Aromaticity.
Reactions of Benzene
Chapter 15
Chapter 15
1
Contents of Chapter 15








Aromaticity
Heterocyclic Compounds
Chemical Consequences of Aromaticity
Molecular Orbital Description of Aromaticity
Reactivity Considerations
Mechanism for Electrophilic Substitution
Halogenation/Nitration/Sulfonation of
Benzene
Friedel–Crafts Reactions
Chapter 15
2
Aromaticity

Benzene is a cyclic compound which
has a planar structure with a delocalized
cloud of p electrons above and below
the plane of the ring
Chapter 15
3
Criteria for Aromaticity
•
There must be an uninterrupted ring of p orbitalbearing atoms leading to a delocalized p cloud



For the p cloud to be cyclic, the molecule must be
cyclic
For the p cloud to be uninterrupted, every ring atom
must have a p orbital
For the p cloud to form, each p orbital must be able to
overlap the p orbital on either side
Chapter 15
4
Criteria for Aromaticity
•
•
The p cloud must have an odd
number of pairs of p electrons, or
(2n+1)•2 = 4n+2 p electrons
Hückel’s rule
Chapter 15
5
Aromaticity

cyclooctatetraene
is nonaromatic

It is not planar
Chapter 15
6
Aromaticity
resonance broken
nonaromatic
2 p electrons
aromatic
Chapter 15
4 p electrons
antiaromatic
7
Aromaticity
Chapter 15
8
Aromaticity


The criteria for aromaticity also can be
applied to polycyclic hydrocarbons
Naphthalene (5 pairs of p electrons),
phenanthrene (7 pairs of p electrons), and
chrysene (9 pairs of p electrons) all are
aromatic
Chapter 15
9
Heterocyclic Compounds


Lone pair can’t be in p orbital because p orbital
used to build p bond with adjacent carbon(s)
The lone pair on pyridine’s nitrogen is in an sp2
hybrid, not part of the 3-pair aromatic p system
Chapter 15
10
Heterocyclic Compounds

In pyrrole the lone pair could be put into either an
sp3 hybrid or a p orbital with bonds in sp2 hybrid

Pyrrole puts the lone pair in a p orbital, making 3
pairs of p electrons (aromatic is more stable)
Chapter 15
11
Antiaromaticity
•
A compound is classified an
antiaromatic if it


Has an uninterrupted planar cyclic p
system
Has an even number of electron pairs
(4n p electrons)
Chapter 15
12
Antiaromaticity

Both cyclobutadiene and
cyclopentadienyl cation are planar p
systems and the number of p electron
pairs is even
Chapter 15
13
Antiaromaticity


An aromatic compound is more stable
than an analogous cyclic compound with
localized electrons
An antiaromatic compound is less stable
than the analogous cyclic compound with
localized electrons
Chapter 15
14
Heterocyclic Compounds

In above structures the N lone pairs could be put into either sp3
hybrids or p orbitals with bonds to N in sp2 hybrids

Both the lone pairs and bonds to N put into into sp3 hybrids,
minimizing antiaromaticity


First structure is forced to be planar so it’s still antiaromatic.
Second structure can become nonplanar but not enough to put pi
bonds perpendicular to each other so it still slightly antiaromatic.
Chapter 15
15
Heterocyclic Compounds

In furan and thiophene there are 2 pairs of
unshared electrons - one is an sp2 hybrid orbital
and one pair is in a p orbital, like pyrrole (3 pairs
of p electrons, aromatic)
Chapter 15
16
Heterocyclic Compounds
Chapter 15
17
Heterocyclic Compounds

Quinoline, indole, imidazole,
purine, and pyrimidine also are
aromatic heterocyclic compounds
Chapter 15
18
Chemical Consequences of
Aromaticity
Chapter 15
19
Chemical Consequences of
Aromaticity

Cyclopentadiene has such a low pKa
because of the stability of the anion
formed when the hydrogen ionizes - the
anion is aromatic
Chapter 15
20
Chemical Consequences of
Aromaticity

Cycloheptatrienyl bromide is ionic
because of the stability of the aromatic
cycloheptatrienyl cation
Chapter 15
21
Reactivity Considerations

The benzene ring consists of a ring with p
electrons above and below

Electrophiles are attracted to a benzene
ring and form a nonaromatic carbocation
intermediate (a cyclohexadienyl cation)
H
+
Y
Y
Chapter 15
carbocation
intermediate
22
Electrophilic Substitution

Electrophilic addition doesn’t occur (would
destroy aromaticity)
Chapter 15
23
Reactivity Considerations
Chapter 15
24
Mechanism for Electrophilic
Substitution Reactions
Chapter 15
25
Halogenation of Benzene
Chapter 15
26
Halogenation of Benzene
Chapter 15
27
Halogenation of Benzene
Chapter 15
28
Nitration of Benzene
Chapter 15
29
Sulfonation of Benzene
Chapter 15
30
Sulfonation of Benzene
Chapter 15
31
Friedel–Crafts Acylation
Chapter 15
32
Friedel–Crafts Alkylation
Chapter 15
33
Friedel-Crafts Alkylation
• Two problems with Friedel–Crafts alkylation

Reaction proceeds through a carbocation which is
subject to rearrangement
Chapter 15
34
Friedel-Crafts Alkylation


The reaction product is more reactive
toward Friedel–Crafts alkylation than the
original reactant, leading to multiple
substitutions
A large excess benzene must be used to
minimize multiple substitutions
Chapter 15
35
Friedel-Crafts Alkylation

Even with primary alkyl halides
rearrangements occur via incipient primary
carbocations
The carbocation never really forms, but the
incipient carbocation remains complexed with
the catalyst and behaves like a primary cation

Chapter 15
36
Alkylation via Acylation
Followed by Reduction

Problems associated with Friedel–
Crafts alkylation can be avoided by
conducting an acylation followed by a
reduction of the carbonyl group to a
methylene group (CH2)
Chapter 15
37
Alkylation via Acylation
Followed by Reduction
• Two methods of reduction available


Clemmensen reduction in acid solution
Wolff-Kishner reduction in basic solution
Chapter 15
38
Alkylation via Acylation
Followed by Reduction

The method of reduction depends on
other groups on the molecule
Chapter 15
39