B.Sc. Sem-I Unit-IV Nomenclature and Isomerism of Aromatic Compounds by Dr Pramod R Padole

1
B.Sc. First year
Students
B.Sc. Semester – I
Unit-IV
Nomenclature and
Isomerism of Aromatic
Compounds:
Structure of Benzene:
Kekule structure and
Molecular orbital structure.
by
Dr Pramod R Padole

Unit IV- Aromatic Hydrocarbons
Orientation: Effect of substituent groups. Activating and
deactivating groups.
Theory of reactivity and orientation on the basis of inductive
and resonance effects (-CH3, -OH, -NO2 and –Cl groups).
D
Nomenclature and Isomerism of Aromatic Compounds:
Structure of Benzene: Kekule structure and Molecular orbital structure.
A
Aromaticity and Huckel’s rule Aromatic, antiaromatic and
non-aromatic systems
B
Mechanism of Electrophilic Aromatic Substitution: Nitration,
Friedal Craft Alkylation and Acylation.Nuclear and Side Chain
Halogination, Birch Reduction
C

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A) Nomenclature and
Compounds:

Do you know?
Aromatic Hydrocarbons & Aromaticity

Aromatic Hydrocarbons & Aromaticity
Aromatic is a Greek word,
meaning,
Aroma = pleasant smell,
i.e. sweet smelling
 The name aromatic is used for characteristic pleasant odor.
 The simplest aromatic hydrocarbon is benzene.
 Benzene is the oldest known organic compound, firstly discovered by
Michael Faraday in 1825.
 Its structure was first proposed by German chemist August Kekule in
1865.
 The source of aromatic hydrocarbons are coke and petroleum.

A] Nomenclature and Isomerism
of Aromatic Compounds.
Nomenclature of Benzene Derivatives
Or
Aromatic Compounds
or
Arenes:
Kekule structure
and Molecular orbital structure

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Aromatic Compounds
The aromatic hydrocarbons are known as arenes
Nomenclature & Isomerism of

pramodpadole@gmail.com By Dr Pramod R Padole
Nomenclature and Isomerism
of Aromatic Compounds:
Arenes:
Definition:
Replacement of one or more hydrogen
atoms of the benzene ring by alkyl, alkenyl,
alkynyl or aryl groups; to form aromatic
hydrocarbons are called arenes or Aromatic
compounds.
OR
The aromatic hydrocarbons are known
as arenes.
CH3 CH2CH3
Methyl-benzene
(Toluene)
Ethyl-benzene

Nomenclature of Benzene Derivatives or
Aromatic Compounds or Arenes:
Q.1) Write structural formula of following compounds. (S-04, 2 Mark)
1) Ortho-Xylene (W-10, 1 Mark), 2) o - Nitro-Phenol,
3) Benzene sulphonic acid, & 4) Mesitylene
Q.2) Write structural formula of following compounds. (W-04, 3 Mark)
1) Picric Acid, 2) p - Nitro-Phenol, 3) Aniline
Q.3) Write structural formula of following compounds. (W-05, 2 Mark)
1) Acetophenone, & 2) m - Xylene
Q.4) Write structural formula of O-Xylene compounds. (W-10, 1 Mark)
Q.5) Draw the position isomers of:- (S-10, 3 Mark)
(i) Xylene & (ii) dichlorobenzene.
Q.6) Write the structural formula of picric acid. (S-17, 1 Mark)
Q.7) Write the structural formula of Benzene sulphonic acid. (S-18, 1 Mark)

Nomenclature of Benzene Derivatives
or Aromatic Compounds or Arenes:
First
Second
Third
Fourth
Mono-substituted benzenes or derivatives
(i.e. one substituent only):
Di-substituted benzenes or derivatives
(i.e. two same or different substituents):
Tri or Polysubstituted benzenes or derivatives:
(i.e. Three or more than three same or
different substituents):
Fused Polycyclic Arenes:

Mono-substituted benzenes
or derivatives:
(i.e., one substituent only)

Br Dr Pramod R Padole
Mono-substituted benzenes or
derivatives (i.e. one substituent only):
•Mono-substituted benzenes are named by prefixing
the name of the substituent group to the word
benzene; certain mono-substituted benzene
derivatives are given special names.
Cl NO2
chlorobenzene Nitro-benzene
OH NH2
Phenol Aniline
CH3
SO3H
Toluene
(methyl-benzene)
Benzene sulphonic acid
COCH3
Acetyl benzene
Acetophenone
(methyl,phenyl ketone)
COOH CHO
Benzoic acid Benzaldehyde
CH2Cl CH2OH
Benzyl chloride Benzyl alcohol

