Aromaticity
© E.V. Blackburn, 2011
Aromatic hydrocarbons
Originally called aromatic due to fragrant odors,
today this seems strange as many possess distinctly
non-fragrant smells!
Their properties differ markedly from those of aliphatic
hydrocarbons.
Aromatic hydrocarbons undergo ionic substitution
whereas aliphatic hydrocarbons undergo free radical
substitution coupled with ionic addition to double and
triple bonds.
© E.V. Blackburn, 2011
Nomenclature
Cl
chlorobenzene
NO 2
nitrobenzene
© E.V. Blackburn, 2011
Nomenclature
CH3
toluene
CO2H
benzoic acid
NH 2
aniline
SO3H
benzenesulfonic
acid
OH
phenol
OCH 3
anisole
© E.V. Blackburn, 2011
Nomenclature
NO 2
Br
Br
Cl
o-dibromobenzene
1,2-dibromobenzene
m-chloronitrobenzene
1-chloro-3-nitrobenzene
NO 2
O2N
p-dinitrobenzene
1,4-dinitrobenzene
© E.V. Blackburn, 2011
Nomenclature
I
Cl
NH 2
p-iodoaniline
4-iodoaniline
Cl
Cl
1,3,5-trichlorobenzene
CH3
O2N
NO 2
NO 2
2,4,6-trinitrotoluene
© E.V. Blackburn, 2011
A few aromatic compounds
CO2CH3
NH 2
CO2CH3
OH
oil of wintergreen
methyl anthanylate grape taste and odor
O
CH2CH2NH 2
O
CH2CH=CH 2
safrole - root beer smell
H3CO
OCH3
OCH3
mescaline - euphoric
© E.V. Blackburn, 2011
Benzene
The molecular formula of benzene is C6H6. How are
the atoms arranged?
In 1865 Kekulé proposed that benzene has a
“cyclohexatriene” structure:-
© E.V. Blackburn, 2011
Benzene
However there are other structures having this molecular
formula:-
Evidence points to the “cyclohexatriene” structure.
© E.V. Blackburn, 2011
Benzene
1. There is only one monosubstituted benzene of
formula C6H5Y - all benzene hydrogens must therefore
be equivalent.
2. There are three disubstituted isomers:Br
Br
Br
Br
Br
Br
© E.V. Blackburn, 2011
Benzene
However......
double single
bond bond
Br
Br
Br
Br
What is the structure of benzene?
What do we learn in the lab?
© E.V. Blackburn, 2011
Reactions of benzene
The benzene ring is very stable - it undergoes
substitution reactions rather than addition reactions.
Br2/CCl4
X
However:
Br2/CCl4
Br
Br
Therefore benzene cannot be a simple triene as it
does not react with bromine in carbon tetrachloride.
© E.V. Blackburn, 2011
Heats of hydrogenation
The heats of hydrogenation and combustion are lower
than predicted for a cyclohexatriene structure.
© E.V. Blackburn, 2011
Heats of hydrogenation
© E.V. Blackburn, 2011
Heats of hydrogenation
© E.V. Blackburn, 2011
Heats of hydrogenation
© E.V. Blackburn, 2011
Heats of hydrogenation
© E.V. Blackburn, 2011
Heats of hydrogenation
The heats of hydrogenation and combustion are lower
than predicted for a cyclohexatriene structure.
The heat of hydrogenation of one mole of benzene is 152
kJ less than that of three moles of cyclohexene.
Benzene is therefore 152 kJ more stable than expected for
“cyclohexatriene.”
© E.V. Blackburn, 2011
The structure of benzene
Benzene is a planar, cyclic molecule containing six
atoms of carbon.
All carbon - carbon distances are 1.397Å and all angles
are 120o.
The Kekulé structure cannot explain the physical and
chemical properties of benzene.
Remember CHEM 261 and the concept of
resonance......
“Whenever a molecule can be represented by 2 or more
structures which differ only in the arrangement of their
electrons, there is resonance.”
© E.V. Blackburn, 2011
Resonance
The structure of benzene is a resonance hybrid of the two
Kekulé structures:
The resonance hybrid is more stable than any one
contributing canonical form (resonance-contributing
structure). This energy, 150 kJ, is called the resonance
energy.
© E.V. Blackburn, 2011
Orbital description of benzene
H
H
bond
H
o
2
sp
120
H
H
H
A planar structure
© E.V. Blackburn, 2011
Orbital description of benzene
© E.V. Blackburn, 2011
Aromatic character
• Compounds whose molecular formulae indicate a
high degree of unsaturation.
• Compounds do not readily undergo addition
reactions.
• Compounds undergo electrophilic substitution
reactions.
• Compounds whose molecules are cyclic and planar.
© E.V. Blackburn, 2011
Hückel’s Rule
Hückel proposed the hypothesis that aromatic
compounds possess molecules containing cyclic
clouds of  electrons delocalised above and below the
plane of the molecule and that the  electron clouds
must contain a total of (4n+2)  electrons.
Therefore, in order to possess aromatic character, the
number of  electrons must be 2 or 6 or 10 etc.
© E.V. Blackburn, 2011
Cyclopentadiene
+
-
cyclopentadienyl cation
.
cyclopentadienyl anion
cyclopentadienyl radical
 electrons: 4
5
6
antiaromatic
aromatic
antiaromaticity: R. Breslow, D.R. Murayama, S. Murahashi,
and R. Grubbs, J. Amer. Chem. Soc., 95, 6688 (1973).
© E.V. Blackburn, 2011
Dicyclopentadienyliron
Ferrocene
Fe
© E.V. Blackburn, 2011
The tropylium cation
• Tropylium bromide, C7H7Br, mp > 200C.
• It is soluble in water but insoluble in non-polar
solvents.
• It forms a precipitate of silver bromide on
addition of AgNO3.
© E.V. Blackburn, 2011
Aromatic character?
+
+
+
+
H
H
N
H
© E.V. Blackburn, 2011
Heme
H2C=HC
CH 3
H3C
CH=CH 2
N
N
Fe
N
H3C
HO 2CH 2CH 2C
N
CH 3
CH 2CH 2CO 2H
Heme is the prosthetic group (non-peptide portion) of
hemoglobin.
© E.V. Blackburn, 2011
Aromatic compounds in
biochemistry
Three amino acids necessary for protein synthesis
contain a benzene ring:
HO
CO2-
CO2-
H
NH 3+
phenylalanine
H
NH 3+
tyrosine
H
N
CO2H +
NH 3
tryptophan
© E.V. Blackburn, 2011
Aromatic compounds in
biochemistry
Humans do not have the biochemical ability to
synthesize the benzene ring. Thus phenylalanine and
tryptophan derivatives are essential in our diet.
CO2H
NH 3+
H
N
CO2H
NH 3+
Tyrosine can be synthesized from phenylalanine in a
reaction catalyzed by phenylalanine hydroxylase.
© E.V. Blackburn, 2011
Aromatic compounds in
biochemistry
Heterocyclic aromatics are present in many biochemical
systems. Thus derivatives of purine and pyrimidine are
essential parts of DNA and RNA.
N
N
N
H
purine
N
N
N
pyrimidine
© E.V. Blackburn, 2011