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1 1
2
Benzene and Aromatic Compounds
• Benzene (C6H6) is the simplest aromatic hydrocarbon (or
arene).
• Benzene has four degrees of unsaturation, making it a
highly unsaturated hydrocarbon.
• Whereas unsaturated hydrocarbons such as alkenes,
alkynes and dienes readily undergo addition reactions,
benzene does not.
Background
3
Benzene and Aromatic Compounds
• Benzene does react with bromine, but only in the presence of
FeBr3 (a Lewis acid), and the reaction is a substitution, not an
addition.
Background
• Proposed structures of benzene must account for its high
degree of unsaturation and its lack of reactivity towards
electrophilic addition.
• August Kekulé proposed that benzene was a rapidly
equilibrating mixture of two compounds, each containing a six-
membered ring with three alternating bonds.
• In the Kekulé description, the bond between any two carbon
atoms is sometimes a single bond and sometimes a double
bond.
4
Benzene and Aromatic Compounds
• These structures are known as Kekulé structures.
Background
• Although benzene is still drawn as a six-membered ring with
alternating bonds, in reality there is no equilibrium between the
two different kinds of benzene molecules.
• Current descriptions of benzene are based on resonance and
electron delocalization due to orbital overlap.
• In the nineteenth century, many other compounds having
properties similar to those of benzene were isolated from natural
sources. Since these compounds possessed strong and
characteristic odors, they were called aromatic compounds. It
should be noted, however, that it is their chemical properties,
and not their odor, that make them special.
5
Benzene and Aromatic Compounds
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 Structure of Benzene
The Kekulé structures satisfy the first two criteria but not the
third, because having three alternating bonds means that
benzene should have three short double bonds alternating with
three longer single bonds.
6
Benzene and Aromatic Compounds
• 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 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 bonds (the electrons).
The Structure of Benzene
7
Benzene and Aromatic Compounds
• Because each bond has two electrons, benzene has six
electrons.
The Structure of Benzene
8
Benzene and Aromatic Compounds
• 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).
The Structure of Benzene
9
Molecular Orbital Model of Benzene
• The concepts of hybridization of atomic orbitals and resonance
provided the first adequate structure of benzene.
• Benzene has a six carbon skeleton in a regular hexagon with C-C-
C angles of 120o. All the carbons are the same length (1.39 Å) as
well as the hydrogens (1.09 Å).
• The hybridization of the C-C bonds is sp2-sp2 whereas the C-H
bond is sp2-1s.
CC
C CC
CH
H
H
H
HH
120o
120o
sp2-sp2
sp2-1s
1.39 Å
1.09 Å
10
Benzene and Aromatic Compounds
• To name a benzene ring with one substituent, name the
substituent and add the word benzene.
Nomenclature of Benzene Derivatives
• Many monosubstituted benzenes have common names
which you must also learn.
11
Benzene and Aromatic Compounds
• 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.
Nomenclature of Benzene Derivatives
ortho-dibromobenzene
or
o-dibromobenzene
or 1,2-dibromobenzene
meta-dibromobenzene
or
m-dibromobenzene
or 1,3-dibromobenzene
para-dibromobenzene
or
p-dibromobenzene
or 1,4-dibromobenzene
12
Benzene and Aromatic Compounds
• 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.
Nomenclature of Benzene Derivatives
13
Benzene and Aromatic Compounds
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.
Nomenclature of Benzene Derivatives
14
Benzene and Aromatic Compounds
• A benzene substituent is called a phenyl group, and it can be
abbreviated in a structure as “Ph-”.
Nomenclature of Benzene Derivatives
• Therefore, benzene can be represented as PhH, and phenol
would be PhOH.
15
Benzene and Aromatic Compounds
• The benzyl group, another common substituent that contains a
benzene ring, differs from a phenyl group.
Nomenclature of Benzene Derivatives
• Substituents derived from other substituted aromatic rings are
collectively known as aryl groups.