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(i.e. two same or different substituents):

By Dr Pramod R Padole
Di-substituted benzenes
or derivatives:
When two
similar
substituents
are present
on the
benzene
ring
Di-substituted
Benzenes
When two
dissimilar
substituents
are present
on the
benzene
ring

a) When two similar substituents are present on
the benzene ring, their positions are indicated by
using the symbols ortho, meta or para before the
prefix.
Thus, the isomeric dimethyl benzenes (Xylenes) are
named as:

prefix.

prefix.
Cl
o-dichloro benzene
(1,2-dichloro benzene)
Cl
NO2
NO2
Br
p-dibromo benzene
(1,4-dibromo benzene)
Br
m-dinitro benzene
(1,3-dinitro benzene)

(i.e. two different substituents):
b) When two dissimilar substituents are present on the
benzene ring, the symbols ortho, meta or para is followed by
names of the groups arranged alphabetically and the root word
benzene is written at the end.
For examples:
Seniority Table:
(1) COOH (Carboxylic group)
(2) COOR (Ester group)
(3) SO3H (Sulphonic acid)
(4) COX (Carboxyl halide) (X=Cl, Br, I)
(5) CONH2 (amide group)
(6) CHO (aldehyde group)
(7) >C=O (ketone or carbonyl group)
(8) OH (Hydroxy group)
(9) O (Ether group)
(10) NH2 (amine group)
(11) X (Cl, Br, I) (Halo group)
(12) NO2 (Nitro group)
(13) >C=C< (alkenyl group)
(14) C C (alkynyl group)

(i.e. two different substituents):
 Some di-substituted benzene derivatives are named by
prefixing the name of the substituent to the special name
of the compound.
 For example:
OH
o-nitro phenol
NO2
OH
o-Cresol
(o-methyl phenol)
CH3
OH
CH3
CH3
OH
m-Cresol
(m-methyl phenol)
p-Cresol
(p-methyl phenol)
CH3
NO2
p-nitro toluene
COOH
NO2
p-nitro benzoic acid
COOH
OH
p-hydroxy benzoic acid
OH
(o-hydroxy benzoic acid)
COOH
Three isomeric forms of Cresol
Salicylic acid
NH2
(o-amino benzoic acid)
COOH
Anthranilic acid

Do you know?
Tri or Polysubstituted benzenes or derivatives:

Tri or Polysubstituted benzenes or
derivatives:
(i.e. Three or more than three same or different substituents):
If there are three or more substituents present on the ring, the arenes are
designated by IUPAC names.
For example,
i) The names of the isomers of the trimethyl benzene are given as:
ii) In tri or poly-substituted benzene derivatives, the relative positions of the
substituents are indicated by numbers.
CH3
1,2,3-trimethyl benzene
CH3
CH3
1
2
3
CH3
CH3
1
2
3
CH3
4
CH3
(Mesitylene)
CH3
1
2
3
H3C 4
5
Cl
1,2,3-trichloro-benzene
Cl
Cl
1
2
3
NO2
2-bromo,4-chloro-nitrobenzene
Br
1
2
3
Cl
4
COOH
3,5-dinitro benzoic acid
NO2
1
2
3
O2N 4
5
OH
2,4,6-trinitro phenol
(Picric acid)
1
2
3
4
5
NO2
NO2
O2N 6
CH3
2,4,6-trinitro toluene
(TNT)
1
2
3
4
5
NO2
NO2
O2N 6

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iv) Fused Polycyclic
Arenes:

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Compounds:

Isomerism of Aromatic Compounds:
 The compounds having same molecular
formula but different structural formula are
called as isomers and this process is called as
isomerism.
(i) Mono-substituted benzene derivatives exist in one form
only.
(ii) The disubstituted benzene derivatives exist in three isomeric
forms depending upon the relative positions of the two
substituents.
 These three isomers are called ortho, meta or para isomers, if
the two groups occupy adjacent (1,2), alternate(1,3) or
diagonal (1,4) positions respectively.
Q.1) Discuss the isomerism in aromatic compounds. (S-12, 4 Mark)
Q. 2) Draw the positions isomers of Xylene (dimethyl benzene). (S-10, 2 Mark)
Q. 3) Draw the positions isomers dihydroxy benzene.
Q.4) The number of disubstituted products possible for benzene is _3__. (W-15, ½ Mark)
a) 2 b) 3 c) 4 d) 5

