Chapter 14: reactions of Benzene and Substituted Benzenes
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Transcript Chapter 14: reactions of Benzene and Substituted Benzenes
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
CH2CH3
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
H3C
H3C
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 pseodopolarized in
an alternating fashion by substituents for product
analysis purposes.
• Electrophiles (+ groups) add to – positions
• Alkyl groups and atoms with lone pairs pseudopolarize
the ring carbon they are attached to +
Chapter 8
32
Electron-donating
Substituents
Chapter 8
33
Electron-withdrawing
Substituents
Atoms with + charge pseudopolarize 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