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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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8 of the 10 best-selling drugs have aromatic moieties.
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Klein, Organic Chemistry 1e 18-2
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Coal contains aromatic rings fused together and joined by
nonromantic moieties.
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Klein, Organic Chemistry 1e 18-3
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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
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Many benzene derivatives have common names that become the
parent name. (memorize these)
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Klein, Organic Chemistry 1e 18-5
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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 (phi) symbol.
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Klein, Organic Chemistry 1e 18-6
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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
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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.
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Klein, Organic Chemistry 1e 18-8
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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.
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Klein, Organic Chemistry 1e 18-9
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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
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In 1866, August Kekul proposed that benzene is a ring comprised
of alternating double and single
bonds.
Kekul suggested that the exchange of double and single bonds was
an equilibrium process.
18.3 Structure of Benzene
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Klein, Organic Chemistry 1e 18-11
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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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Klein, Organic Chemistry 1e 18-12
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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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Klein, Organic Chemistry 1e 18-13
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Heats of hydrogenation can be used to quantify aromatic
stability.
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Klein, Organic Chemistry 1e 18-14
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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
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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
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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
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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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Klein, Organic Chemistry 1e 18-18
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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
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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.
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Consider a 7-membered ring.
If six pi electrons are present, what charge will be
necessary?
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Klein, Organic Chemistry 1e 18-21
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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
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Heteroatoms (atoms other than C or H) can also be part of an
aromatic ring.
If the heteroatoms lone pair is necessary, it will be included
in the HCKEL number of pi electrons.
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Klein, Organic Chemistry 1e 18-23
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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
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The difference in electron density can also be observed by
viewing the electrostatic potential
maps.
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Klein, Organic Chemistry 1e 18-25
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Will the compounds below be aromatic, antiaromatic, or non
aromatic?
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Klein, Organic Chemistry 1e 18-26
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Klein, Organic Chemistry 1e 18-27
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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
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HOWEVER, benzylic positions can readily be fully oxidized.
The benzylic position needs to have at least one proton attached
to undergo oxidation.
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Klein, Organic Chemistry 1e 18-29
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Permanganate can also be used as an oxidizing reagent.
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Klein, Organic Chemistry 1e 18-30
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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
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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
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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 18-33
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Klein, Organic Chemistry 1e 18-34
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Give necessary reagents for the reactions below.
18.6 Reactions at the Benzylic
Position
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Klein, Organic Chemistry 1e 18-35
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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
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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
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Vinyl side groups can be selectively reduced.
H is just slightly less than the expected 120 kJ/mol expected
for a C=C CC conversion.
WHY are less forceful conditions required?
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Klein, Organic Chemistry 1e 18-38
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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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Klein, Organic Chemistry 1e 18-39
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Like alkenes, benzene can undergo the BIRCH reduction.
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Klein, Organic Chemistry 1e 18-40
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Like alkenes, benzene can undergo the BIRCH reduction.
Draw the final product.
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Klein, Organic Chemistry 1e 18-41
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The presence of an electron donating alkyl side group or EWD
groups provides different regioselectivity. Why?
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Klein, Organic Chemistry 1e 18-42
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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
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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
_______ cm1.
a. 34, 100150, 14501600 b. 6.5-8.5, 100150, 3000-3100 c. 34,
100150, 2250 d. 6.5-8.5, 100150, 14501600
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18.8 Spectroscopy of Aromatic
Compounds
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Klein, Organic Chemistry 1e 18-45
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Klein, Organic Chemistry 1e 18-46
IR spectra for ethylbenzene:
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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.
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NMR of Aromatics
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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.
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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.
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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.
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Graphite, Buckyballs, and
Nanotubes
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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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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