Isomerism of Aromatic Compounds:
Dimethyl benzenes are given the special name of Xylenes.
Q.1) Explain: Position isomerism in Xylene. (S-16, 2 Mark)
For example, the position isomers of xylene are-
For example, the position isomers of dihydroxy benzene
are-
X
o-isomer
(1,2-disubstituted benzene)
Adjacent position
Y
1
2
X
1
2
3
Y
4
X
Y
1
2
3
m-isomer
Alternate position
p-isomer
Diagonal position
1
2
1
2
3
4
1
2
3
CH3
1,2-dimethyl benzene
(o-xylene)
CH3
CH3
(m-xylene)
CH3
CH3
(p-xylene)
CH3
OH
Catechol
OH
OH
OH
OH
Hydroquinone
(p-quinol)
OH
Resorcinol
1
2
1
2
3
4
1
2
3

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Kekule’s Cyclic
Structure for
Benzene:
Cyclic nature of benzene
Structure
of Benzene

Kekule’s Cyclic Structure for Benzene:
Q.1) Discuss Kekule’s Structure of Benzene (Structure of Benzene).
(S-04, W-04, W-07, S-14, W-16 & S-18, 4 Mark)
Q.2) How does benzene reacts with H2 in presence of Ni catalyst.
(S-06, W-06, S-07, W-15, W-18 & S-19, 2 Mark)
Q.3) How can you account for- (W-06, 3 Mark), ( W-08 & S-10, 2 Mark)
i) Cyclic nature of benzene, ii) Presence of three double bonds in benzene
& iii) Alternate position of the double bonds.
Q.4) How will you obtain Benzene tri-ozonide from benzene. (W-07, 2 Mark)
Q.5) Discuss the stability of benzene with reference to resonance theory. (S-08, 3 Mark)
Q.6) How will you convert benzene to benzene-hexachloride (B.H.C.). (W-08, 2 Mark)
Q.7) How will you prove: (W-09, W-13 & S-15, 4 Mark)
i) Cyclic nature of benzene, & ii) Presence of three double bonds in benzene
Q.8) Complete the following reaction. (S-09, 2 Mark)
Q.9) Complete the following reaction ( Convert benzene to benzene-hexachloride). (S-09, 2 Mark)
Q.10) What happens when, Benzene is hydrogenated in the presence of Ni catalyst? (W-11, 2 Mark)
Q.11) How will you convert: Benzene to benzene-hexa chloride? (S-12, 2 Mark)
Q.12) The molecular formula of benzene is __________. (W-15, ½ Mark)
Q.13) Discuss / Explain the structure of Benzene as proposed by Kekule. (W-15 & W-17, 4 Mark)
Q.14) Benzene on reduction with H2 / Ni catalyst gives mainly: (S-17, ½ Mark)
(a) Cyclohexane (b) 1,4-cyclohexadiene (c) n-hexane (d) Hexatriene
+ 3 H2 ?
Ni /
+ 3 Cl2 ?
Sun light

Structure of Benzene or Kekule’s Structure for Benzene:
 German chemist, August Kekule in 1865 proposed
the structure for benzene (C6H6 ).
 According to him, benzene molecule is the hexagon
of six carbon atoms by means of alternate single &
double bonds.
 Each C-atom is attached to one H-atom.
or or
Kekule's Structure for Benzene
C
C
C
C
C
C
H
H
H
H
H
H

Facts in support Kekule’s Structure for
Benzene
5) Kekule’s
Dynamic Formula
6) Resonance structure
of Benzene
&
Stability
2) Cyclic Nature
of Benzene
3) Presence of three
(>C=C<)double bonds
in benzene
4) Alternate position of
(>C=C<) double bonds
1) Molecular Formula
Facts in
Support

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Facts in support
Kekule’s Structure for Benzene:
(1) Molecular formula:
From the molecular weight and elemental analysis
determination showed that, the molecular formula of benzene
is C6H6.
(2) Cyclic Nature of benzene:
Catalytic hydrogenation of Benzene:
Or Preparation of cyclohexane form benzene:
When benzene is heated with hydrogen in presence of Ni /
Pt / Pd, (catalytic hydrogenation) under pressure; to form
cyclohexane.

LOGO
Facts in support
(1) Molecular formula:
From the molecular weight and elemental analysis
determination showed that, the molecular formula of benzene
is C6H6.
(a) Catalytic hydrogenation of Benzene:
Or Preparation of cyclohexane form benzene:
When benzene is heated with hydrogen in presence of Ni /
Pt / Pd, (catalytic hydrogenation) under pressure; to form
cyclohexane.

