Benzene
Organic Chemistry, Third Edition
• Benzene (C6H6) (not to be confused with Benzine) is the
simplest aromatic hydrocarbon (arene).
• Benzene has four degrees of unsaturation, making it a highly
unsaturated hydrocarbon.
• Unlike most unsaturated hydrocarbons, benzene does not
undergo addition reactions.
Janice Gorzynski Smith
University of Hawai’i
Chapter 17-18
Lecture Outline
Prepared by Layne A. Morsch
The University of Illinois - Springfield
• Benzene reacts with bromine in the presence of a Lewis acid
to give a substitution product.
Adapted for 2302266/272 by Tirayut Vilaivan
Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.
1
Historical Background
• Some possible structures of benzene
• August Kekulé proposed that benzene was a rapidly equilibrating
mixture of two compounds, each containing a six-membered ring
with three alternating π bonds.
• In the Kekulé structures, the bond between any two carbon atoms is
sometimes a single bond and sometimes a double bond.
• Although benzene is usually drawn as a six-membered ring with
alternating π bonds, in reality there is no equilibrium between the
two different kinds of benzene molecules.
• Current descriptions of benzene are based on resonance and
3
electron delocalization due to orbital overlap.
2
Structure of Benzene
Any structure for benzene must account for the following facts:
1. It contains a six-membered ring and three additional
degrees of unsaturation.
2. It is planar.
3. All C–C bond lengths are equal.
• The Kekulé structures satisfy the first two criteria but not the
third, because having three alternating π bonds means that
benzene should have three short double bonds alternating
with three longer single bonds.
4
Resonance Hybrid of Benzene
Representations of Benzene
• The resonance description of benzene consists of two
equivalent Lewis structures, each with three double bonds
that alternate with three single bonds.
• The true structure of benzene is a resonance hybrid of the two
Lewis structures, with the dashed lines of the hybrid
indicating the position of the π bonds.
• We will use one of the two Lewis structures and not the hybrid
in drawing benzene. This will make it easier to keep track of
the electron pairs in the π bonds (the π electrons).
• Because each π bond has two electrons, benzene has six π
electrons.
• In benzene, the actual bond length (1.39 Å) is intermediate
between the carbon–carbon single bond (1.53 Å) and the
carbon–carbon double bond (1.34 Å).
5
Benzene Bond Lengths
6
Physical Properties of Benzene
• Overlap of six adjacent p orbitals creates two rings of edensity, one above and one below the plane of the benzene
ring.
• These π electrons make benzene electron-rich and reactive
with strong electrophiles.
• A colorless liquid of pleasant aroma with b.p. 80.1 oC
• Has a high m.p. of 5.5 oC and density 0.8765 g/cm3 at 20 oC
(cf. toluene m.p. = -95 oC)
• Flammable, insoluble in water
• Used mainly as an intermediate
to make other chemicals and
as solvents
• Is a known carcinogen
Figure 17.1
Entry: BENZEN from CCDC database
7
Left: Ice vs solid benzene
Above: Packing of benzene molecules in
the solid state.
8
Nomenclature of Benzene Derivatives
Naming Substituted Benzenes
• There are three different ways that two groups can be
attached to a benzene ring, so a prefix—ortho, meta, or para—
can be used to designate the relative position of the two
substituents.
• To name a benzene ring with one substituent, name the
substituent and add the word benzene.
• Many monosubstituted benzenes have common names which
you must also learn.
ortho-dibromobenzene
or
o-dibromobenzene
or 1,2-dibromobenzene
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meta-dibromobenzene
or
m-dibromobenzene
or 1,3-dibromobenzene
para-dibromobenzene
or
p-dibromobenzene
or 1,4-dibromobenzene
10
Nomenclature of Benzene Derivatives
Rules for Naming Benzene Derivatives
• If the two groups on the benzene ring are different,
alphabetize the names of the substituents preceding the word
benzene.
• If one substituent is part of a common root, name the
molecule as a derivative of that monosubstituted benzene.
For three or more substituents on a benzene ring:
1. Number to give the lowest possible numbers around the ring.
2. Alphabetize the substituent names.
3. When substituents are part of common roots, name the
molecule as a derivative of that monosubstituted benzene. The
substituent that comprises the common root is located at C1.
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12
Naming Benzene as a Substituent
Benzyl and Aryl Groups
• A benzene substituent is called a phenyl group, and it can be
abbreviated in a structure as “Ph-”.