16
Benzene and Aromatic Compounds
Four structural criteria must be satisfied for a compound
to be aromatic.
The Criteria for Aromaticity—Hückel’s Rule
[1] A molecule must be cyclic.
To be aromatic, each p orbital must overlap with p orbitals
on adjacent atoms.
17
Benzene and Aromatic Compounds
The Criteria for Aromaticity—Hückel’s Rule
[2] A molecule must be planar.
All adjacent p orbitals must be aligned so that the
electron density can be delocalized.
Since cyclooctatetraene is non-planar, it is not aromatic,
and it undergoes addition reactions just like those of other
alkenes.
18
19
Benzene and Aromatic Compounds
The Criteria for Aromaticity—Hückel’s Rule
[3] A molecule must be completely conjugated.
Aromatic compounds must have a p orbital on every atom.
20
Benzene and Aromatic Compounds
The Criteria for Aromaticity—Hückel’s Rule
[4] A molecule must satisfy Hückel’s rule, and contain
a particular number of electrons.
Benzene is aromatic and especially stable because it
contains 6 electrons. Cyclobutadiene is antiaromatic and
especially unstable because it contains 4 electrons.
Hückel's rule:
21
Benzene and Aromatic Compounds
The Criteria for Aromaticity—Hückel’s Rule
Note that Hückel’s rule refers to the number of electrons,
not the number of atoms in a particular ring.
22
Benzene and Aromatic Compounds
The Criteria for Aromaticity—Hückel’s Rule
1. Aromatic—A cyclic, planar, completely conjugated
compound with 4n + 2 electrons.
2. Antiaromatic—A cyclic, planar, completely conjugated
compound with 4n electrons.
3. Not aromatic (nonaromatic)—A compound that lacks
one (or more) of the following requirements for
aromaticity: being cyclic, planar, and completely
conjugated.
Considering aromaticity, a compound can be classified in
one of three ways:
23
Benzene and Aromatic Compounds
The Criteria for Aromaticity—Hückel’s Rule
Note the relationship between each compound type and a similar
open-chained molecule having the same number of electrons.
24
Benzene and Aromatic Compounds
The Criteria for Aromaticity—Hückel’s Rule
• 1H NMR spectroscopy readily indicates whether a
compound is aromatic.
• The protons on sp2 hybridized carbons in aromatic
hydrocarbons are highly deshielded and absorb at 6.5-8
ppm, whereas hydrocarbons that are not aromatic
absorb at 4.5-6 ppm.
25
Benzene and Aromatic Compounds
Examples of Aromatic Rings
• Completely conjugated rings larger than benzene are
also aromatic if they are planar and have 4n + 2
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.
26
O
N
i)ii) iii)
vii)vii
v)iv)
vi) ix) x)
Aromatic Heterocyclic Compound
Aromatic Compound
Which of the following compounds are Aromatic, anti Aromatic and non aromatic Compounds
27
28
Benzene and Aromatic Compounds
Examples of Aromatic Rings
• [10]-Annulene has 10 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 electrons
can’t delocalize over the entire ring and it is not
aromatic.
29 29
Electrophilic Aromatic Substitution
• The characteristic reaction of benzene is electrophilic aromatic
substitution—a hydrogen atom is replaced by an electrophile.
Background
30 30
• Benzene does not undergo addition reactions like other
unsaturated hydrocarbons, because addition would yield
a product that is not aromatic.
• Substitution of a hydrogen keeps the aromatic ring
intact.
• There are five main examples of electrophilic aromatic
substitution.
31 31
32 32
• 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:
• * The intermediate cyclohexadienyl cation thus formed is, in general called arenium ion (or some times a σ sigma
complex
σ sigma complex
33 33
• The first step in electrophilic aromatic substitution forms a
carbocation, for which three resonance structures can be
drawn. To help keep track of the location of the positive
charge:
34 34
• The energy changes in electrophilic aromatic substitution are
shown below:
35 35
• 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.
