbon

1. 1
Late Ku. Durga K. Banmeru Science College,
LONAR DIST. BULDANA (Maharashtra), India.
Section –II
Unit-4
Aromatic Hydrocarbons
“Structure of Benzene”
B. Sc. Ist year Sem-Ist
Subject:- Chemistry

2. 2
Dr. Suryakant B. Borul
(M.Sc., M.Phil., Ph.D.)
Head Of Department
Department of Chemistry
Late Ku. Durga K. Banmeru Science College,
Lonar
Teacher Profile

3. Introduction-
 The word “aroma” means fragrance that is pleasant smell.
 Benzene is the parent hydrocarbon of aromatic compounds and aromatic
hydrocarbons are those which contain one or more benzene rings.
 These are also called as arenes.
“AROMATIC HYDROCARBONS”
CH3
Benzene Naphthalene Anthracene
Phenanthracene Toluene
For Example-

4.  Structure of Benzene
The structures benzene suggested by German chemist, August Kekule is the
most accepted.
According to Kekule, structure of Benzene molecule is made up of
hexagone of six carbon atoms joined alternately by single and double with one
hydrogen atom attached to each carbon.
H
H
H
H
H
H
Kekule structure of Benzene

5. Evidence in support of structure of Benzene
proposed by Kekule
1) Molecular Formula-
From elemental analysis, percentage composition and molecular weight
determination, the molecular formula of benzene is C6H6.
2) Cyclic Structure-
on catalytic hydrogenation, benzene yield cyclohexane. Formation of
cyclohexane inducts that all the carbon atoms are present in cyclic form i.e.
benzene must be six membered ring compound.
C6H6 + 3H2
Ni catalyst
100oC
C6H12
Cyclohexane
Benzene

6. 3) Position of Hydrogen-
Benzene gives only one mono-substituted product and three di-substituted
products. This indicated that all the six hydrogen are equivalent. Keeping apart
the fourth valency of each carbon, structure of benzene, mono-substituted
product and di-substituted products are as follows;
X
X
Y
X
Y
X
Y
Benzene Monosubstituted derivatives
Disubstituted derivatives

7. 4) Presence of three double bonds-
Benzene adds on three molecules of chlorine to give benzene hexachloride
(BHC). It is used as powerful insecticide.
C6H6 + 3Cl2
Ni catalyst
100oC
C6H12Cl6
H Cl
H
Cl
H
Cl
HCl
Cl
H
Cl
H
Benzene hexacholoride ( BHC)
Benzene

8. 5) Position of double bonds-
Presence of three alternate double bonds can be proved by ozonolysis
reaction. When Benzene reacts with ozone which to form triozonide , which on
hydrolysis gives three molecules of glyoxal.
H
H
H
H
H
H
+ 3O2 O
O
O
O
O
O
O
O
O
H
H
H
H
H
H
Hydrolysis
CHO
CHO
3 + OH23
Benzene triozonide
Glycoxal

9. 6) Conformation of structure by synthesis-
Benzene molecule can be further confirmed by its synthesis from acetylene.
Benzene is obtained by passing acetylene gas through red hot tube.
CH
CH
CH
CH
CH
CH
+
Red hot tube
Acetylene
Benzene

10. Objections to Kekule’s Structure
Structure of benzene as proposed by Kekule give two possible ortho
disubstituted products as shown;
1, 2-Disubstituted deriv ativ es
X
Y
X
Y
1 2
3
4
5
6
1
2
3
4
5
6
1, 6-Disubstitutedderivatives
However, in actual practice, only one ortho disubstituted product is
obtained. Kekule himself discarded this objection by saysing that the position of
double bonds is not fixed but double bonds are mobiles.