Chapter 14
Aromatic Compounds
Created by
Professor William Tam & Dr. Phillis Chang
Ch. 14 - 1
1. The Discovery of Benzene

Benzene:

In 1825, Faraday isolated benzene
from a compressed illuminating gas
that had been made by pyrolyzing
whale oil
or
Ch. 14 - 2

In 1834, a German chemist, Eilhardt
Mitscherlich, synthesized benzene by
heating benzoic acid with calcium oxide
COOH
+ CaO
heat
+ CaCO3
Ch. 14 - 3
In 19th century, organic compounds
were classified as being either
aliphatic or aromatic
 Aliphatic
● The chemical behavior of a
compound was “fatlike”
 Aromatic
● The compound had a low
hydrogen-to-carbon ratio and it was
“fragrant”

Ch. 14 - 4
2. Nomenclature of Benzene
Derivatives

Naming monosubstituted benzenes
● In many simple compounds, benzene is
the parent name and the substituent is
simply indicated by a prefix
F
Cl
Br
NO2
Fluorobenzene Chlorobenzene Bromobenzene Nitrobenzene
Ch. 14 - 5
● For other simple and common
compounds, the substituent and the
benzene ring taken together may form a
commonly accepted parent name
CH3
O
Toluene
H
Phenol
O
H
N
H
SO3H
Benzenesulfonic acid
Aniline
O
CH3
Anisole
O
OH
Benzoic acid
Acetophenone
Ch. 14 - 6

Naming disubstituted benzenes
● When two substituents are present,
their relative positions are indicated by
the prefixes ortho-, meta-, and para(abbreviated o-, m-, and p-) or by the
use of numbers
Br
Br
Br
1,2-Dibromobenzene
(o-dibromobenzene)
ortho
Br
Br
1,3-Dibromobenzene
(m-dibromobenzene)
meta
Br
1,4-Dibromobenzene
(p-dibromobenzene)
para
Ch. 14 - 7
● Other examples
CH3
NO2
COOH
2-Nitrobenzoic acid
(o-Nitrobenzoic acid)
Cl
OH
3-Methylphenol
(m-Methylphenol)
CH3
4-Chlorotoluene
(p-Chlorotoluene)
(1-Chloro-4-methylbenzene)
Ch. 14 - 8
● The dimethylbenzenes are often
called xylenes
CH3
CH3
CH3
1,2-Dimethylbenzene
(o-xylene)
H3C
CH3
1,3-Dimethylbenzene
(m-xylene)
CH3
1,4-Dimethylbenzene
(p-xylene)
Ch. 14 - 9

Naming benzene rings with more than
two groups
● If more than two groups are present on
the benzene ring, their positions must
be indicated by the use of numbers
● The benzene ring is numbered so as to
give the lowest possible numbers to
the substituents
Cl
1
6
Br
2
Cl
3
5
4
Cl
1,2,3-Trichlorobenzene
1
6
2
3
5
4
Br
Br
1,2,4-Tribromobenzene
(not 1,3,4-Tribromobenzene)
Ch. 14 - 10
● When more than two substituents
are present and the substituents
are different, they are listed in
alphabetical order
Cl
1
6
2
F
3
5
4
Br
4-Bromo-1-chloro-2-fluorobenzene
Ch. 14 - 11
● When a substituent is one that,
together with the benzene ring
gives a new base name, that
substituent is assumed to be in
position 1 and the new parent
name is used
Cl
COOH
3
Cl
4
2
5
1
6
H3C
OH
3,5-Dichlorophenol
2
1
6
3
5
4
Br
5-Bromo-2-methylbenzoic acid
Ch. 14 - 12
● When the C6H5 group is named as a
substituent, it is called a phenyl
group
● A hydrocarbon composed of one
saturated chain and one benzene
ring is usually named as a
derivative of the larger structural
unit. However, if the chain is
unsaturated, the compound may be
named as a derivative of that chain,
regardless of ring size
Ch. 14 - 13
● Examples
Butylbenzene
Isopropylbenzene
2
4
1
1
3
trans-1-Phenyl-1-butene
3
2
5
4
7
6
8
(R)-3-Phenyloctane
Ch. 14 - 14
● Benzyl is an alternative name for
the phenylmethyl group. It is
sometimes abbreviated Bn
Cl
The benzyl group
(the phenylmethyl group)
Benzyl chloride
(phenylmethyl chloride
or BnCl)
Ch. 14 - 15
3. Reactions of Benzene
Br2
CCl 4
Br2
CCl 4
Br
Br
No Reaction
Ch. 14 - 16
1. OsO4
2. NaHSO3
1. OsO4
2. NaHSO3
OH
OH
No Reaction
Ch. 14 - 17
H+
OH
H2O
H+
H2O
No Reaction
Ch. 14 - 18
H2/Ni
25oC, 1 atm
H2/Ni
high temperature
and pressure
Ch. 14 - 19