Halogenation
36 36
• Chlorination proceeds by a similar mechanism.
37 37
• 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.
Nitration and Sulfonation
38 38
• Generation of the electrophile in nitration requires
strong acid.
39 39
• Generation of the electrophile in sulfonation requires
strong acid.
40 40
• In Friedel-Crafts alkylation, treatment of benzene with an
alkyl halide and a Lewis acid (AlCl3) forms an alkyl
benzene.
Friedel-Crafts Alkylation and Friedel-Crafts Acylation
41 41
• In Friedel-Crafts acylation, a benzene ring is treated with
an acid chloride (RCOCl) and AlCl3 to form a ketone.
• Because the new group bonded to the benzene ring is
called an acyl group, the transfer of an acyl group from
one atom to another is an acylation.
42 42
Friedel-Crafts Alkylation and Friedel-Crafts Acylation
43 43
44 44
• In Friedel-Crafts acylation, the Lewis acid AlCl3 ionizes the
carbon-halogen bond of the acid chloride, thus forming a
positively charged carbon electrophile called an acylium ion,
which is resonance stabilized.
• The positively charged carbon atom of the acylium ion then goes
on to react with benzene in the two step mechanism of
electrophilic aromatic substitution.
45 45
Three additional facts about Friedel-Crafts alkylation
should be kept in mind.
[1] Vinyl halides and aryl halides do not react in Friedel-
Crafts alkylation.
46 46
[2] Rearrangements can occur.
These results can be explained by carbocation
rearrangements.
47 47
48 48
Rearrangements can occur even when no free carbocation
is formed initially.
49 49
[3] Other functional groups that form carbocations can
also be used as starting materials.
50 50
Each carbocation can then go on to react with benzene to
form a product of electrophilic aromatic substitution. For
example:
51 51
Starting materials that contain both a benzene ring and an
electrophile are capable of intramolecular Friedel-Crafts
reactions.
52 52
Formylation
It is the substitution of formyl group -CHO in the benzen ring.
Formylation can be brought about by
• Gattermann Koch Reaction
• Formyl Fuoride
Gattermann Koch Reaction
When carbon monoxide is passed through a solution of
Benzene containing anhydrous AlCl3, CuCl and HCl
formylation CO + HCl Cl H
O
H Cl
O
AlCl3 H
O
H
O
+
+
AlCl4
O
H
53
Formylation of benzene occurs when formyl fluorde reacts with it in the presence of BF3 in
ether. The reaction is carried out at low temperature
Formylation Using Formyl Fluoride
H
O
+
O
H
F
BF3HBF4+
54
Carboxylation
It is the substation of carboxylic group (-COOH) in the
benzene ring. For example carboxylation of benzene takes
place on treatment with Phosgene gas in the presence of
AlCl3
Cl Cl
O
AlCl3 Cl
O
+
+
AlCl4
O
ClBF3
Cl Cl
O
O
OHAlCl3
H2O
Cl
OH
O
Cl
O
ClH2O
O
OH
Mechanism
55
56 56
1) Why is benzene less reactive than an alkene?
The pi electrons of benzene are delocalized over 6
atoms, thus making benzene more stable and less
available for electron donation.
While an alkene’s electrons are localized between two
atoms, thus making it more nucleophillc and more
reactive toward electrophiles.
57 57
2) Show how the other two resonance structures can be
deprotonated in step two of electrophillic aromatic
substitution. H
E
BE
H
E
BE
58 58
H
E
BE
59 59
3) Draw a detailed mechanism of the chlorination of
benzene.
Cl ClFeCl3 Cl Cl FeCl3
Formation of Electrophile
Cl Cl FeCl3
HH
Cl
H
Cl
H
Cl
+ FeCl4
Electrophillic Additon
60 60
H
Cl
FeCl4
Cl
+ HCl + FeCl3
Deprotonation
61 61
4) Draw stepwise mechanism for the sulfonation of A.