• The benzyl group, another common substituent that contains
a benzene ring, differs from a phenyl group.
• Therefore, benzene can be represented as PhH, and phenol
would be PhOH.
• Substituents derived from other substituted aromatic rings
are collectively known as aryl groups (abbreviated as Ar).
13
14
• Benzene derivatives show several characteristic frequencies
• C-H Stretching occurs near 3030 cm-1
• Stretching motions of the ring give bands at 1450-1600 cm-1 and
two bands near 1500 and 1600 cm-1
• Substituted benzenes show characteristic bands at 650-850 cm-1.
Br
672, 684, 735
C-H stretching
Overtone bands
C=C stretching
Out-of-plane C-H bending
Br
Br
746
Br
Br
727, 770
Br
15
Br
806
810
IR spectra were obtained from http://sdbs.riodb.aist.go.jp/sdbs.
1H
13C
NMR
NMR
Interesting Aromatic Compounds
Br
• Benzene and toluene, the simplest aromatic hydrocarbons
obtained from petroleum refining, are useful starting materials for
synthetic polymers.
• They are also two of the components of the BTX mixture added to
gasoline to boost octane ratings.
• Compounds containing two or more benzene rings that share
carbon–carbon bonds are called polycyclic aromatic
hydrocarbons (PAHs).
• Naphthalene, the simplest PAH, is the active ingredient in
mothballs.
Br
Br
Br
Br
Br
Br
18
17
NMR spectra were obtained from http://sdbs.riodb.aist.go.jp/sdbs.
Benzo[a]pyrene, a Common PAH
Stability of Benzene
• Benzo[a]pyrene, produced by the incomplete oxidation of
organic compounds in tobacco, is found in cigarette smoke.
• When ingested or inhaled, benzo[a]pyrene and other similar
PAHs are oxidized to carcinogenic products.
• Consider the heats of hydrogenation of cyclohexene,
1,3-cyclohexadiene and benzene, all of which give
cyclohexane when treated with excess hydrogen in the
presence of a metal catalyst.
Figure 17.3
19
20
Heats of Hydrogenation for Benzene
Unusual Reactivity of Benzene
Figure 17.6
• Benzene’s unusual behavior is not limited to hydrogenation.
• Benzene does not undergo addition reactions typical of other
highly unsaturated compounds, including conjugated dienes.
• Benzene does not react with Br2 to yield an addition product.
• Instead, in the presence of a Lewis acid, bromine substitutes for
a hydrogen atom, yielding a product that retains the benzene
ring.
• The huge difference between the hypothetical and observed
heats of hydrogenation for benzene cannot be explained solely
on the basis of resonance and conjugation.
• The low heat of hydrogenation of benzene means that benzene is
especially stable—even more so than conjugated polyenes.
• This unusual stability is characteristic of aromatic compounds.
21
22
The Criteria for Aromaticity
The Criteria for Aromaticity
Four structural criteria must be satisfied for a compound
to be aromatic:
1. A molecule must be cyclic.
2. A molecule must be planar.
• All adjacent p orbitals must be aligned so that the π electron
density can be delocalized.
• To be aromatic, each p orbital must overlap with p orbitals on
adjacent atoms.
• Cyclooctatetraene behaves rather like normal alkenes.
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24
The Criteria for Aromaticity
The Criteria for Aromaticity
4. A molecule must satisfy Hückel’s rule, and contain a
particular number of π electrons.
3. A molecule must be completely conjugated.
• Aromatic compounds must have a p orbital on every atom.
Hückel's rule:
• Benzene is aromatic and especially stable because it contains
6 π electrons.
• Cyclobutadiene is antiaromatic and especially unstable
because it contains 4 π electrons.
25
Aromatic, Antiaromatic, and Nonaromatic
• Considering aromaticity, a compound can be classified in one
of three ways:
1. Aromatic—A cyclic, planar, completely conjugated
compound with 4n + 2 π electrons.
2. Antiaromatic—A cyclic, planar, completely conjugated
compound with 4n π electrons.
3. Not aromatic (nonaromatic)—A compound that lacks one (or
more) of the following requirements for aromaticity; being
cyclic, planar, and completely conjugated.
27
26
Larger Aromatic Rings
• Completely conjugated rings larger than benzene are also
aromatic if they are planar and have 4n + 2 π electrons.
