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• AROMATIC compounds or ARENES include
benzene and benzene derivatives.
– Aromatic compounds are quite common.
– Many aromatic compounds were originally isolated from
fragrant oils.
– However, many aromatic compounds are odorless.
18.1 Intro to Aromatic Compounds
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Klein, Organic Chemistry 1e 18-1
• 8 of the 10 best-selling drugs have aromatic
moieties.
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Wiley & Sons, Inc.
Klein, Organic Chemistry 1e 18-2
• Coal contains aromatic rings fused together and
joined by nonromantic moieties.
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Klein, Organic Chemistry 1e 18-3
• Benzene is generally the parent name for
monosubstituted derivatives.
18.2 Nomenclature of Benzene
Derivatives
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Klein, Organic Chemistry 1e 18-4
• Many benzene derivatives have common names
that become the parent name. (memorize these)
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Wiley & Sons, Inc.
Klein, Organic Chemistry 1e 18-5
• If the substituent is larger than the ring, the
substituent becomes the parent chain.
• Aromatic rings are often represented with a Ph (for
phenyl) or with a φ (phi) symbol.
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Wiley & Sons, Inc.
Klein, Organic Chemistry 1e 18-6
• The common name for dimethyl benzene
derivatives is XYLENE.
– What do ORTHO, META, and PARA mean?
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Klein, Organic Chemistry 1e 18-7
• Locants are required for rings with more than 2
substituents.
1. Identify the parent chain (generally the aromatic
ring):
– Often a common name can be the parent chain.
2. Identify and name the substituents.
Copyright 2012 John
Wiley & Sons, Inc.
Klein, Organic Chemistry 1e 18-8
3. Number the parent chain
– A substituent that is part of the parent
name must be assigned locant
NUMBER 1.
4. List the numbered substituents
before the parent name in
alphabetical order:
– Ignore prefixes (except iso) when
ordering alphabetically.
– Complete the name for the molecule
above.
Copyright 2012 John
Wiley & Sons, Inc.
Klein, Organic Chemistry 1e 18-9
Klein, Organic Chemistry 1e
Section: 18.2 What is the correct name for the following?
a. o-bromoaniline
b. 2-bromoaniline
c. 1-amino-2-bromobenzene
d. all of the above
NH2
Br
• In 1866, August Kekulé proposed that benzene is
a ring comprised of alternating double and single
bonds.
– Kekulé suggested that the exchange of double and
single bonds was an equilibrium process.
18.3 Structure of Benzene
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Wiley & Sons, Inc.
Klein, Organic Chemistry 1e 18-11
• We now know that the aromatic structures are
resonance contributors rather than in equilibrium.
• Sometimes the ring is represented with a circle in
it
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Wiley & Sons, Inc.
Klein, Organic Chemistry 1e 18-12
• The stability that results from a ring being
aromatic is striking.
– Recall that in general, alkenes readily undergo addition
reactions.
– Aromatic rings are stable enough that they do not
undergo such reactions.
18.4 Stability of Benzene
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Wiley & Sons, Inc.
Klein, Organic Chemistry 1e 18-13
– Heats of hydrogenation can be used to quantify
aromatic stability.
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Klein, Organic Chemistry 1e 18-14
• Molecular orbital (MO) theory can help us explain
aromatic stability.
• The six atomic p-orbitals of benzene overlap to
make six MOs.
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Klein, Organic Chemistry 1e 18-15
Summary of Critical Points of MO Theory
• MO Theory is good for describing excited states,
ions, exotic species that defy Lewis structures.
• Mix n atomic orbitals to form n molecular orbitals
– each orbital has max occupation = 2 electrons
– Fill from lowest energy MO
– An MO has NO EFFECT until it contains an electron
– For each bonding MO, there exists an anti-bonding
MO that can exactly cancel the bonding MO
• Most likely excitation moves one electron from
HOMO to LUMO
• we can use FROST CIRCLES to predict the
relative MO energies.
18.4 Stability of Benzene
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Klein, Organic Chemistry 1e 18-17
• Use the FROST CIRCLES below to explain the
4n+2 rule.
– Note that the number of bonding orbitals is always an
odd number; aromatic compounds will always have an
odd number of electron pairs.
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Wiley & Sons, Inc.
Klein, Organic Chemistry 1e 18-18
• Some rings must carry a formal charge to be
aromatic.
• Consider a 5-membered ring.
18.5 Aromatic Compounds Other
Than Benzene
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Klein, Organic Chemistry 1e 18-19
• The pKa value for cyclopentadiene is much lower
than typical C-H bonds. WHY?
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Klein, Organic Chemistry 1e 18-20
vs.
• Consider a 7-membered ring.
– If six pi electrons are present, what charge will be
necessary?
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Wiley & Sons, Inc.
Klein, Organic Chemistry 1e 18-21
Klein, Organic Chemistry 1e
Which of the following are aromatic?
A. B. C.
a. A
b. B
c. C
d. A and B
e. B and C
f. A and C
g. none of the above
h. all of the above
• Heteroatoms (atoms other than C or H) can also
be part of an aromatic ring.
– If the heteroatom’s lone pair is necessary, it will be
included in the HÜCKEL number of pi electrons.
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Klein, Organic Chemistry 1e 18-23
• If the lone pair is necessary to make it aromatic,
the electrons will not be as basic.
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Klein, Organic Chemistry 1e 18-24
pKa=5.2
pKa=0.4
• The difference in electron density can also be
observed by viewing the electrostatic potential
maps.
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Wiley & Sons, Inc.
Klein, Organic Chemistry 1e 18-25
• Will the compounds below be aromatic,
antiaromatic, or non aromatic?