SO3
H2SO4
SO3H
a
b
O
S
O O
H OSO3H SO3H + HSO4
Formation of Electrophile
62 62
H
SO3H
RR
H
SO3H
R
H
SO3H
R
H
SO3H
Electrophillic Addition
R
H
SO3H
HSO4R
SO3H
+ H2SO4
Deprotonation
63 63
5) What product is formed when benzene is reacted with
each of the following alkyl halides?
+ (CH3)2CHClAlCl3
a) CH(CH3)2
+
Cl
AlCl3
b)
64 64
+AlCl3
H3CH2C
Cl
O
c) O
CH2CH3
65 65
6) What acid chloride is necessary to produce each
product from benzene using a Friedal-Crafts acylation?
O
CH2CH2CH(CH3)2
a)
Cl
O
CH2CH2CH(CH3)2
O
b) O
Cl
66 66
O
c)
O
Cl
67 67
7) Draw a stepwise mechanism for the following
friedal-Crafts alkylation?
+ CH3CH2Cl
AlCl3
CH2CH3
CH3CH2Cl AlCl3 H3CH2C Cl AlCl3
Formation of Electrophile
68 68
H3CH2C Cl AlCl3
H
H
CH2CH3
H
CH2CH3
H
CH2CH3
Electrophillic Additoon
H
CH2CH3
AlCl3Cl
CH2CH3
+ HCl + AlCl3
Protonation
69 69
8) Which of these halides are reactive in a Friedal-Crafts
alkylation?
Br
A
Br
B
Br
CBr
D
Br
B
Br
D
Look at the carbon to which the halogen is
attached and determine its hybridization. If sp2 its
unreactive, while sp3 is reactive.
70 70
9) Draw a stepwise mechanism for the following
reaction.
+ (CHe)2CHCH2Cl
AlCl3
C(CH3)3
+ HCl + AlCl3
H3C
CH3
H
H2C Cl
AlCl3H3C
CH3
H
H2C Cl AlCl3
H3C
CH3
CH3+ AlCl4
Formation of Electrophile
1,2 H shift
71 71
H3C
CH3
CH3
H H C(CH3)3 H C(CH3)3 H C(CH3)3
Electrophillic Additon
H C(CH3)3
AlCl4
C(CH3)3
+ AlCl3 + HCl
Deprotonation
72 72
10) Draw the product of each reaction
+H2SO4
a)
+H2SO4
(H3C)2C CH2
b)
C(CH3)3
73 73
+H2SO4
OH
c)
+H2SO4
OHd)
74 74
Substituted Benzenes
Many substituted benzene rings undergo electrophilic
aromatic substitution.
Each substituent either increases or decreases the
electron density in the benzene ring, and this affects the
course of electrophilic aromatic substitution.
75 75
Considering inductive effects only, the NH2 group
withdraws electron density and CH3 donates electron
density.
76 76
Resonance effects are only observed with substituents
containing lone pairs or bonds.
An electron-donating resonance effect is observed
whenever an atom Z having a lone pair of electrons is
directly bonded to a benzene ring.
77 77
• An electron-withdrawing resonance effect is observed in
substituted benzenes having the general structure
C6H5-Y=Z, where Z is more electronegative than Y.
• Seven resonance structures can be drawn for
benzaldehyde (C6H5CHO). Because three of them place a
positive charge on a carbon atom of the benzene ring,
the CHO group withdraws electrons from the benzene
ring by a resonance effect.
78 78
• 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.
• For example, alkyl groups donate electrons by an
inductive effect, but they have no resonance effect
because they lack nonbonded electron pairs or bonds.
• Thus, any alkyl-substituted benzene is more electron
rich than benzene itself.
79 79
• The inductive and resonance effects in compounds having the
general structure C6H5-Y=Z (with Z more electronegative than Y)
are both electron withdrawing.
80 80
• These compounds represent examples of the general
structural features in electron-donating and electron
withdrawing substituents.
81 81
Electrophilic Aromatic Substitution and Substituted
Benzenes.