• Hydrocarbons containing a single ring with alternating double
and single bonds are called annulenes.
• To name an annulene, indicate the number of atoms in the ring
in brackets and add the word annulene.
28
Hückel’s Rule and Number of π Electrons
• [10]-Annulene has 10 π electrons, which satisfies Hückel's
rule, but a planar molecule would place the two H atoms
inside the ring too close to each other.
• Thus, the ring puckers to relieve this strain.
• Since [10]-annulene is not planar, the 10 π electrons cannot
delocalize over the entire ring and it is not aromatic.
29
8.9 ppm
5.18 ppm
-1.4 ppm
10.58 ppm
Crystal structure of [16]-annulene
Entry: ANNULE01 from CCDC database
Crystal structure of [18]-annulene 30
Entry: ANULEN from CCDC database
Fused Ring Aromatics
Heterocyles
• Two or more six-membered rings with alternating double and
single bonds can be fused together to form PAHs.
• There are two different ways to join three rings together, forming
anthracene and phenanthrene.
• Heterocycles containing oxygen, nitrogen or sulfur, can also
be aromatic.
• With heteroatoms, we must determine whether the lone pair is
localized on the heteroatom or part of the delocalized π
system.
• An example of an aromatic heterocycle is pyridine.
• As the number of fused rings increases, the number of resonance
structures increases. Naphthalene is a hybrid of three resonance
structures whereas benzene is a hybrid of two.
31
32
Aromatic Heterocycles
Ionic Aromatic Compounds
• Pyrrole is another example of an aromatic heterocycle. It contains
a five-membered ring with two π bonds and one nitrogen atom.
• Pyrrole has a p orbital on every adjacent atom, so it is completely
conjugated.
• Pyrrole has six π electrons—four from the π bonds and two from
the lone pair.
• Pyrrole is cyclic, planar, completely conjugated, and has 4n + 2 π
electrons, so it is aromatic.
33
Cyclopentadienyl Compounds
• Both negatively and positively charged ions can be aromatic
if they possess all the necessary elements.
• We can draw five equivalent resonance structures for
the cyclopentadienyl anion.
34
Cyclopentadienyl Anion
• Cyclopentadiene itself is not aromatic because it is not fully
conjugated.
• Having the “right” number of electrons is necessary for a
species to be unusually stable by virtue of aromaticity.
• Thus, although five resonance structures can also be drawn
for the cyclopentadienyl cation and radical, only the
cyclopentadienyl anion has 6 π electrons, a number that
satisfies Hückel’s rule.
35
• However, cyclopentadiene is much more acidic (pKa = 15)
than most hydrocarbons because its conjugate base is
aromatic and therefore, very stable.
• Loss of a proton causes the cyclopentadienyl anion to
become fully conjugated and consequently aromatic.
36
Tropylium Cation
The Basis of Hückel’s Rule
• The tropylium cation is a planer carbocation with three
double bonds and a positive charge contained in a sevenmembered ring.
• Because the tropylium cation has three π bonds and no other
nonbonded electron pairs, it contains six π electrons and is
fully conjugated, thereby satisfying Hückel’s rule.
37
π Molecular Orbitals for Benzene
• Why does the number of π electrons determine whether a
compound is aromatic?
• The basis of aromaticity can be better understood by
considering orbitals and bonding.
38
Inscribed Polygon Method of Predicting
Aromaticity
• Since each of the six carbon atoms in benzene has a p orbital,
six atomic p orbitals combine to form six π MOs.
Figure 17.9
39
40
Inscribed Polygon Method of Predicting
Aromaticity
Inscribed Polygon Method of Predicting
Aromaticity
Figure 17.10
• This method works for all monocyclic completely conjugated
systems regardless of ring size.
• The total number of MOs always equals the number of vertices of
the polygon.
• The inscribed polygon method is consistent with Hückel's 4n + 2
rule—there is always one lowest energy bonding MO that can
hold two π electrons and the other bonding MOs come in
41
degenerate pairs that can hold a total of four π electrons.
42
Diamond and Graphite
Reactions of Aromatic Compounds
• The two most common elemental forms of carbon are
diamond and graphite.
• Their physical characteristics are very different because their
molecular structures are very different.
• The most typical reaction of aromatic compounds is
electrophilic aromatic substitution.
E+
E
H
+
+
H
• Benzene and derivatives undergoes addition reaction under
forcing conditions.
