Lecture sheet-1
Instructor: Dr. Biplab Kumar Das
Assistant Professor, Pharmacy, NSU
Aromatic compounds: Benzene and its derivatives
The term aromatic compounds stands for the whole series of compounds,
which contain one or more benzene rings in their molecule. With the
introduction of the new name of ‘Arenes’ for all aromatic hydrocarbons
(Benzene, Naphthalene, Anthracene, etc.), a precise definition may be
given as arenes and their derivatives.
Discovery of Benzene
•Isolated in 1825 by Michael Faraday who determined C: H ratio to be 1:1.
•Synthesized in 1834 by Eilhard Mitscherlich who determined molecular
formula to be C6H6.
•Other related compounds with low C: H ratios had a pleasant smell, so
they were classified as aromatic.
Structure of Benzene
The structure of benzene has been derived as follows:
(1) Molecular formula: Elemental analysis and, molecular weight
determination showed that benzene had the molecular formula C6H6. This
indicated that benzene was a highly unsaturated compound (compare it
with n-hexane, C6H14).
(2) Straight-Chain Structure Not Possible. Benzene could be constructed
as a straight chain or ring compound having double (C=C) and/or triple
bonds. But benzene did not behave like alkenes or alkynes. It did not
decolorize bromine in carbon tetrachloride or cold aqueous potassium
permanganate. It did not add water in the presence of acids.
Br2/CCl4
No reaction
Dilute cold
Benzene
No reaction
KMnO4
H2O/K+
No reaction
He a t
(3) Evidence of cyclic structure. (a) Substitution of Benzene. Benzene
reacted with bromine in the presence of FeBr3 (catalyst) to form
bromobenzene. The fact that only one monobromo and no isomeric
products were obtained indicated that all six hydrogen atoms in benzene
were identical. This could be possible only if benzene had a cyclic
structure of six carbons and to each carbon was attached one hydrogen.
(b) Addition of Hydrogen. Benzene added three moles of hydrogen in the
presence of nickel catalyst to give cyclohexane. This confirmed the cyclic
structure of benzene and also showed the presence of three carboncarbon double bonds.
(4) Kekule’s Structure for Benzene.
Proposed in 1866 by Friedrich Kekulé, shortly after
suggested. Benzene according to this proposal
cyclohexatriene.
multiple bonds were
was simple, 1,3,5-
H
H
C
C
C
H
C
C
H
H
C
H
=
>
There were two objections: (i) If the Kekule’s structure was correct, there
should exist two ortho isomers of dibromobenzene. In one isomer, the two
bromine atoms should be on carbons that are connected by a double
bond, as shown in the structure (a). In the other isomer, the bromines
should be on carbons connected by a single bond as in the structure (b).
But, only one ortho-dibromobenzene could be prepared.
Br
Br
2Br2
Br
Br
+
FeBr3
(a)
(b)
To overcome this objection, Kekule further suggested that benzene was a
mixture of two forms (1 and 2) in rapid equilibrium.
1
2
ii. Kekule’s structures failed to explain why benzene with three double
bonds did not give addition reactions like other alkenes. For example,
benzene did not react with HBr or Br2 in CCl4.
(5)Resonance Description of Benzene: Benzene can be represented by
two resonance structures 1, 2 and one resonance hybrid structure 3. The
resonance structures 1 and 2 are not actual structures of the benzene
molecule. None of these structures adequately represents the molecule.
All single bonds in 1 are double bonds in structure 2. That’s why the
hybrid structure is considered as the correct structure of benzene.
Resonance hybrid is more stable than any of its contributing structures.
For benzene, the stability due to resonance is so great that -bonds of the
molecule will normally resist breaking. This explains lack of reactivity of
benzene towards addition.
(6) Molecular Orbital Structure for Benzene: All six-carbon atoms in
benzene are sp2 hybridized. The sp2 hybrid orbitals overlap with each
other and with s orbitals of the six hydrogen atoms forming C-C and C-H
 bonds. Since the s-bonds result from the overlap of planar sp2 orbitals,
all carbon and hydrogen atoms in benzene lie in the same plane. All sbonds in benzene lie in one plane and all bond angles are 120°. Each sp2
hybridized C in the ring has an unhybridized p orbital perpendicular to the
ring, which overlaps around the ring.
=>
Stability of benzene and Resonance energy of benzene
Benzene’s special stability is due to the formation of the delocalized 
molecular orbital. The magnitude of this extra stability can be estimated
by measuring the changes in heat of hydrogenations that are associated
with reactions. Hydrogenation of cyclohexane evolves 28.6 kcal/mole, a
value typical for hydrogenation of alkenes.
