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Chapter 17
Reactions of Aromatic Compounds
Jo BlackburnRichland College, Dallas, TX
Dallas County Community College District2003,Prentice Hall
Organic Chemistry , 5th Edition
L. G. Wade, Jr.
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Chapter 17 2
Electrophilic
Aromatic SubstitutionElectrophile substitutes for a hydrogen on
the benzene ring.
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Chapter 17 3
Mechanism
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Chapter 17 4
Bromination of Benzene• Requires a stronger electrophile than Br 2.
• Use a strong Lewis acid catalyst, FeBr 3.
Br Br FeBr 3 Br Br FeBr 3 Animation
Br Br FeBr 3
H
H
H
H
H
H
H
H
H
H
HH
Br + + FeBr 4
_
Animation
Animation Br
HBr +=>
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Chapter 17 5
Energy Diagram
for Bromination
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Chapter 17 6
Chlorination
and Iodination• Chlorination is similar to bromination.
Use AlCl3 as the Lewis acid catalyst.
• Iodination requires an acidic oxidizingagent, like nitric acid, which oxidizes the
iodine to an iodonium ion.
H+
HNO3 I21/2 I
+ NO2 H2O+ ++ +
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Chapter 17 7
Nitration of BenzeneUse sulfuric acid with nitric acid to form
the nitronium ion electrophile.
H O N
O
O
H O S O H
O
O
+ HSO4
_ H O N
OH
O+
H O N
OH
O+
H2O + N
O
O
+NO2+ then forms asigma complex with
benzene, loses H+ to
form nitrobenzene. =>
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Chapter 17 8
Sulfonation
Sulfur trioxide, SO3, in fuming sulfuric acid
is the electrophile.
S
O
O OS
O
O OSO
O OSO
O O
+ + +
_
_ _
S
O
OO
H
SO
O
OH
+
_
S
HOO
O
benzenesulfonic a
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Chapter 17 9
Desulfonation
• All steps are reversible, so sulfonic acid
group can be removed by heating in
dilute sulfuric acid.
• This process is used to place deuterium
in place of hydrogen on benzene ring.
Benzene-d 6
=>
D
D
D
D
D
D
D2SO4/D2O
large excess
H
H
H
H
H
H
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Chapter 17 10
Nitration of Toluene
• Toluene reacts 25 times faster than benzene.
The methyl group is an activator.
• The product mix contains mostly ortho and
para substituted molecules.
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Chapter 17 11
Sigma Complex
Intermediate
is more
stable if
nitrationoccurs at
the ortho
or para
position.
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Chapter 17 12
Energy Diagram
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Chapter 17 13
Activating, O-, P -
Directing Substituents• Alkyl groups stabilize the sigma complex
by induction, donating electron density
through the sigma bond.• Substituents with a lone pair of electrons
stabilize the sigma complex by resonance.
OCH3
H
NO2
+
OCH3
H
NO2
+
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Chapter 17 14
Nitration of Anisole
Ortho attack
Meta attack
Para attack
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Chapter 17 15
The Amino Group
Aniline reacts with bromine water (without a
catalyst) to yield the tribromide. Sodium
bicarbonate is added to neutralize theHBr that’s also formed.
NH2
Br 23
H2O, NaHCO3
NH2
Br
Br
Br
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Chapter 17 16
Summary of
Activators
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Chapter 17 17
Deactivating Meta-
Directing Substituents• Electrophilic substitution reactions for
nitrobenzene are 100,000 times slower than for benzene.
• The product mix contains mostly themeta isomer, only small amounts of theortho and para isomers.
• Meta-directors deactivate all positionson the ring, but the meta position is lessdeactivated.
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Chapter 17 18
Ortho Substitution
on Nitrobenzene
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Chapter 17 19
Para Substitution
on Nitrobenzene
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Chapter 17 20
Meta Substitution
on Nitrobenzene
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Chapter 17 21
Energy Diagram
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Chapter 17 22
Structure of Meta-
Directing Deactivators
• The atom attached to the aromatic ring
will have a partial positive charge.
• Electron density is withdrawn inductively
along the sigma bond, so the ring is less
electron-rich than benzene.
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Chapter 17 23
Summary of Deactivators
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Chapter 17 24
More Deactivators
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Chapter 17 25
Halobenzenes
• Halogens are deactivating toward
electrophilic substitution, but are ortho,
para-directing!
• Since halogens are very electronegative,
they withdraw electron density from the
ring inductively along the sigma bond.
• But halogens have lone pairs of electrons
that can stabilize the sigma complex by
resonance. =>
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Chapter 17 26
Sigma Complex
for Bromobenzene
Br
E+
Br
H
E
(+)
(+)(+)
Ortho attack
+ Br
E
+
Br
H E
+
(+)
(+)(+)
Para attack
Ortho and para attacks produce a bromonium ion
and other resonance structures.
=>
Meta attack
Br
E+
Br
H
H
E
+
(+)
(+) No bromonium ionpossible with meta attack.
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Chapter 17 27
Energy Diagram
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Chapter 17 28
Summary of
Directing Effects
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Chapter 17 29
Multiple Substituents
The most strongly activating substituent
will determine the position of the next
substitution. May have mixtures.
OCH3
O2 N
SO3
H2SO
4
OCH3
O2 N
SO3H
OCH3
O2 N
SO3H
+
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Chapter 17 30
Friedel-Crafts Alkylation
• Synthesis of alkyl benzenes from alkyl
halides and a Lewis acid, usually AlCl3.
• Reactions of alkyl halide with Lewis acidproduces a carbocation which is the
electrophile.
• Other sources of carbocations:alkenes + HF or alcohols + BF3.
