COURSE NAME: Chemistry of Aromatic Compounds COURSE CODE: 4022142-3
Chapter
INTRODUCTION 1
By the end of this chapter, you should understand:
1. The definition of aromatic compounds.
2. How to differentiate between the aromatic, anti-aromatic
and non-aromatic compounds.
The classification of organic compounds is based on the structure of molecules.
3
Introduction
Aromatic Compounds
Unsaturated cyclic molecules which have additional
stability due to the arrangement of π- electrons
5
Kekule `s structure of benzene
* Benzene is a cyclic hexagonal planer structure of six c- atoms with alternating single and double bonds.
* Benzene is 1,3,5-cyclohexatriene.
* Benzene is existed in two resonating structures.
Structure of Benzene
X
X
In the light of the proposed structure of benzene, it should give two ortho disubsitituted isomers and in reality only one isomer is observed.
X
X
The two ortho substituents are attached to one double bond
The two ortho substituents are attached to one single bond
Objection
7
Resonance theory
“The benzene ring can be represented by two equivalent resonating structures and the truly structure is the hybrid one” The hybrid structure can explain the formation of single ortho-disubstituted isomer.
Resonance hybrid structure of benzene
* The resonance theory also explains why the six carbon-carbon bonds are equivalents; the value of each c-c bond is shorter than normal single bond and longer than the double bond
1.54 Ao 1.34 Ao 1.40 Ao
Single bond C-C bond in Double bond
benzene
* It also explains the high stability of benzene towards addition 1.34
1.40
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H H
+ H2 catalyst H = - 26.5 to 30.0 kcal/mole
The resonance theory also explained the high stability of benzene ring due to the delocalization of π-electrons.
H2
H = 28.6 kj/mol
Observed H = 28.6 kj/mol
2H2
H = 57.2 kj/mol
Observed H = 55.4 kj/mol
3H2
H = 85.8 kj/mol
Observed H = 49.8 kj/mol Resonance Energy = 85.8 - 49.8 = 36 kj/mol
Delocalization of pi-electrons
Molecular orbital description
Criteria for Aromaticity
11
Huckel’s Rule
The aromatic compounds must contain (4n+2) p electrons (n = integer number, 0, 1, 2, …..)
p electrons = 6 = 4n + 2 n = 1 (integer number)
Other Aromatic systems
N N
Quinoline IndoleH
10 p electrons Naphthaline
N N
H Pyridine
S O
Pyrrole Thiophene
6 p electrons N
N
Pyrimidine Furan
The features of Antiaromatic compounds
•Cyclic, planar, conjugated system of p-electrons (double, single, double, single, ……….)
•contain (4n) p-electrons (n = integer number 1, 2, 3, …..).
H H H H
H
H H H
Cyclopentadiene Non-aromatic
Cyclopentadienyl anion Aromatic (p-electrons = 6)
Cyclopentadienyl cation Antiaromatic (p-electrons = 4)
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If the compounds did not fulfill the requirements of
aromatic or antiaromatic characters it will be Non-aromatic Compounds
Annulenes: Monocyclic hydrocarbons with conjugated system.
A prefix in brackets indicates the number of carbons in the ring.