Hypothetical 1,3,5-cyclohexatriene
+ 3H2
36 kcal extra stability
(resonance energy) of
benzene
1,3-cyclohexadiene
+
2H2
+ 2H2
Cyclohexene
+
H2
55.4 kcal
85.8 kcal
estimated
49.8 kcal
26.6 kcal
(Cyclohexane)
Figure: Heats of hydrogenation of one mole of some cyclic compounds
Hydrogenation of both double bonds of 1,3-cyclohexadiene evolves
54.4 kcal/mole, approximately double the amount observed for
cyclohexene. The molecule of 1,3,5-cyclohexatriene containing three
ordinary double bonds is hypothetical-any efforts to produce it yields
benzene. We would, however, expect complete hydrogenation of the
unknown 1,3,5-cyclohexatriene to evolve approximately 3x28.6 or
85.8 kcal/mole. But hydrogenation of benzene gives only 49.8
kcal/mole. This 36 kcal difference between the heat evolved in the
hydrogenation of benzene and that estimated for hydrogenation of a
compound with three ordinary double bonds is the added stability.
This added stability is sometimes called Resonance energy.
Resonance energy is a measure of how much more stable a
resonance hybrid structure is than its extreme resonance
structures.
Aromaticity (HUCKEL RULE)
The aromatic compounds undergo substitution reactions rather than
addition reactions. This characteristic behavior is called aromatic
character or aromaticity. Aromaticity is a property of the sp2
hybridized planar rings in which the p orbitals (one on each atom)
allow cyclic delocalization of  electrons. Aromatic compounds
apparently contain alternate double and single bonds in a cyclic
structure, and resemble benzene in chemical behavior. They undergo
substitution
Criteria for aromaticity
1. An aromatic compound is cyclic and planar.
2. Each atom in an aromatic ring has a p orbital. These p orbitals must be
parallel so that a continuous overlap is possible around the ring.
3. The cyclic p molecular orbital (electron cloud) formed by overlap of p
orbitals must contain (4n + 2)  electrons, where n = integer 1,2,3 etc. This
is known as Huckel rule.
Benzene
Aromatic
Cycloheptatriene
Non-aromatic
Ctclooctatetraene
Non-aromatic
Annulenes
•All cyclic conjugated hydrocarbons were proposed to be aromatic.
•However, cyclobutadiene is so reactive that it dimerizes before it can be
isolated.
•And cyclooctatetraene adds Br2 readily.
Energy Diagram for Benzene
• The six electrons fill three bonding pi orbitals.
• All bonding orbitals are filled (“closed shell”), an extremely stable
arrangement.
Aromatic Requirements: Aromaticity
•Structure must be cyclic with conjugated
pi bonds.
•Each atom in the ring must have an unhybridized p orbital.
•The p orbitals must overlap continuously around the ring. (Usually
planar structure)
•Compound is more stable than its open-chain counterpart.
Anti- and Nonaromatic
Antiaromatic and Nonaromatic compounds
•Antiaromatic compounds are cyclic, conjugated, with overlapping p orbitals
around the ring, but the energy of the compound is greater than its openchain counterpart.
•Nonaromatic compounds do not have a continuous ring of overlapping p
orbitals and may be nonplanar.
Hückel’s Rule
•If the compound has a continuous
4N + 2 electrons, it is aromatic.
•If the compound has a continuous
4N electrons, it is antiaromatic.
ring of overlapping p orbitals and has
ring of overlapping p orbitals and has
Physical and Chemical properties of Benzene
Physical:
1. Colorless liquid, bp 80.1 °C, mp 5.5 °C.
2. Insoluble in water and forms the upper of two layers when mixed,
miscible with alcohol, ether and chloroform.
3. A good solvent for many organic and inorganic substances e.g.,
fat, resins, sulphur and iodine.
4. Vapors are highly toxic which on inhalation produce loss of
consciousness. Poisoning in the long run can prove fatal, destroying
the red and white blood cells.
5. It burns with a bluish flame.
6. With its derivatives shown characteristic IP spectrum. Thw two
bands near 1600 cm-1 and 1500 cm-1 have been correlated with the
stretching of the carbon-carbon bonds of the aromatic ring. The sharp
bands bear 3030 cm-1 are caused by aromatic C-H bond.
Chemical reactions of benzene:
The principal types of reactions of benzene are:
# Electrophilic substitution reactions
# Addition reactions
# Oxidation reactions.
Pyridine
• Heterocyclic aromatic compound.
• Nonbonding pair of electrons in sp2 orbital, so weak base, pKb
= 8.8.
=
>
Pyrrole
Also aromatic, but lone pair of electrons is delocalized, so much
weaker base.
=>
Basic or Nonbasic?
N
N
N
N
N
N H
N
H
N
Pyrimidine has two basic
nitrogens.
Imidazole has one basic
nitrogen and one nonbasic
Purine?
Other Heterocyclics
Fused Ring Hydrocarbons
Anthracene
Phenanathrene
Reactivity of Polynuclear Hydrocarbons
As the number of aromatic rings increases, the resonance energy per
ring decreases, so larger PAH’s will add Br2.
H
Br
H
Br
Br
H
H
Br
(mixture of cis and trans isomers)
Fused Heterocyclic Compounds
Common in nature, synthesized for drugs.