Chapter Nine
Benzene and Its Derivatioves
Key words
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Aromatic/Anti-Aromatic ring
Huckel Rules
Electrophilic Aromatic Substitution
Friedel Crafts Alkylation
Friedel Crafts Acylation
Para Substitution
Meta Substitution
Ortho Substitution
Aromaticity—Hückel’s Rule
Four structural rules must be satisfied for a compound to
be aromatic.
1. A molecule must be cyclic.
To be aromatic, each p orbital must overlap with p orbitals on
adjacent atoms.
2
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.
3
Aromaticity—Hückel’s Rule
3.
A molecule must be completely conjugated.
Aromatic compounds must have a p orbital on every atom.
4
Aromaticity—Hückel’s Rule
4.
A molecule must satisfy Hückel’s rule, and contain
a particular number of  electrons.
Hückel's rule:
Benzene is aromatic and especially stable because it contains
6  electrons. Cyclobutadiene is antiaromatic and especially
unstable because it contains 4  electrons.
5
Aromaticity—Hückel’s Rule
Note that Hückel’s rule refers to the number of  electrons,
not the number of atoms in a particular ring.
NUMBER OF π ELECTRONS THAT SATISFY HUCKEL’S RULE
n
4n + 2
0
2
1
6
2
10
3
14
4
18
5 etc…
22
6
Aromaticity—Hückel’s Rule
Considering aromaticity, a compound can be classified in
one of three ways:
1. Aromatic—A cyclic, planar, completely
compound with 4n + 2  electrons.
conjugated
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.
7
The Basis of Hückel’s Rule
• Why does the number of  electrons determine whether a
compound is aromatic?
• The basis of aromaticity can be better understood by
considering orbitals and bonding.
8
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.
9
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.
10
Examples of Aromatic Rings
• Two or more six-membered rings with alternating double and single
bonds can be fused together to form polycyclic aromatic
hydrocarbons (PAHs).
• As the number of fused rings increases, the number of resonance
structures increases.
11
Examples of Aromatic Rings
• Heterocycles containing oxygen, nitrogen or sulfur, can also be aromatic.
• With heteroatoms, we must determine whether the lone pair is localized
on the heteroatom or part of the delocalized  system.
• An example of an aromatic heterocycle is pyridine.
12
Examples of Aromatic Rings
• Pyrrole is another example of an aromatic heterocycle. It contains a
five-membered ring with two  bonds and one nitrogen atom.
• Pyrrole has a p orbital on every adjacent atom, so it is completely
conjugated.
• Pyrrole has six  electrons—four from the  bonds and two from the
lone pair.
• Pyrrole is cyclic, planar, completely conjugated, and has 4n + 2 
electrons, so it is aromatic.
13
Examples of Aromatic Rings
Both negatively and positively charged ions can be aromatic if they possess
all the necessary elements.
We can draw five equivalent resonance structures for the
cyclopentadienyl anion.
14
Examples of Aromatic Rings
• Of the three species below, only the cyclopentadienyl anion satisfies
Hückel’s rule.
15
Examples of Aromatic Rings
• Cyclopentadiene is more acidic than many hydrocarbons because its
conjugate base is aromatic.
• The pKa of cyclopentadiene is 15, much lower (acidic) than the pKa of any
C—H bond discussed thus far.
• The cyclopentadienyl anion is both aromatic and resonance stabilized, so
it is a very stable base.
• Cyclopentadiene itself is not aromatic because it is not fully conjugated.
16
Examples of Aromatic Compounds
Porphyrin Ring
Electrophilic Aromatic Substitution
H
aromatic
resonance
stabilized
aromatic
Resonance delocalization of cationic intermediate:
a) Sulfonation
Re-aromatization
Sulfonation differs from other substitutions since it is reversible
b) Nitration
Nitronium ion
c) Halogenation
Cl
Cl
Cl
Fe Cl
Cl
Cl
Cl Fe Cl
Cl
+
Cl
Cl
Cl
Cl
Fe Cl
Cl
d) Friedel-Crafts Alkylation
R
C l
A l C l
C l
C l
E le c r to p h ile , E =
R
C l
C l
A l C l
C l
C l
R
R
H
C l
A l C l
C l
+
R
+
R -C l
A lC l3
H
+
R
E le c tr o p h ile =
–
C l A lC l3
B
H
R
C l A lC l3
+
–
R
C l A lC l3
R
R
+
H r e p la c e d b y R , R = a lk y l g r o u p
–
d) Friedel-Crafts Alkylation
Carbocation formation can lead to REARRANGEMENTS!!
e) Friedel-Crafts Acylation
R C
O
C l
A l C l
C l
O
C l
E le c r to p h ile , E =
A c y liu m
O
io n
C R
R C
O
C R
C l
A l C l
C l
+
C l
C l
C l
A l C l
C l
While Friedel-Crafts acylation can be used to make aryl ketones, it cannot be used
to make corresponding aldehydes due to the instability of formyl choride.
Limitations of Friedel-Crafts Alkylation
• There are two major limitations on F-C alkylations.
• It is practical only with stable carbocations, such as 2° and 3° carbocations.
• It fails on benzene rings bearing one or more strongly electron-withdrawing groups.
O
O
O
O
O
CH
CR
COH
COR
CNH2
SO3 H
C N
NO2
NR3
CF3
CCl3
+
Other Benzene Alkylations
• Generation of carbocations
• Treating an alkene with a protic acid, most commonly H2SO4 or H3PO4.
+
H
O
H H
+
E
=
c
a
rb
o
c
a
tio
n
H
H2SO4 or H3PO4
H
H2SO4 or H3PO4
Product after carbocation
rearrangement
Other Benzene Alkylations
Treating an alcohol with H2SO4 or H3PO4.