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Klein, Organic Chemistry 1e 18-26
Copyright 2012 John
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Klein, Organic Chemistry 1e 18-27
• A carbon that is attached to a benzene
ring is BENZYLIC.
• Recall that aromatic rings and alkyl
groups are not easily oxidized.
18.6 Reactions at the Benzylic
Position
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Klein, Organic Chemistry 1e 18-28
• HOWEVER, benzylic positions can readily be fully
oxidized.
– The benzylic position needs to have at least one proton
attached to undergo oxidation.
Copyright 2012 John
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Klein, Organic Chemistry 1e 18-29
• Permanganate can also be used as an oxidizing
reagent.
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Klein, Organic Chemistry 1e 18-30
• BENZYLIC positions have similar
reactivity to allylic positions.
– Benzylic positions readily undergo free radical
bromination.
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Klein, Organic Chemistry 1e 18-31
• Once the benzylic position is substituted with a
bromine atom, a range of functional group
transformations are possible.
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Klein, Organic Chemistry 1e 18-32
• Once the benzylic position is substituted with a
bromine atom, a range of functional group
transformations are possible.
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Wiley & Sons, Inc.
Klein, Organic Chemistry 1e 18-33
Copyright 2012 John
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Klein, Organic Chemistry 1e 18-34
• Give necessary reagents for the reactions below.
18.6 Reactions at the Benzylic
Position
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Klein, Organic Chemistry 1e 18-35
Klein, Organic Chemistry 1e
Section: 18.6 What is the correct order of reagents to
achieve the following synthesis?
a. 1) CH3CH2CH2MgBr 2) H2O
b. NBS, light, CCl4
c. HBr
d. NaBr
e. 1) LAH 2) H2O
O Br
1) A
2) E, B
3) E, C
4) E, D
5) E, B, C
• Under forceful conditions, benzene can be
reduced to cyclohexane.
– Is the process endothermic or exothermic? WHY?
– WHY are forceful conditions required?
18.7 Reduction of the Aromatic
Moiety
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Klein, Organic Chemistry 1e 18-37
• Vinyl side groups can be selectively reduced.
– ΔH is just slightly less than the expected –120 kJ/mol
expected for a C=C C–C conversion.
– WHY are less forceful conditions required?
Copyright 2012 John
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Klein, Organic Chemistry 1e 18-38
• Note that the BIRCH reduction product has sp3
hybridized carbons on opposite ends of the ring.
18.7 Reduction of the Aromatic
Moiety
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Wiley & Sons, Inc.
Klein, Organic Chemistry 1e 18-39
• Like alkenes, benzene can undergo the BIRCH
reduction.
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Klein, Organic Chemistry 1e 18-40
• Like alkenes, benzene can undergo the BIRCH
reduction.
– Draw the final product.
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Wiley & Sons, Inc.
Klein, Organic Chemistry 1e 18-41
– The presence of an electron donating alkyl side group
or EWD groups provides different regioselectivity. Why?
Copyright 2012 John
Wiley & Sons, Inc.
Klein, Organic Chemistry 1e 18-42
Klein, Organic Chemistry 1e
Section: 18.7 What is the product(s) of the following
reaction.
A. C.
B. D.
NO2
Na, CH3OH NH3
NO2
NO2
NO2
NO2
Klein, Organic Chemistry 1e
Section: 18.8
In 1H NMR spectrometry, aromatic C-H bonds show peaks
at _______ ppm.
In C NMR spectrometry, aromatic carbons show peaks at
_______ ppm.
In IR spectroscopy, aromatic C-H bonds show peaks at
_______ cm–1.
a. 3–4, 100–150, 1450–1600
b. 6.5-8.5, 100–150, 3000-3100
c. 3–4, 100–150, 2250
d. 6.5-8.5, 100–150, 1450–1600
18.8 Spectroscopy of Aromatic
Compounds
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Klein, Organic Chemistry 1e 18-45
Copyright 2012 John
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Klein, Organic Chemistry 1e 18-46
• IR spectra for ethylbenzene:
NMR of Aromatics
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Klein, Organic Chemistry 1e 18-47
– Recall from Section 16.5 how the anisotropic
effects of an aromatic ring affect NMR shifts.
NMR of Aromatics
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Wiley & Sons, Inc.
Klein, Organic Chemistry 1e 18-48
– The integration and splitting of protons in the
aromatic region of the 1H NMR (≈7 ppm) in
often very useful.
NMR of Aromatic Compounds
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Klein, Organic Chemistry 1e 18-49
– Because of possible ring symmetry, the number
of signals in the 13C NMR (≈100-150 ppm)
generally provides structural information.
Copyright 2012 John
Wiley & Sons, Inc.
Klein, Organic Chemistry 1e 18-50
– For the molecule below, predict the shift for the 13C signals, and predict the shift, integration,
and multiplicity for the 1H NMR signals.
Graphite, Buckyballs, and
Nanotubes
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Wiley & Sons, Inc.
Klein, Organic Chemistry 1e 18-51
• Graphite consists of layers of sheets of
fused aromatic rings.
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Klein, Organic Chemistry 1e 18-52
• Buckyballs are C60 spheres made of
interlocking aromatic rings.
– Fullerenes come in other sizes such as C70.
– How are Buckyballs aromatic when they are not
FLAT?
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Wiley & Sons, Inc.
Klein, Organic Chemistry 1e 18-53
• Fullerenes can also be made into tubes
(cylinders).
• Single, double, and multi-walled carbon nanotubes
have many applications:
– Conductive Plastics, Energy Storage, Conductive
Adhesives, Molecular Electronics, Thermal Materials,
Fibres and Fabrics, Catalyst Supports, Biomedical
Applications