Benzene undergoes substitution but
not addition
Br
Br2
(an addition)
CCl 4
Br
(C6H10Br2)
(C6H10)
H
Br
Br2
(a substitution)
FeBr3
(a Lewis acid)
(C6H6)
(C6H5Br)
Ch. 14 - 20
4. The Kekulé Structure for
Benzene
H
H
H
C
C
C
C
C
C
H
or
H
H
The Kekulé formula for benzene
Ch. 14 - 21
6
5
1
6
Br
and
4
3
2
Br
5
4
3
1
2
Br
Br
These 1,2-dibromobenzenes
do not exist as isomers
6
5
4
3
1
2
Br
Br
6
X
5
4
3
1
Br
2
Br
There is no such equilibrium between
benzene ring bond isomers
Ch. 14 - 22
Br2
No Reaction
Br2
Br
Br
Ch. 14 - 23
5. The Thermodynamic Stability
of Benzene

Since p bonds are formed from side-way
overlap of p orbitals, p electron clouds are
above & below the plane of the double bond
p-electrons above
and below ring
Ch. 14 - 24
Ch. 14 - 25
6.
Modern Theories of the Structure
of Benzene
6A. The Resonance Explanation of the
Structure of Benzene
All C C bond lengths the same (1.39
Å) (compare with C–C single bond 1.54
Å, C=C double bond 1.34 Å)
 Extra stabilization due to resonance 
aromatic
Ch. 14 - 26


3-D structure
p-electrons above
and below ring
● Planar structure
● All carbons sp2 hybridized
Ch. 14 - 27
6B. The Molecular Orbital Explanation
of the Structure of Benzene
Ch. 14 - 28
Ch. 14 - 29
7. Hückel’s Rule: The 4n + 2 p
Electron Rule


Hückel’s rule is concerned with
compounds containing one planar ring in
which each atom has a p orbital as in
benzene
Planar monocyclic rings containing 4n + 2 p
electrons, where n = 0, 1, 2, 3, and so on
(i.e., rings containing 2, 6, 10, 14 . . . etc. p
electrons), have closed shells of delocalized
electrons like benzene and have substantial
resonance energies
Ch. 14 - 30

Hückel’s rule states that planar
monocyclic rings with 2, 6, 10,
14 . . . delocalized electrons
should be aromatic
Ch. 14 - 31
7A. How To Diagram the Relative
Energies of p Molecular Orbitals in
Monocyclic Systems Based on
Hückel’s Rule
antibonding p orbitals
nonbonding p orbital
bonding p orbitals
Polygon in circle
Energy levels of MOs
Type of p orbital
Ch. 14 - 32

The p molecular orbitals that cyclooctatetraene
would have if it were planar. Notice that, unlike
benzene, this molecule is predicted to have two
nonbonding orbitals, and because it has eight p
electrons, it would have an unpaired electron in
each of the two nonbonding orbitals. Such a
system would not be expected to be aromatic.
Ch. 14 - 33

The bonds of cyclooctatetraene are
known to be alternately long and short;
X-ray studies indicate that they are
1.48 and 1.34 Å, respectively
Ch. 14 - 34
7B. The Annulenes

Hückel’s rule predicts that annulenes
will be aromatic if their molecules have
4n + 2 p electrons and have a planar
carbon skeleton
Ch. 14 - 35