H
O
H H
O
H+
H
O
H
2
O
+ H
2
+
E
=ca
rb
o
ca
tio
n
H O
+
H
2
S O
4
o r H
3
P O
H
2
O
4
O H
+
H
H
2
S O
4
o r H
3
P O
H
2
4
P r o d u c t a fte r c a r b o c a tio n
re a rra n g e m e n t
O
Oxidation of Benzylic Carbons
• Benzene is unaffected by strong oxidizing agents such as H2CrO4 and
KMnO4.
• Halogen and nitro substituents are unaffected by these reagents.
• An alkyl group with at least one hydrogen on the benzylic carbon is
oxidized to a carboxyl group.
C
O
H
2
H
C
rO
2
4
H
e
a
t
H
O
C
2
C
O
H
2
H
C
r
O
2
4
C
O
H
2
H
e
a
t
C
O
H
2
2
e
q
u
i
v
a
l
e
n
t
s
C O 2H
H 2 C rO
H eat
4
Samples reactions for Electrophillic Aromatic Substitution :CO2H
KMnO4, 
Br2, FeBr3
then H+
or H2CrO4
Br
Cl
AlCl3
Br
conc. H2SO4
SO3H
HNO3, H2SO4
NO2
Br2, FeBr3
O
Cl
Cl
O
NO REACTION
AlCl3
AlCl3
O
Cl
AlCl3
NO REACTION
Activating/Deactivating Effects of Substituents (Reactivity)
Relative Rates of Electrophilic Substitution
Two step mechanism, carbocation intermediate
Stability of the carbocation intermediate determines overall rate
electron-donating groups increase rate
electron-withdrawing groups decrease rate
Electron Withdrawal and Donation from/to
Hyperconjugative Electron Donation
Resonance Electron Donation
Inductive Electron Withdrawal
Relative Reactivity of Substituted Benzenes
Relative Reactivity of Substituted Benzenes
Di- and Polysubstitution
• Existing groups on a benzene ring influence further substitution in
both orientation and rate.
• Orientation
• Certain substituents direct new substitution preferentially toward
ortho-para positions, others direct preferentially toward meta
positions.
• Rate
• Certain substituents are activating toward further substitution,
others are deactivating toward further substitution.
Directing Effects of Substituents (Selectivity)
Directing Effects of Substituents (Selectivity)
Directing Effects of Substituents (Selectivity)
Effect of substitution on carbocation intermediates:
•
Carbocation-stabilizing substituents stabilize the ortho and para substitution pathways
and are therefore ortho/para directing.
•
Carbocation-destabilizing substituents destabilize the ortho and para substitution
pathways and are therefore meta directing.
Substituent Effects
Methoxy and hydroxy substituents are so strongly activating.
O C H
O C H
3
O C H
3
B r
B r2
C H
A n is o le
3
3
+
C O O H
B r
p - B r o m o a n is o le
4 %
o - B r o m o a n is o le
9 6 %
Nitration mechanism
(Assume para substitution)
OCH3
OCH3
+
NO2
NO2
r e s o n a n c e c o n tr ib u tin g s tr u c tu r e s
OCH3
OCH3
OCH3
OCH3
1
NO2
NO2
NO2
M O ST STABLE
O 2N
H
H O
H
R e a r o m a tiz a tio n
Contributor places positive charge on the most
electronegative atom (oxygen) and outside the ring
OCH3
M A JO R
NO2
Nitration mechanism
(Assume meta substitution)
OCH3
OCH3
+
NO2
SLO W
NO2
R e a r o m a tiz a tio n
OCH3
OCH3
NO2
H
OCH3
NO2
H
NO2
H
O N L Y 3 r e s o n a n c e c o n tr ib u tin g s tr u c tu r e s
H 2O
fa s t
OCH3
m e ta p r o d u c t
V E R Y L IT T L E
NO2
Nitration mechanism
EW G
NO2
NO2
(Assume meta substitution)
NO2
NO2
HNO3
+
+
NO2
m e ta p ro d u c t
m a jo r, 9 3 %
NO2
o
H 2S O 4, 1 0 0 C
n itra tio n
N O
o rth o a n d p a ra < 7 %
N O
2
+
N O
NO2
2
2
NO
2
R e a r o m a tiz a tio n
NO
NO
2
NO
H
NO
2
NO
2
2
NO
2
H
2
H
H 2O
r e s o n a n c e c o n tr ib u tin g s tr u c tu r e s
NO
2
m e ta p ro d u c t
m a jo r , 9 3 %
N O
2
Nitration mechanism
(Assume para substitution)
N O
2
N O
2
N O
2
2
N O
2
v e r y s lo w !!!
+
N O
N O
2
r e s o n a n c e c o n tr ib u tin g s tr u c tu r e s
R e a r o m a tiz a tio n
N O 2
N O 2
N O 2
+ c h a rg e n e x t
to s tro n g E W G
s o n o t fo rm e d
Contributor places positive charge on adjacent atoms
So only two resonace contributing structures left!!!
H
N O
2
N O
2
N O
2
H 2O
N O T FO R M ED
Examples
OCH
3
H N O 3, H 2S O
O
B r2, F e B r3
4
Examples
HNO 3 , H 2SO 4
O
Conc. fuming
H 2 SO 4
End of Chapter Nine
Benzene and Its Derivatioves
Key words








Aromatic/Anti-Aromatic ring
Huckel Rules
Electrophilic Aromatic Substitution
Friedel Crafts Alkylation
Friedel Crafts Acylation
Para Substitution
Meta Substitution
Ortho Substitution