Aromatic compounds
Dr AKM Shafiqul Islam
Aromaticity
hydrocarbons
aliphatic
alkanes alkenes alkynes
aromatic
 Aliphatic compounds: open-chain compounds
and ring compounds that are chemically similar to
open-chain compounds. Alkanes, alkenes,
alkynes, dienes, alicyclics, etc.
 Aromatic compounds: unsaturated ring
compounds that are far more stable than they
should be and resist the addition reactions typical
of unsaturated aliphatic compounds. Benzene and
related compounds.
Aromatic Compounds
 Aromatic was originally used to described some
fragrant compounds in early 19th century.
 Later, aromatic was used for specific chemical
behavior – cyclic unsaturated compounds that
undergo substitution rather than addition.
 Current: aromatic compounds distinguished from
aliphatic compounds by a special electronic
configuration.
Benzene
 This aromatic hydrocarbon was first discovered in 1825 but
its structure was not generally agreed upon until 1946.
Facts about benzene:
 Formula = C6H6
 Isomer number:
one monosubstituted isomer C6H5Y known
two disubstituted isomers C6H4Y2 known
 Benzene resists addition reaction, undergoes substitution
reactions.
 Heats of hydrogenation and combustion are far lower than
they should be.
 From X-ray, all of the C—C bonds in benzene are the same
length and intermediate in length between single and
double bonds.
Reagent
Cyclohexene
Benzene
KMnO4
oxidation
no reaction
Br2/CCl4
addition
no reaction
HI
addition
no reaction
H2/Ni
reduction
no reaction
Benzene + 3 H2, Ni, room temp.  NR
Benzene +
3 H2, Ni, 200oC, 1500 psi  cyclohexane
Although highly unsaturated, benzene does not react like
alkenes, dienes, cycloalkenes, or alkynes (addition
reactions) rather it undergoes substitution reactions
instead.
Kekule Structure of Benzene
 Open chain structure not possible
-CH=CH-CH=CH-CH=CH-
 Cyclic structure of benzene proposed by
Kekule (1872)
H
C
HC
CH
HC
CH
C
H
Benzene
The Lewis representation of benzene suggests that we
deal with a six-membered ring of carbon atoms that are
held together by alternating single and double bonds.
H
H
H
H
H
H
C: 6 • 4 = 24 valence electrons
H: 6 • 1 = 6 valence electrons
---------------------------------------Total
= 30 electrons
= 15 bonds
Benzene
This implies that we should observe alternating
short (1.33 A) and long (1.54 A) bond lengths.
Measurements indicate that all bond lengths are the
same (1.39 A). Again, we need to draw resonance
structures.
Aromatic Compounds
 Aromatic compound: a hydrocarbon that contains one or
more benzene-like rings
– arene: a term used to describe aromatic compounds
– Ar-: a symbol for an aromatic group derived by removing an -H from
an arene
– Kekulé structure for benzene (1872)
H
H
H
C
C
C
C
C
C
H
H
H
A Kek ulé structure
A Keku lé structu re
show ing all atoms as a line-angle formula
 But experiments show that the Kekulé
structure is not correct. All C-C bonds are
identical and benzene does not undergo
addition reactions typical of double bonds.
 A correct description is given by resonance
theory or by orbital models – valence bond
or molecular orbital.
Benzene
 Resonance structure for benzene (1930s)
– the theory of resonance, developed by Linus Pauling,
provided the first adequate description of the structure of
benzene
– according to the theory of resonance, certain molecules
and ions are best described by writing two or more Lewis
structures; the real molecule or ion is a resonance hybrid
of these structures
– each individual Lewis structure is called a contributing
structure
– we show that the real molecule is a resonance hybrid of
the two or more Lewis structures by using a doubleheaded arrow between them
Benzene
– here are two contributing structures for benzene
H
H
C
C
H
C
C
H
C
C
H
H
H
H
C
C
H
C
C
H
C
C
H
H
– the resonance hybrid has some of the
characteristics of each Lewis contributing
structure
– the length of a carbon-carbon bond in benzene,
for example, is midway between that of a carboncarbon single bond and a double bond
Delocalized electrons are not confined between two
adjacent bonding atoms, but actually extend over
three or more atoms.
Hybrid Resonance Structure
The Criteria for Aromacity: Hückel's
Rule
 A molecule must be cyclic, planar,
completely conjugated and contain a
particular number of p electorn. That is,
they are the kind of hydrocarbons
treated by Hückel's rule.
Hückel's rule
1.
2.
3.
4.
A molecule must be cyclic
a molecule must be planer
a molecule must be conjugated
a molecule must satisfy Hückel's rule and
contain a particular number of p electron
Aromaticity and the 4n + 2 Rule
 Huckel’s rule (based on calculations) – a planar cyclic
molecule with conjugated double bonds has aromatic
stability if it has 4n+ 2 p electrons,
where n is 0,1,2,3,4 (any integer)
 For n=1: 4n+2 = 6; benzene is stable and the electrons are
delocalized
Hückel's Rule
among planar, monocyclic, completely
conjugated polyenes, only those with 4n + 2
p electrons possess special stability (are
aromatic)
n
4n+2
0
2
1
6
2
10
3
14
4
18
benzene!
Hückel's Rule
 Hückel restricted his analysis to planar,
completely conjugated, monocyclic polyenes
 he found that the p molecular orbitals of
hese compounds had a distinctive pattern
 one p orbital was lowest in energy, another
was highest in energy, and the others
were arranged in pairs between the highest
and the lowest
Sources of Aromatic Hydrocarbons
- from high temperature distillation of coal tar
- heating petroleum at high temp and pressure w/
catalyst
The Phenyl Group
 When a benzene ring is a substituent, the
term phenyl is used (for C6H5 )
– You may also see “Ph” or “f” in place of “C6H5”
 “Benzyl” refers to “C6H5CH2
”
Disubstituted Benzene Names
To indicate relative position on a benzene ring:
– ortho- (o) on adjacent carbons (1,2)
– meta- (m) separated by one carbon (1,3)
– para- (p) separated by two carbons (1,4)
Heterocyclic Aromatic Compounds
N
••
••
N
••
••
O
S
Furan
Thiophene
••
••
H
Pyridine
Pyrrole
Exercise
O
S
a.
g.
e.
c.
H
N
H
O
N
N
b.
f.
d.
O
N
H
N
O
N
h.
N
Thank you
very much