Aromaticity.
Reactions of Benzene
Chapter 8
Chapter 8
1
Contents of Chapter 15










Aromaticity
Heterocyclic Compounds
Chemical Consequences of Aromaticity
Nomenclature
Reactivity Considerations
Mechanism for Electrophilic Substitution
Halogenation/Nitration/Sulfonation of Benzene
Friedel–Crafts Reactions
Substituent Effects
Retrosynthetic Analysis
Chapter 8
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 8
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 8
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 8
5
Aromaticity

cyclooctatetraene
is nonaromatic

It is not planar
Chapter 8
6
Aromaticity
resonance broken
nonaromatic
2 p electrons
aromatic
Chapter 8
4 p electrons
antiaromatic
7
Aromaticity
Chapter 8
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 8
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 8
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 8
11
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

Because the lone pairs give these rings an EVEN number of pi
electrons these rings are not aromatic

In these cases the lone pairs are put into sp3 hybrid orbitals
because nature doesn’t like even numbers of pairs of pi electrons
in cyclic pi systems
Chapter 8
12
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 8
13
Heterocyclic Compounds
Chapter 8
14
Heterocyclic Compounds

Quinoline, indole, imidazole,
purine, and pyrimidine also are
aromatic heterocyclic compounds
Chapter 8
15
Chemical Consequences of
Aromaticity
Chapter 8
16
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 8
17
Chemical Consequences of
Aromaticity

Cycloheptatrienyl bromide is ionic
because of the stability of the aromatic
cycloheptatrienyl cation
Chapter 8
18
Naming Monosubstituted
Benzenes
Br
Cl
NO2
bromobenzene chlorobenzene nitrobenzene
Chapter 8
C H 2CH 3
ethylbenzene
19
Naming Common
Monosubstituted Benzenes
OMe
anisole
CH3
OH
NH2
toluene
phenol
aniline
CHO
styrene
COOH
benzaldehyde benzoic acid
Chapter 8
CN
benzonitrile
20
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 8
carbocation
intermediate
21
Electrophilic Substitution

Electrophilic addition doesn’t occur (would
destroy aromaticity)
Chapter 8
22
Reactivity Considerations
Chapter 8
23
Mechanism for Electrophilic
Substitution Reactions
Chapter 8
24
Halogenation of Benzene
Chapter 8
25
Nitration of Benzene
Chapter 8
26
Anilines From Nitrobenzenes
• Anilines (aminobenzenes) are always made from nitrobenzenes.
• Anilines decompose and make black tar when exposed to
electrophilic aromatic substitution (EAS) reaction conditions
• For this reason nitrobenzenes are converted to anilines in the very
LAST step AFTER all other groups are added by EAS reactions
• There are several ways to convert nitrobenzenes to anilines but this
course teaches H2 and Pd/C
Chapter 8
27
Sulfonation of Benzene
Chapter 8
28
Friedel–Crafts Acylation
Chapter 8
29
Friedel–Crafts Alkylation
Chapter 8
30
Electron-donating
Substituents
O
CH3
O
CH3
H 3C
H 3C
O
O
O
CH3
Resonance contributors increase electron density in ortho
and para positions. Overall electron density is bigger.
Chapter 8
31
Electron-donating
Substituents
• A pi system can be considered to be polarized in an
alternating fashion by substituents for product analysis
purposes.
• Electrophiles (+ groups) add to – positions
• Alkyl groups and atoms with lone pairs polarize the ring
carbon thhey are attached to +
Chapter 8
32
Electron-donating
Substituents
Chapter 8
33
Electron-withdrawing
Substituents
Atoms with + charge polarize the attached ring carbon
negative (–)
Chapter 8
34
Electron-withdrawing
Substituents
Chapter 8
35
Naming Disubstituted
Benzenes
Chapter 8
36
Naming Disubstituted
Benzenes
Chapter 8
37
Retrosynthetic Analysis
• Work backwards from an aromatic compound to figure out
how to make it
• First if amino group (NH2) is present work it backwards to
nitro (NO2)
• Next remove one substituent and polarize the ring according
to positions of other substituents
• If the substituent you removed came from a – carbon remove
another substituent, polarize the ring again, and repeat
• If remaining substituents don’t agree on ring polarization or
latest removed substituent doesn’t remove from a – carbon
replace removed substituent and try to remove another one.
• Continue until all substituents have been removed
• Reverse the retrosynthetic analysis to figure out what
reagents to add to benzene in what order
Chapter 8
38
Retrosynthetic Analysis
Work out synthesis of 4-bromo-3-chloroaniline
using retrosynthetic analysis
Chapter 8
39