All these (4n + 2)p, planar
annulenes are aromatic
(4n + 2) p
planar annulenes:
Benzene
[6]Annulene
[14]Annulene
(aromatic)
[18]Annulene
(aromatic)
Ch. 14 - 36

Non-planar (4n + 2)p annulenes are
antiaromatic
H
H
4
5
6
[10]Annulenes
(None are aromatic
because none are planar)
Ch. 14 - 37

(4n)p non-planar annulenes are
antiaromatic
Cyclobutadiene
[4]Annulene
[8]Annulene
[16]Annulene
Ch. 14 - 38
7C. NMR Spectroscopy: Evidence for
Electron Delocalization in
Aromatic Compounds
The 1H NMR spectrum of benzene
consists of a single unsplit signal at
d 7.27
 The signal occurs at relatively high
frequency, which is compelling
evidence for the assertion that the p
electrons of benzene are delocalized

Ch. 14 - 39

The circulation of p electrons in
benzene creates an induced magnetic
field that, at the position of the protons,
reinforces the applied magnetic field.
This reinforcement causes the protons
to be strongly deshielded and to have a
relatively high frequency (d ~ 7)
absorption
Ch. 14 - 40
Ch. 14 - 41
H
H
H
H
(d 9.3)
H
H
H
H
H
H
H
(d -3.0)
H
H
H
H
H
H
H
Ch. 14 - 42
7D. Aromatic Ions
pka = 36
pka = 16
H
H
H
H
Ch. 14 - 43
H
H
H
Bu Li
(a strong base)
6 p electrons
 aromatic
H
H
sp3
strong
H
base
sp2
Ch. 14 - 44
H
H
- H+
8 p electrons
H
H
- H
6 p electrons
(aromatic)
Ch. 14 - 45
7E. Aromatic, Antiaromatic, and
Nonaromatic Compounds

An aromatic compound has its p
electrons delocalized over the entire
ring and it is stabilized by the pelectron delocalization
Ch. 14 - 46


One way to evaluate whether a cyclic
compound is stabilized by delocalization of p
electrons through its ring is to compare it
with an open-chain compound having the
same number of p electrons
Based on sound calculations or experiments
● If the ring has lower p-electron energy, then
the ring is aromatic
● If the ring and the chain have the same pelectron energy, then the ring is
nonaromatic
● If the ring has greater p-electron energy than
the open chain, then the ring is
antiaromatic
Ch. 14 - 47

Cyclobutadiene
p-electron
energy increases
1,3-Butadiene
4 p electrons

+ H2
Cyclobutadiene
4 p electrons
(antiaromatic)
Benzene
p-electron
energy decreases
1,3,5-Hexatriene
6 p electrons
+ H2
Benzene
6 p electrons
(aromatic)
Ch. 14 - 48
8. Other Aromatic Compounds
8A. Benzenoid Aromatic Compounds

Benzenoid polycyclic aromatic hydrocarbons
consist of molecules having two or more
benzene rings fused together
8
Naphthalene
C10H8
1
8
9
7
2
7
2
6
3
6
3
5
5
4
6
7
5
4
Phenanthrene
C14H10
1
8
3
2
9
1
10
10
4
9
8
Anthracene
C14H10
7
10
6
1
2
5
3
4
Pyrene
C16H10
Ch. 14 - 49
8B. Nonbenzenoid Aromatic
Compounds
(Azulene)
Ch. 14 - 50
8C. Fullerenes
Ch. 14 - 51
9.

Heterocyclic Aromatic Compounds
Cyclic compounds that include an
element other than carbon are called
heterocyclic compounds
4
4
5
3
6
2
N
1
Pyridine
(electronically
related to
benzene)
5
4
3
2
N1
3
4
2
5
3
2
5
O1
S1
Furan
Thiophene
H
Pyrrole
(electronically related to
cyclopentadienyl anion)
Ch. 14 - 52

Examples of useful heterocyclic
aromatic compounds HO
HOOC
H
N
S
O
Penicillin
(antibiotic)
N
S
Serotonin
H
(neurotransmitter)
N
O
O
COOH
H
O
N
N
N
O
Nitrofurantoin
(urinary antibacterial)
O
N
N
O
O2N
NH2
N
N
H
O
S
O
N
N
"Viagra"
Ch. 14 - 53

