Download Aromatic Compounds

Aromatic Compounds
 Reactions of Benzene
Even though benzene is highly unsaturated it does not undergo
any of the regular reactions of alkenes such as addition or
oxidation
Benzene can be induced to react with bromine if a Lewis acid
catalyst is present however the reaction is a substitution and not
an addition

Benzene produces only one monobrominated compound, which indicates that all
6 carbon-hydrogen bonds are equivalent in benzene
Chapter 14
2
 The Kekule Structure for Benzene
Kekule was the first to formulate a reasonable representation of
benzene
The Kekule structure suggests alternating double and single
carbon-carbon bonds


Based on the Kekule structure one would expect there to be two different 1,2dibromobenzenes but there is only one
Kekule suggested an equilibrium between these compounds to explain this
observation but it is now known no such equilibrium exists
Chapter 14
3
 The Stability of Benzene
Benzene is much more stable than would be expected based on
calculations for “cyclohexatriene”



A reasonable prediction for the heat of hydrogenation of hypothetical
cyclohexatriene is -360 kJ mol-1 (3 times that of cyclohexene, -120 kJ mol-1 )
The experimentally determined heat of hydrogenation for benzene is -280 mol-1,
152 kJ mol-1 more stable than hypothetical cyclohexatriene
This difference is called the resonance energy
Chapter 14
4
 Modern Theories of the Structure of Benzene
 The Resonance Explanation of the Structure of Benzene
Structures I and II are equal resonance contributors to the real
structure of benzene



Benzene is particularly stable because it has two equivalent and important
resonance structures
Each carbon-carbon bond is 1.39 Å, which is between the length of a carboncarbon single bond between sp2 carbons (1.47Å) and a carbon-carbon double
bond (1.33 Å)
Often the hybrid is represented by a circle in a hexagon (III)
Chapter 14
5
 The Molecular Orbital Explanation of the Structure of
Benzene
The carbons in benzene are sp2 hybridized with p orbitals on all 6
carbons (a)

The p orbitals overlap around the ring (b) to form a bonding molecular orbital with
electron density above and below the plane of the ring (c)
There are six p molecular orbitals for benzene
Chapter 14
6
 Huckel’s Rule: The 4n+2p Electron Rule
Planar monocyclic rings with a continuous system of p orbitals
and 4n + 2p electrons are aromatic (n = 0, 1, 2, 3 etc)


Aromatic compounds have substantial resonance stabilization
Benzene is aromatic: it is planar, cyclic, has a p orbital at every carbon, and 6 p
electrons (n=1)
There is a polygon-and-circle method for deriving the relative
energies of orbitals of a system with a cyclic continuous array of p
orbitals



A polygon corresponding to the ring is inscribed in a circle with one point of the
polygon pointing directly down
A horizontal line is drawn where vertices of the polygon touch the circle - each
line corresponds to the energy level of the p MOs at those atoms
A dashed horizontal line half way up the circle indicates the separation of bonding
and antibonding orbitals
Benzene has 3 bonding and 3 antibonding orbitals

All the bonding orbitals are full and there are no electrons in antibonding orbitals;
benzene has a closed shell of delocalized electrons and is very stable
Chapter 14
7
Cyclooctatetraene has two nonbonding orbitals each with one
electron

This is an unstable configuration; cyclooctatetraene adopts a nonplanar
conformation with localized p bonds to avoid this instability
Chapter 14
8
 Aromatic Ions
Cyclopentadiene is unusually acidic (pKa = 16) because it becomes
the aromatic cyclopentadienyl anion when a proton is removed

Cyclopentadienyl anion has 6 p electrons in a cyclic, continuous p-electron
system, and hence follows the 4n + 2 rule for aromaticity
Cycloheptatriene is not aromatic because its p electrons are not
delocalized around the ring (the sp3-hybridized CH2 group is an
“insulator”)