Cl
3 Cl2
Cl
H
H H
H
H
Cl
Cl
then EtOH
hv
Cl
Cl
mixture of isomers
"Birch Reduction"
O
O
HO
H2, Ni cat
OH
43
resorcinol
H
Na, liq NH3
900 psi (60 atm)
OH
cyclohexane-1,3-dione
O
44
H H
• Benzene ring carrying a leaving group undergoes
nucleophilic aromatic substitution. This is a difficult
reaction and usually requires one or more of electron
withdrawing group on the benzene ring.
Nu
Electrophilic Aromatic Substitution
• The characteristic reaction of benzene is electrophilic aromatic
substitution—a hydrogen atom is replaced by an electrophile.
-
Nu
X
+
X-
Z
Z
• Since benzene is especially stable, reactions that keep the
aromatic ring intact are favored.
difficult without Z group!
• C-H bonds in benzene is quite acidic and can be
deprotonated by alkyl lithium.
n
C4H9Li
Li
H
+
C4H10
phenyl lithium
• Benzene ring can be oxidized to give carbonyl
compounds or carboxylic acid derivatives and CO2.
O
O
CrO3
HOAc
O
1,4-naphthoquinone
Air
O
V2O5, ∆
O
Phthalic anhydride
45
Examples of Electrophilic Aromatic Substitution
Figure 18.1
46
Mechanism of Substitution
• Regardless of the electrophile used, all electrophilic aromatic
substitution reactions occur by the same two-step mechanism—
addition of the electrophile E+ to form a resonance-stabilized
carbocation, followed by deprotonation with base, as shown
below:
47
48
Resonance-Stabilized Aromatic Carbocation
• The first step in electrophilic aromatic substitution forms a
carbocation known as “benzenonium ion”, for which three
resonance structures can be drawn.
• To help keep track of the location of the positive charge:
Energy Diagram for Electrophilic Aromatic
Substitution
Figure 18.2
49
Halogenation
50
Mechanism of Electrophilic Halogenation
• In halogenation, benzene reacts with Cl2 or Br2 in the
presence of a Lewis acid catalyst, such as FeCl3 or FeBr3, to
give the aryl halides chlorobenzene or bromobenzene,
respectively.
• Analogous reactions with I2 and F2 are not synthetically
useful because I2 is too unreactive and F2 reacts too violently.
• Chlorination proceeds by a similar mechanism.
51
52
Friedel–Crafts Alkylation
Nitration and Sulfonation
• In Friedel–Crafts alkylation, treatment of benzene with an
alkyl halide and a Lewis acid (AlCl3) forms an alkyl benzene.
• Nitration and sulfonation introduce two different functional
groups into the aromatic ring.
• Nitration is especially useful because the nitro group can be
reduced to an NH2 group.
53
• The electrophile is a carbocation (or carbocation-like species
for 1o halides), which is generated by ionization of the halides.
54
Limitation of Friedel–Crafts Alkylation
[1] Vinyl halides and aryl halides do not react in Friedel–
Crafts alkylation.
[2] Rearrangements can occur.
• The carbocation can also be generated by various other means.
O
2
H+
+
HO
Used in production of polycarbonates
HO
OH
"bisphenol A"
[3] Not applicable to electron-deficient aromatic and
aromatic amine substrates
55
[4] Multiple FC alkylation is likely to occur.
56
Friedel–Crafts Acylation
Substituent Effects of Substituted Benzenes
• In Friedel–Crafts acylation, a benzene ring is treated with an acid
chloride (RCOCl) and AlCl3 to form a ketone.
• Each substituent either increases or decreases the electron
density in the benzene ring, and this affects the course of
electrophilic aromatic substitution.
• Donation of electron density to the ring makes benzene
more electron rich.
• Withdrawal of electron density from the ring makes
benzene less electron rich.
Q: What is the
electrophile in
Friedel-Crafts
acylation?
• There is no rearrangement in FC acylation, so it may be used for
synthesis of alkylbenzenes that cannot be obtained by FC alkylation.
AlCl3
Cl
• Electrophilic substitution on an already substituted benzene
produces isomers, some of which are favored over others.
O
O
+
• Many substituted benzene rings undergo electrophilic
aromatic substitution.
Zn/Hg
H3O+
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58
isobutylbenzene
Inductive and Resonance Effects
• To predict whether a substituted benzene is more or less
electron rich than benzene itself, we must consider the net
balance of both the inductive and resonance effects.