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Chapter 17 31
Examples of
Carbocation Formation
CH3 CH CH3
Cl
+ AlCl3
CH3
C
H3C H
Cl AlCl3+ _
H2C CH CH3
HFH3C CH CH3
F+
_
H3C CH CH3
OHBF3
H3C CH CH3
OH BF3+
H3C CH CH3
++ HOBF3
_
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Chapter 17 32
Formation of
Alkyl Benzene
C
CH3
CH3
H+
H
H
CH(CH3)2+
H
H
CH(CH3)2
B
F
F
F
OH
CH
CH3
CH3
+
HF
BF
OHF
=>
+
-
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Chapter 17 33
Limitations of
Friedel-Crafts• Reaction fails if benzene has a substituent
that is more deactivating than halogen.
• Carbocations rearrange. Reaction of benzene with n-propyl chloride and AlCl3
produces isopropylbenzene.
• The alkylbenzene product is more reactivethan benzene, so polyalkylation occurs.
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Chapter 17 34
Friedel-Crafts
Acylation• Acyl chloride is used in place of alkyl
chloride.
• The acylium ion intermediate isresonance stabilized and does not
rearrange like a carbocation.
• The product is a phenyl ketone that isless reactive than benzene.
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Chapter 17 35
Mechanism of Acylation
R C
O
Cl AlCl3 R C
O
AlCl3Cl+ _
R C
O
AlCl3Cl+ _ AlCl4 +
_ +R C O R C O
+
C
O
R +
H
C
H
O
R
+Cl AlCl3
_ C
O
R +
HCl
AlCl3
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Chapter 17 36
Clemmensen Reduction
Acylbenzenes can be converted to
alkylbenzenes by treatment with
aqueous HCl and amalgamated zinc.
+ CH3CH2C
O
Cl1) AlCl3
2) H2O
C
O
CH2CH3Zn(Hg)
aq. HCl
CH2CH2CH3
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Chapter 17 37
Gatterman-Koch
Formylation• Formyl chloride is unstable. Use a high
pressure mixture of CO, HCl, and catalyst.
• Product is benzaldehyde.
CO + HCl H C
O
ClAlCl3/CuCl
H C O+
AlCl4
_
C
O
H
+ CO
H+ HCl+
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Chapter 17 38
Nucleophilic
Aromatic Substitution• A nucleophile replaces a leaving group
on the aromatic ring.
• Electron-withdrawing substituents
activate the ring for nucleophilic
substitution.
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Chapter 17 39
Examples of
Nucleophilic Substitution
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Chapter 17 40
Addition-Elimination
Mechanism
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Chapter 17 41
Benzyne Mechanism
• Reactant is halobenzene with no electron-withdrawing groups on the ring.
• Use a very strong base like NaNH2.
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Chapter 17 42
Benzyne Intermediate
CH3
HH
H NH2
CH3
HH
H NH2
_
or
CH3
HH
H
NH2
CH3
HH
H
NH2 _
meta-toluidine para-toluidine =>
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Chapter 17 43
Chlorination of Benzene
• Addition to the benzene ring may occur with high heat and pressure (or light).
• The first Cl2 addition is difficult, but the
next 2 moles add rapidly.• The product, benzene hexachloride, is
an insecticide.
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Chapter 17 44
Catalytic Hydrogenation
• Elevated heat and pressure is required.
• Possible catalysts: Pt, Pd, Ni, Ru, Rh.
•Reduction cannot be stopped at anintermediate stage.
=>
CH3
CH3
Ru, 100°C1000 psi3H2,
CH3
CH3
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Chapter 17 45
Birch Reduction:
Regiospecific
• A carbon with an e--withdrawing group
is reduced.
• A carbon with an e--releasing group
is not reduced.
C
O
OH Na, NH3
CH3CH2OH
C
O
O
H
_
OCH3 Li, NH3
(CH3)3COH, THF
OCH3
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Chapter 17 46
Birch Mechanism
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Chapter 17 47
Side-Chain Oxidation
Alkylbenzenes are oxidized to benzoic
acid by hot KMnO4 or Na2Cr 2O7/H2SO4.
CH(CH3)2
CH CH2
KMnO4, OH-
H2O, heat
COO
COO
_
_
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Chapter 17 48
Side-Chain Halogenation
• Benzylic position is the most reactive.
• Chlorination is not as selective as
bromination, results in mixtures.• Br 2 reacts only at the benzylic position.
=>
CHCH2CH3
Br
h Br 2,
CH2CH2CH3
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Chapter 17 49
SN1 Reactions
• Benzylic carbocations are resonance-
stabilized, easily formed.
• Benzyl halides undergo SN1 reactions.
CH2Br CH3CH2OH, heat
CH2OCH2CH3
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Chapter 17 50
SN2 Reactions
•Benzylic halides are 100 times morereactive than primary halides via SN2.
• Transition state is stabilized by ring.
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Chapter 17 51
Reactions of Phenols
• Some reactions like aliphatic alcohols:
phenol + carboxylic acid ester
phenol + aq. NaOH phenoxide ion
• Oxidation to quinones: 1,4-diketones.
OH
CH3
Na2Cr 2O7, H2SO4
CH3
O
O
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Chapter 17 52
Quinones
• Hydroquinone is used as a developer for film. It reacts with light-sensitized
AgBr grains, converting it to black Ag.• Coenzyme Q is an oxidizing agent
found in the mitochondria of cells.
=>
El hili
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Chapter 17 53
Electrophilic
Substitution of Phenols• Phenols and phenoxides are highly reactive.
• Only a weak catalyst (HF) required for
Friedel-Crafts reaction.
• Tribromination occurs without catalyst.
• Even reacts with CO2.O
_
CO2, OH-
O
C
O
O _
_
H+
OH
C
O
OH
salicylic acid
=>