Lose of hydride produces the aromatic cycloheptatrienyl cation (tropylium cation)
Chapter 14
9
 Other Aromatic Compounds
 Benzenoid Aromatic Compounds
Polycyclic benzenoid aromatic compounds have two or more
benzene rings fused together
Chapter 14
10
Naphthalene can be represented by three resonance structures


The most important resonance structure is shown below
Calculations show that the 10 p electrons of napthalene are delocalized and that it
has substantial resonance energy
Pyrene has 16 p electrons, a non-Huckel number, yet is known to
be aromatic

Ignoring the central double bond, the periphery of pyrene has 14 p electrons, a
Huckel number, and on this basis it resembles the aromatic [14]annulene
Chapter 14
11
 Fullerenes
Buckminsterfullerene is a C60 compound shaped like a soccer ball
with interconnecting pentagons and hexagons


Each carbon is sp2 hybridized and has bonds to 3 other carbons
Buckminsterfullerene is aromatic
Analogs of “Buckyballs” have been synthesized (e.g. C70)
Chapter 14
12
 Heterocyclic Aromatic Compounds
Heterocyclic compounds have an element other than carbon as a
member of the ring
Example of aromatic heterocyclic compounds are shown below

Numbering always starts at the heteroatom
Pyridine has an sp2 hybridized nitrogen



The p orbital on nitrogen is part of the aromatic p system of the ring
The nitrogen lone pair is in an sp2 orbital orthogonal to the p orbitals of the ring;
these electrons are not part of the aromatic system
The lone pair on nitrogen is available to react with protons and so pyridine is
basic
Chapter 14
13
The nitrogen in pyrrole is sp2 hybridized and the lone pair resides
in the p orbital


This p orbital contains two electrons and participates in the aromatic system
The lone pair of pyrrole is part of the aromatic system and not available for
protonation; pyrrole is therefore not basic
In furan and thiophene an electron pair on the heteroatom is also
in a p orbital which is part of the aromatic system
Chapter 14
14
 Nomenclature of Benzene Derivatives
Benzene is the parent name for some monosubstituted benzenes;
the substituent name is added as a prefix
For other monosubstituted benzenes, the presence of the
substituent results in a new parent name
Chapter 14
15
When two substituents are present their position may be indicated
by the prefixes ortho, meta, and para (o, m and p) or by the
corresponding numerical positions
Dimethyl substituted benzenes are called xylenes
Chapter 14
16
Numbers must be used as locants when more than two
substituents are present



The lowest possible set of numbers should be given to the substituents
The substituents should be listed in alphabetical order
If one of the substituents defines a parent other than benzene, this substituent
defines the parent name and should be designated position 1
Chapter 14
17
The C6H5- group is called phenyl when it is a substituent



Phenyl is abbreviated Ph or F
A hydrocarbon with a saturated chain and a benzene ring is named by choosing
the larger structural unit as the parent
If the chain is unsaturated then it must be the parent and the benzene is then a
phenyl substituent
The phenylmethyl group is called a benyl (abbreviated Bz)
Chapter 14
18
Reactions of Aromatic Compounds
 Electrophilic Aromatic Substitution
Arene (Ar-H) is the generic term for an aromatic hydrocarbon

The aryl group (Ar) is derived by removal of a hydrogen atom from an arene
Aromatic compounds undergo electrophilic aromatic substitution
(EAS)

The electrophile has a full or partial positive charge
Chapter 14
20
 A General Mechanism for Electrophilic Aromatic
Substitution: Arenium Ion Intermediates
Benzene reacts with an electrophile using two of its p electrons

This first step is like an addition to an ordinary double bond
Unlike an addition reaction, the benzene ring reacts further so that
it may regenerate the very stable aromatic system
In step 1 of the mechanism, the electrophile reacts with two p
electrons from the aromatic ring to form an arenium ion

The arenium ion is stabilized by resonance which delocalizes the charge
In step 2, a proton is removed and the aromatic system is
regenerated
Chapter 14
21
The energy diagram of this reaction shows that the first step is
highly endothermic and has a large DG‡ (1)