LOGO
Facts in support
(a) Catalytic hydrogenation of Benzene Or Preparation of cyclohexane form benzene:
When benzene is heated with hydrogen in presence of Ni / Pt / Pd, (catalytic
hydrogenation) under pressure; to form cyclohexane.
 Since, hydrogenation reaction do not change the C-atoms of benzene
(i.e. six C-atoms)
 Cyclohexane is a six membered ring compound ( i.e. six C-atoms); hence
the structure of benzene must be a cyclic one ( or nature) containing six
C-atoms.

LOGO
Facts in support
(b) On substitution of Benzene to form one and only
one mono-substituted product:
Or Preparation of bromo benzene:
When benzene is reacted with bromine in presence of FeBr3 (or FeCl3)
as a catalyst (Halogen carrier); to form bromo-benzene.
Formation of one and only one mono-substituted benzene is
possible only when benzene has cyclic structure & one hydrogen is
attached to each C-atom.
Br2
H Br
FeCl3 / FeBr3 / Fe / AlCl3
H-Br
+
Benzene
.ie. Halogen carrier
+
bromo-benzene
Br Br

3) Presence of three (C=C)
double bonds in benzene:
Under suitable condition, when addition of three molecules of hydrogen
& chlorine on benzene; to form cyclohexane and Benzene hexa-
chloride (B.H.C.) respective products.
In above reaction, it is clearly indicated that, in suitable condition,
addition of three molecules of hydrogen & chlorine resp. in benzene is
possible, due to the presence of three C=C bonds in the benzene ring.
H2
3
Ni / Pt / Pd
, under Pressure
Cyclohexane
+
Benzene
C
C
C
C
C
C
H
H
H
H
H
H
Benzene
+ 3 Cl2
C
C
C
C
C
C
H
H
H
H
H
H
Cl
Cl
Cl
Cl
Cl
Cl
sun light
Benzene hexa-chloride (B.H.C.)
(or Hexachloro-cyclohexane)
B.H.C. is insecticides, rarely used, harmful effects

4) Alternate position of (C=C) double bonds:
Or Preparation of Benzene tri-ozonide
or Preparation of Glyoxal
or Reaction with O3 & H2O:
 When benzene on ozonolysis; to form
Benzene tri-ozonide (unstable).
C
C
C
C
C
C
H
H
H
H
H
H
Benzene
+ O3
ozonolysis
C
C
C
C
C
C
H
H
H
H
H
H
O
O
O
O
O
O
O
O
O
Benzene tri-ozonide (unstable)

4) Alternate position of (C=C) double bonds:
Or Preparation of Benzene tri-ozonide
or Preparation of Glyoxal
or Reaction with O3 & H2O:
 Benzene tri-ozonide, which is further reacts with water in
presence of Zn; to form three glyoxal molecules, indicates
that the presence of three C=C bonds in benzene are
present in alternate positions.
C
C
C
C
C
C
H
H
H
H
H
H
O
O
O
O
O
O
O
O
O
Benzene tri-ozonide (unstable)
3 H2O
CHO
CHO
H2O2
+ in presence of Zn
3 + 3
Glyoxal

LOGO
5) Kekule’s Dynamic Formula or
Objection to Kekule’s Structure:
Kekule suggested that, the double bond in benzene
are not fixed, but mobile (delocalization of π e- s) &
there exists an equilibrium between 1,2 & 1,6-
disubstituted products and hence could not be
separated.
Br
ortho
(1,2)
Br
1
2
Br
Br 1
2
3
4
3
4
5
6
5
6
ortho
(1,6)
(Note for Drawbacks of Kekule structure:
In actual practice only one ortho di-substituted product is possible.)

LOGO
Resonance:
 The double bond keep on changing their positions and this
is called Resonance.
 Resonance is also called mesomerism.
6) Resonance Structure of Benzene
and Stability:
Or Resonance energy and Stability of Benzene:
 Consider benzene as the resonance hybrid of the
two resonance structures.
I II
Resonance structure of Benzene Resonance hybrid

LOGO
6) Resonance Structure of Benzene and Stability:
Note: Stability of Benzene is inversely proportional to the value
of Heat of Hydration, i.e., less value of Heat of Hydration
means compound is most stable (more stability).