• Alkyl groups donate electrons by an inductive effect, but they
have no resonance effect because they lack nonbonded
electron pairs or π bonds.
• Thus, any alkyl-substituted benzene is more electron rich
than benzene itself.
• The same analysis can be done with groups other than alkyl.
Substituent Effects on Electrophilic
Aromatic Substitution
• Electrophilic aromatic substitution is a general reaction of all
aromatic compounds, including polycyclic aromatic
hydrocarbons, heterocycles, and substituted benzene
derivatives.
• A substituent affects two aspects of the electrophilic aromatic
substitution reaction:
1. The rate of the reaction: A substituted benzene reacts
faster or slower than benzene itself.
2. The orientation: The new group is located either ortho,
meta, or para to the existing substituent.
• The identity of the first substituent determines the
position of the second incoming substituent.
59
60
Categorizing Directors and Activators
Categorizing Directors and Activators
• All substituents can be divided into three general types:
61
Energy Diagrams Comparing Substitution Rates
• The energy diagrams below illustrate the effect of electronwithdrawing and electron-donating groups on the transition
state energy of the rate-determining step.
62
Methyl Carbocation Stabilization
• A CH3 group directs electrophilic attack ortho and para to itself
because an electron-donating inductive effect stabilizes the
carbocation intermediate.
Figure 18.6 Energy diagrams comparing the rate of electrophilic substitution of substituted benzenes
63
64
Nitro Carbocation Stabilization
Amine Carbocation Stabilization
• An NH2 group directs electrophilic attack ortho and para to itself
because the carbocation intermediate has additional resonance
stabilization.
• With the NO2 group (and all meta directors) meta attack occurs
because attack at the ortho and para position gives a
destabilized carbocation intermediate.
65
Friedel–Crafts Reactions and Ring Activation
• Treatment of benzene with an alkyl halide and AlCl3 places an
electron-donor R group on the ring.
66
Aromatics with Two Directing Groups
Case #1
• Since R groups activate the ring, the alkylated product
(C6H5R) is now more reactive than benzene itself towards
further substitution, and it reacts again with RCl to give
products of polyalkylation.
Case #2
• Polysubstitution does not occur with Friedel–Crafts acylation.
Case #3
67
68
Synthesis of Benzene Derivatives
Reactions at Side Chains: Halogenation
• Pathway I, in which bromination precedes nitration, yields
the desired product.
• Pathway II yields the undesired meta isomer.
• Benzylic C–H bonds are weaker than most other sp3 hybridized
C–H bonds, because homolysis forms a resonance-stabilized
benzylic radical.
• As a result, alkyl benzenes undergo selective bromination at
the weak benzylic C–H bond under radical conditions to form
the benzylic halide.
69
70
Reactions at Side Chains: Oxidation
Ionic and Radical Halogenation
• Alkyl benzenes undergo two different reactions depending on
the reaction conditions:
• Arenes containing at least one benzylic C–H bond are oxidized
with KMnO4 to benzoic acid.
• Substrates with more than one alkyl group are oxidized to
dicarboxylic acids.
• With Br2 and FeBr3 (ionic conditions), electrophilic aromatic
substitution occurs, resulting in replacement of H by Br on the
aromatic ring to form ortho and para isomers.
• Compounds without a benzylic hydrogen are inert to oxidation.
• With Br2 and light or heat (radical conditions), substitution of H
by Br occurs at the benzylic carbon of the alkyl group.
71
72
Reactions at Side Chains: Reduction of C=O
• Ketones formed as products of Friedel–Crafts acylation can be
reduced to alkyl benzenes by two different methods:
1. The Clemmensen reduction—uses zinc and mercury in the
presence of strong acid.
2. The Wolff–Kishner reduction—uses hydrazine (NH2NH2) and
strong base (KOH).
Reactions at Side Chains: Reduction of NO2
• A nitro group (NO2) that has been introduced on a benzene ring
by nitration with strong acid can readily be reduced to an amino
group (NH2) under a variety of conditions.
73
74
Synthesis of Acetaminophen
Multistep Synthesis
• p-Nitrobenzoic acid from benzene.
Retrosynthetic
analysis
Synthesis
p-aminophenol
Acetaminophen
(p-acetamidophenol)
75
Adapted from: http://commons.wikimedia.org/wiki/File:Synthesis_of_paracetamol_from_phenol.svg
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