The first step requires the loss of aromaticity of the very stable benzene ring,
which is highly unfavorable
The first step is rate-determining
The second step is highly exothermic and has a small DG‡ (2)

The ring regains its aromatic stabilization, which is a highly favorable process
Chapter 14
22
 Halogenation of Benzene
Halogenation of benzene requires the presence of a Lewis acid
Fluorination occurs so rapidly it is hard to stop at
monofluorination of the ring

A special apparatus is used to perform this reaction
Iodine is so unreactive that an alternative method must be used
Chapter 14
23
In the step 1 of the mechanism, bromine reacts with ferric bromide
to generate an electrophilic bromine species
In step 2, the highly electrophilic bromine reacts with p electrons
of the benzene ring, forming an arenium ion
 In step 3, a proton is removed from the arenium ion and
aromaticity is regenerated

The FeBr3 catalyst is regenerated
Chapter 14
24
 Nitration of Benzene
Nitration of benzene occurs with a mixture of concentrated nitric
and sulfuric acids

The electrophile for the reaction is the nitronium ion (NO2+)
Chapter 14
25
 Sulfonation of Benzene
Sulfonation occurs most rapidly using fuming sulfuric acid
(concentrated sulfuric acid that contains SO3)

The reaction also occurs in conc. sulfuric acid, which generates small quantities
of SO3, as shown in step 1 below
Chapter 14
26
Sulfonation is an equilibrium reaction; all steps involved are
equilibria


The sulfonation product is favored by use of concentrated or fuming sulfuric acid
Desulfonation can be accomplished using dilute sulfuric acid (i.e. with a high
concentration of water), or by passing steam through the reaction and collecting
the volatile desulfonated compound as it distils with the steam
Chapter 14
27
 Friedel-Crafts Alkylation
An aromatic ring can be alkylated by an alkyl halide in the
presence of a Lewis acid

The Lewis acid serves to generate a carbocation electrophile
Chapter 14
28
Primary alkyl halides probably do not form discreet carbocations
but the primary carbon in the complex develops considerable
positive charge
Any compound that can form a carbocation can be used to
alkylate an aromatic ring
Chapter 14
29
 Friedel-Crafts Acylation
An acyl group has a carbonyl attached to some R group
Friedel-Crafts acylation requires reaction of an acid chloride or
acid anhydride with a Lewis acid such as aluminium chloride
Chapter 14
30
Acid chlorides are made from carboxylic acids
Chapter 14
31
 Effects of Substituents on Reactivity and
Orientation
The nature of groups already on an aromatic ring affect both the
reactivity and orientation of future substitution




Activating groups cause the aromatic ring to be more reactive than benzene
Deactivating groups cause the aromatic ring to be less reactive than benzene
Ortho-para directors direct future substitution to the ortho and para positions
Meta directors direct future substitution to the meta position
 Activating Groups: Ortho-Para Directors
All activating groups are also ortho-para directors

The halides are also ortho-para directors but are mildly deactivating
The methyl group of toluene is an ortho-para director

Toluene reacts more readily than benzene, e.g. at a lower temperatures than
benzene
Chapter 14
32
The methyl group of toluene is an ortho-para director
Amino and hydroxyl groups are also activating and ortho-para
directors

These groups are so activating that catalysts are often not necessary
Alkyl groups and heteroatoms with one or more unshared electron
pairs directly bonded to the aromatic ring will be ortho-para
directors (see chart on slide 22)
Chapter 14
33
 Deactivating Groups: Meta Directors
Strong electron-withdrawing groups such as nitro, carboxyl, and
sulfonate are deactivators and meta directors
 Halo Substitutents: Deactivating Ortho-Para Directors
Chloro and bromo groups are weakly deactivating but are also
ortho, para directors

In electrophilic substitution of chlorobenzene, the ortho and para products are
major:
Chapter 14
34
 Classification of Substitutents
Chapter 14
35
 Theory of Substituent Effects on Electrophilic
Substitution
 Reactivity: The Effect of Electron-Releasing and
Electron-Withdrawing Groups
Electron-releasing groups activate the ring toward further reaction