LOGO
6) Resonance Structure of Benzene and Stability:
Resonance Energy = Energy of Resonating structures - Energy of Resonance hybrid
i.e, Resonance Energy = Calculated value - Actual value
Resonance Energy = - 86 - ( - 50 )
Resonance Energy = - 36 Kcal/mol
In fact, the actual ( observed ) Heat of Hydrogenation is only
≈ - 50 Kcal/mol, which is 36 Kcal/mol lower than calculated
( or predicted) ΔHo cal for Benzene.
So, the low Heat of Hydrogenation of benzene means that
benzene is especially stable, due to resonance structure.
This un-usual stability is characteristic of aromatic
compounds.
-----*****-----

LOGO
Q.1) Discuss / Explain the Molecular orbital diagram / picture /
structure of benzene. (W-04, W-05, W-08, S-09, W-09,
W-10, W-11, W-14, S-17 & W-19, 4 Mark)
Q.2) Draw the orbital picture of benzene. ( S-05, 4 Mark)
Q.3) Explain on the basis of orbital picture of benzene,
how many σ and π-bonds are present in benzene.
(S-07, 2 Mark)
Q.4) Describe Molecular orbital structure (Picture) of benzene.
(S-11, W-13 & S-15, 4 Mark)
Molecular Orbital Structure /
Picture / M.O. diagram of
Benzene:

LOGO
Molecular orbital structure of Benzene:
M.O. of
Benzene
1
4
2
3
5
Representation of
Benzene
Molecular Formula
E.C. &
Hybridization of C
E.C. of H
Structure
Formation of
delocalized pi (π)
bonds M.O. diagram
Formation of sigma (σ)
bonds M.O. diagram

LOGO
Molecular orbital structure of
Benzene:
Molecular formula of Benzene is C6H6
 According to MOT, the sigma (σ) bonds and pi (π) bonds
molecular orbital diagram / structure of Benzene are as,
E.C. of H1
1s
&

LOGO
In Benzene, all carbon undergoes sp2-hybridisation.
1
All ring atoms in benzene ( Six carbon) contains
three sp2 H.O’s
2
sp2 hybrid orbital of each C-atom is half-filled (singly filled)
3
The unhybridised p-orbital (i.e., 2-pz) of each carbon atom
is half-filled (singly filled)
4
According to molecular orbital theory (MOT);
Benzene ring is planar due to sp2-hybridisation
(All C & H atoms are in one plane).
5

LOGO
Formation of sigma (σ ) bonds M.O. diagram:
 Signally filled three sp2 hybrid orbital’s of each carbon atom forms σ–bonds
with adjacent carbon atoms (C-C) bond by linear (axial or co-axial) overlap
of sp2 (C)-sp2 (C) hybrid orbital’s and a σ–bond with hydrogen atom
( C-H bond) by linear overlap of sp2 hybrid orbital of carbon with s-orbital of
hydrogen atom (i.e., sp2- s overlap).
 All C-C & C-H σ–bonds lie in a one plane, i.e., benzene molecule is
planar.

LOGO
Formation of sigma (σ ) bonds M.O. diagram:
 The σ-bond skeleton of six carbon and six hydrogen atoms is shown
in the following figure.
 The benzene molecule is a flat molecule with bond angle of 1200.
It is a symmetrical molecule.
 So, total twelve sigma (σ) bonds are present in the benzene.

LOGO
Formation of pi (π) bonds M.O. diagram:
 After forming three σ-bonds by each carbon atom, but each
carbon atom is left with one unhybridised p-orbital
containing one electron each.
 The p-orbitals of carbon atoms are perpendicular to the
plane of the molecule ( Benzene ring or σ-bonds) and
parallel to each other & they overlap laterally (sideways)
with each other; to form π- bonds (π- M.O’s.).

LOGO
 Because, each p-orbital has two lobes or electron clouds (π- M.O’s.);
one lying above and the other below the plane of the atoms (like
sandwitched picture between these electron clouds). π- M.O. contains
six π-e- s, which is called as aromatic sexlet.
 Benzene is aromatic compound because of these six π-e- s & it obeys
Huckel’s (4n+2), π-e- s rule.
a) Overlap of p-orbitals of carbon atoms of benzene; to form
a delocalised π- M.O.
So, total three π-bonds are present in the benzene.

LOGO
C C
C
C
C
C H
H
H
H
H H
b) Delocalised - M.O. of benzene. The electron cloud above and below the plane of the ring.
Upper -M.O.
Lower -M.O.
 -M.O.
Common representation of Benzene
(Circle represents six delocalized e-
s).
Sandwitched picture between these electron clouds
The six electrons, one from each of the carbon atoms, can
be anywhere in this delocalised π-molecular orbital.
This is called as delocalisation of electrons.
This results in the formation of stronger bonds and
more stable molecule.

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B.Sc. Sem-I Unit-IV Nomenclature and Isomerism of Aromatic Compounds by Dr Pramod R Padole