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1
Chapter 17
Reactions of
Aromatic Compounds
Jo BlackburnRichland College, Dallas, TX
Dallas County Community College District 2006, Prentice Hall
Organic Chemistry, 6th EditionL. G. Wade, Jr.
Chapter 17 2
Electrophilic
Aromatic Substitution
Electrophile substitutes for a hydrogen on
the benzene ring.
=>
2
Chapter 17 3
Mechanism
Step 1: Attack on the electrophile forms the sigma complex.
=>
Step 2: Loss of a proton gives the substitution product.
Chapter 17 4
Bromination of Benzene• Requires a stronger electrophile than Br2.
• Use a strong Lewis acid catalyst, FeBr3.
Br
HBr+
Br Br FeBr3 Br Br FeBr3
+ -
Br Br FeBr3
H
H
H
H
H
H
H
H
H
H
HH
Br+ + FeBr4
_+ -
=>
3
Chapter 17 5
Comparison with Alkenes
• Cyclohexene adds Br2, ∆H = -121 kJ
• Addition to benzene is endothermic, not
normally seen.
• Substitution of Br for H retains
aromaticity, ∆H = -45 kJ.
• Formation of sigma complex is rate-
limiting. =>
Chapter 17 6
Energy Diagram
for Bromination
=>
4
Chapter 17 7
Chlorination and Iodination
• Chlorination is similar to bromination. Use AlCl3 as the Lewis acid catalyst.
• Iodination requires an acidic oxidizing agent, like nitric acid, which oxidizes the iodine to an iodonium ion.
H+
HNO3 I21/2 I
+NO2 H2O+ ++ +
=>
Chapter 17 8
Nitration of BenzeneUse sulfuric acid with nitric acid to form
the nitronium ion electrophile.
NO2+ then forms a
sigma complex withbenzene, loses H+ to
form nitrobenzene. =>
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
+
5
Chapter 17 9
Sulfonation
Sulfur trioxide, SO3, in fuming sulfuric
acid is the electrophile.
S
O
O OS
O
O OS
O
O OS
O
O O
+ + +
_
_ _
S
O
OO
H
S
O
O
OH
+
_
S
HOO
O
benzenesulfonic acid
=>
Chapter 17 10
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-d6
=>
D
D
D
D
D
D
D2SO4/D2O
large excess
H
H
H
H
H
H
6
Chapter 17 11
Nitration of Toluene
• Toluene reacts 25 times faster than benzene.
The methyl group is an activating group.
• The product mix contains mostly ortho and para substituted molecules.
=>
Chapter 17 12
Sigma Complex
Intermediate is more stable if
nitration occurs at the orthoor paraposition.
=>
7
Chapter 17 13
Energy Diagram
=>
Chapter 17 14
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
+
=>
8
Chapter 17 15
Substitution on Anisole
=>
Chapter 17 16
The Amino Group
Aniline, like anisole, reacts with bromine
water (without a catalyst) to yield the
tribromide. Sodium bicarbonate is added
to neutralize the HBr that’s also formed.
=>
9
Chapter 17 17
Summary ofActivators
=>
Chapter 17 18
Deactivating Meta-Directing Substituents
• Electrophilic substitution reactions for nitrobenzene are 100,000 times slower than for benzene.
• The product mix contains mostly the meta isomer, only small amounts of the ortho and para isomers.
• Meta-directors deactivate all positions on the ring, but the meta position is lessdeactivated.
=>
10
Chapter 17 19
Ortho Substitutionon Nitrobenzene
=>
Chapter 17 20
Para Substitution on Nitrobenzene
=>
11
Chapter 17 21
Meta Substitutionon Nitrobenzene
=>
Chapter 17 22
Energy Diagram
=>
12
Chapter 17 23
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.
=>
Chapter 17 24
Summary of Deactivators
=>
13
Chapter 17 25
More Deactivators
=>
Chapter 17 26
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. =>
14
Chapter 17 27
Sigma Complex
for Bromobenzene
Ortho and para attacks produce a bromonium ion
and other resonance structures.
=>
No bromonium ion
possible with meta attack.
Chapter 17 28
Energy Diagram
=>
15
Chapter 17 29
Summary of Directing Effects
=>
Chapter 17 30
Multiple Substituents
The most strongly activating substituent
will determine the position of the next
substitution. May have mixtures.
OCH3
O2N
SO3
H2SO4
OCH3
O2N
SO3H
OCH3
O2N
SO3H
+
=>
16
Chapter 17 31
Friedel-Crafts Alkylation
• Synthesis of alkyl benzenes from alkyl
halides and a Lewis acid, usually AlCl3.
• Reactions of alkyl halide with Lewis acid produces a carbocation which is the
electrophile.
• Other sources of carbocations:
alkenes + HF, or alcohols + BF3.
=>
Chapter 17 32
Examples ofCarbocation 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
_
=>
17
Chapter 17 33
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
=>
+
-
Chapter 17 34
Limitations ofFriedel-Crafts
• Reaction fails if benzene has a substituent
that is more deactivating than halogen.
• Carbocations rearrange. Reaction of benzene with n-propyl chloride and AlCl3produces isopropylbenzene.
• The alkylbenzene product is more reactive
than benzene, so polyalkylation occurs.
=>
18
Chapter 17 35
Friedel-CraftsAcylation
• Acyl chloride is used in place of alkyl
chloride.
• The acylium ion intermediate is resonance stabilized and does not
rearrange like a carbocation.
• The product is a phenyl ketone that is
less reactive than benzene.
=>
Chapter 17 36
Mechanism of Acylation
C
O
R
+
H
C
H
O
R
+
Cl AlCl3
_C
O
R +
HCl
AlCl3
=>
19
Chapter 17 37
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
=>
Chapter 17 38
Gatterman-KochFormylation
• 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
+ C
O
H+ HCl+
=>
20
Chapter 17 39
NucleophilicAromatic Substitution
• A nucleophile replaces a leaving group
on the aromatic ring.
• Electron-withdrawing substituentsactivate the ring for nucleophilic
substitution.
=>
Chapter 17 40
Examples ofNucleophilic Substitution
=>
21
Chapter 17 41
Addition-EliminationMechanism
=>