• 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.
82 82
• Consider toluene—Toluene reacts faster than benzene
in all substitution reactions.
• The electron-donating CH3 group activates the benzene
ring to electrophilic attack.
• Ortho and para products predominate.
• The CH3 group is called an ortho, para director.
83 83
• Consider nitrobenzene—It reacts more slowly than
benzene in all substitution reactions.
• The electron-withdrawing NO2 group deactivates the
benzene ring to electrophilic attack.
• The meta product predominates.
• The NO2 group is called a meta director.
84 84
All substituents can be divided into three general types:
85 85
86 86
• Keep in mind that halogens are in a class by
themselves.
• Also note that:
87 87
• To understand how substituents activate or deactivate the ring,
we must consider the first step in electrophilic aromatic
substitution.
• The first step involves addition of the electrophile (E+) to form a
resonance stabilized carbocation.
• The Hammond postulate makes it possible to predict the relative
rate of the reaction by looking at the stability of the carbocation
intermediate.
88 88
• The principles of inductive effects and resonance effects can
now be used to predict carbocation stability.
89 89
90 90
Orientation Effects in Substituted Benzenes
• There are two general types of ortho, para directors and
one general type of meta director.
• All ortho, para directors are R groups or have a
nonbonded electron pair on the atom bonded to the
benzene ring.
• All meta directors have a full or partial positive charge
on the atom bonded to the benzene ring.
91 91
To evaluate the effects of a given substituent, we can use
the following stepwise procedure:
92 92
• A CH3 group directs electrophilic attack ortho and para to itself
because an electron-donating inductive effect stabilizes the
carbocation intermediate.
93 93
• An NH2 group directs electrophilic attack ortho and para to itself
because the carbocation intermediate has additional resonance
stabilization.
94 94
• With the NO2 group (and all meta directors) meta attack occurs
because attack at the ortho and para position gives a
destabilized carbocation intermediate.
95 95
Figure 18.7 The reactivity and directing
effects of common substituted
benezenes
96
12)Identify each group as having an electron
donating or electron withdrawing inductive effect.
a) CH3CH2CH2CH2-
Electron donating
b) Br-
Electron withdrawing
c) CH3CH2O-
Electron withdrawing
97
14)Identify as electron donating or electron withdrawing.
a) OCH3
Lone pair on oxygen, electron
donating
b) I Halogen, electron
withdrawing
c) C(CH3)3 Alkyl group, electron
donating
98
OCH3
CH3CH2Cl
AlCl3
15)Predict the products.
a) OCH3
CH2CH3
+
OCH3
H3CH2C
b) Br
HNO3
H2SO4
Br
NO2
+
Br
O2N
99
c)
NO2
Cl2
FeCl3
NO2
Cl
100
16)Predict the products when reacted with HNO3 and
H2SO4. Also state whether the reactant is more or less
reactive than benzene.
a) OO
NO2
N
b) N
O2N
Less
Less
101
Cl
c)
Cl
NO2
+
Cl
O2N
Less
d)
OHOH
NO2
+
OH
O2N
More
102
CH2CH3
CH2CH3
NO2
+
CH2CH3
O2N
More
d)
103
17)Label each compound as less or more reactive
than benzene.
a)
C(CH3)3
more
OH
OH
b)
more
104
c) O
OCH2CH3 less
N(CH3)3
d)
less
105
18) Rank each group in order of increasing reactivity.
Cl OCH3
a)
2 3 1
NO2 CH3
b)
2 3 1
106 106
Disubstituted Benzenes
1. When the directing effects of two groups reinforce, the
new substituent is located on the position directed by
both groups.
107 107
2. If the directing effects of two groups oppose each
other, the more powerful activator “wins out.”
108 108
Synthesis of Benzene Derivatives
In a disubstituted benzene, the directing effects indicate
which substituent must be added to the ring first.
Let us consider the consequences of bromination first
followed by nitration, and nitration first, followed by
bromination.
109