Electron-releasing groups stabilize the transition state of the first step of
substitution and lead to lower DG‡ and faster rates of reaction
Electron-withdrawing groups deactivate the ring toward further
reaction

Electron-withdrawing groups destabilize the transition state and lead to higher
DG‡ and slower rates of reaction
Chapter 14
36
The following free-energy profiles compare the stability of the first
transition state in electrophilic substitution when various types of
substitutents are already on the ring

These substitutents are electron-withdrawing, neutral (e.g., H), and electrondonating
Chapter 14
37
 Inductive and Resonance Effects: Theory of Orientation
The inductive effect of some substituent Q arises from the
interaction of the polarized bond to Q with the developing positive
charge in the ring as an electrophile reacts with it

If Q is an electron-withdrawing group then attack on the ring is slowed because
this leads to additional positive charge on the ring
The following are some other groups that have an electronwithdrawing effect because the atom directly attached to the ring
has a partial or full positive charge
Chapter 14
38
The resonance effect of Q refers to its ability to increase or
decrease the resonance stabilization of the arenium ion

When Q has a lone pair on the atom directly attached to the ring it can stabilize
the arenium by contributing a fourth resonance form
Electron-donating resonance ability is summarized below
Chapter 14
39
 Ortho-Para Directing Groups
Many ortho-para directors are groups that have a lone pair of
electrons on the atom directly attached to the ring
Chapter 14
40
 Meta-directing Groups
All meta-directing groups have either a partial or full positive
charge on the atom directly attached to the aromatic ring
The trifluoromethyl group destabilizes the arenium ion
intermediate in ortho and para substitution pathways

The arenium ion resulting from meta substitution is not so destabilized and
therefore meta substitution is favored
Chapter 14
41
Activating groups having unshared electrons on the atom bonded
to the ring exert primarily a resonance effect



The aromatic ring is activated because of the resonance effect of these groups
They are ortho-para directors because they contribute a fourth important
resonance form which stabilizes the arenium ion in the cases of ortho and para
substitution only
The fourth resonance form that involves the heteroatom is particularly important
because the octet rule is satisfied for all atoms in the arenium ion
Chapter 14
42
Halo groups are ortho-para directors but are also deactivating


The electron-withdrawing inductive effect of the halide is the primary influence
that deactivates haloaromatic compounds toward electrophilic aromatic
substitution
The electron-donating resonance effect of the halogen’s unshared electron pairs
is the primary ortho-para directing influence
Chapter 14
43
 Ortho-Para Direction and Reactivity of Alkylbenzenes
Alkyl groups activate aromatic rings by inductively stabilizing the
transition state leading to the arenium ion
Alkyl groups are ortho-para directors because they inductively
stabilize one of the resonance forms of the arenium ion in ortho
and para substitution
Chapter 14
44
 Oxidation of the Side Chain
Alkyl and unsaturated side chains of aromatic rings can be
oxidized to the carboxylic acid using hot KMnO4
Chapter 14
45
Heteroaromatic compounds
Heterocyclic aromatic compounds – five
membered rings
Pyrrole is not a base
The dipole-moments of pyrrole and pyrrolydine
N
H
O
benzofurane
indole
S
benzothiophene
Heterocyclic aromatic compounds six membered
rings
Chapter 14
51
The pyridinium ion is a stronger acid than the piperidinium
on, therefore itself is a weaker base
Quinoline and isoquinoline are also known as
benzopyridines
Chapter 14
53
Imidazole
Chapter 14
54
Porphirin ring
Nitrogen containing heterocyclic
aromatic compounds
N
N
N
H
N
N
Purine
Pyrimidine
O
NH2
N
H
N
N
N
H
N
N
Thymine
NH2
H
N
O
H
Adenine
O
N
N
N
N
N
H
Guanine
NH2
N
H
Cytosine
O