Aromaticity
X
X = O, S
N
H
N
X
H

6p e : aromatic
Ch. 14 - 54

Aromaticity
● Evidence: 1H NMR shift
H
H
H
Z
(2.5 ppm)
Z
H
Z
(3.4 ppm)
H
d (ppm)
O
7.3
6.2
NH
6.4
6.2
S
7.1
7.0
d (5.5 ppm)
H d (7.4 ppm)
Ch. 14 - 55

Basicity of nitrogen-containing
heterocycles
N
Order of Basicity:
pKa of the
conjugate acid:
>
>
N
N
H
H
11.2
7
>
N
N
H
5.2
0.4
(c.f. Et3N, pKa of the conjugate acid = 9.7)
Ch. 14 - 56

Basicity of nitrogen-containing
heterocycles
H
+ H+
N
N
H
H
N
H
H
(lost of aromaticity)
H
N
N
H Imidazole
(a very common base
in organic synthesis)
H
H
N
+ H+
H
N
N
N
N
N
H
H
H
(still aromatic)
Ch. 14 - 57
Non-basic
nitrogen
X
4
N
N
H
H
N
(aromatic)
5
1
N
N
3
H
N
H+ X
(aromatic)
H
2
H
X
basic
nitrogen
H
N
N
(aromatic)
6 p electrons
H
N
+
N
X
H
(non-aromatic)
4 p electrons
Ch. 14 - 58
10. Aromatic Compounds in
Biochemistry

Two amino acids necessary for protein
synthesis contain the benzene ring
O
O
O
NH3
Phenylalanine
O
HO
NH3
Tyrosine
Ch. 14 - 59

Derivatives of purine and pyrimidine
are essential parts of DNA and RNA
1
5
N
2
7
6
4
N
5
N
8
3
N
4
N9
6
3
1
N
2
H
Purine
Pyrimidine
Ch. 14 - 60

Nicotinamide adenine dinucleotide, one
of the most important coenzymes in
biological oxidations and reductions, includes
both a pyridine derivative (nicotinamide) and
a purine derivative (adenine) in its structure
O
Nicotinamide
H2N
Adenine
O
N
P
OH HO
Ribose
O

O
O
N

N
P
O
NH2
N
N
O
Pyrophosphate
O
OH
OH
Ribose
Ch. 14 - 61
11. Spectroscopy of Aromatic
Compounds
11A. 1H NMR Spectra

The ring hydrogens of benzene
derivatives absorb downfield in the
region between d 6.0 and d 9.5 ppm
11B. 13C NMR Spectra
 The carbon atoms of benzene rings
generally absorb in the d 100–170 ppm
region of 13C NMR spectra
Ch. 14 - 62
Ch. 14 - 63
N
N
N
N
(d)
(d)
(c)
(c)
H
O
A
H
O
B
H
O
C
H
O
D
Ch. 14 - 64
N
N
N
N
(d)
(d)
(c)
(c)
H
O
E
H
O
F
H
O
G
H
O
H
Ch. 14 - 65
11C. Infrared Spectra of Substituted
Benzenes
Ch. 14 - 66
11D. Ultraviolet–Visible Spectra of
Aromatic Compounds
Octyl-4-N-N-dimethylaminobenzoate
(Padimate O)
O
max 310 nm
O
Me2N
2-Ethylhexyl 4-methoxycinnamate
(Parsol MCX)
max 310 nm
O
O
MeO
Ch. 14 - 67
O
OH
O
O
OMe
2-Hydroxy-4-methoxybenzophenone
(Oxybenzone)
max 288 and 325 nm
H
OH
Homomenthyl salicylate
(Homosalate)
max 309 nm
O
NC
O
2-Ethylhexyl 2-cyano3,3-diphenylacrylate
(Octocrylene)
max 310 nm
Ch. 14 - 68
11E. Mass Spectra of Aromatic
Compounds
R
CH2
m/2 = 91
m/2 = 91
Y
m/2 = 77
Ch. 14 - 69
 END OF CHAPTER 14 
Ch. 14 - 70