Organic Chemistry, 6th Edition
L. G. Wade, Jr.
Chapter 17
Reactions of
Aromatic Compounds
Electrophilic
Aromatic Substitution
Electrophile substitutes for a hydrogen on
the benzene ring.
Chapter 17
2
Mechanism
Step 1: Attack on the electrophile forms the sigma complex.
Step 2: Loss of a proton gives the substitution product.
=>
Chapter 17
3
Bromination of Benzene
• Requires a stronger electrophile than Br2.
• Use a strong Lewis acid catalyst, FeBr3.
Br Br
FeBr3
Br
H
+
Br
H
Br
FeBr3
H
H
H
+
H
Br
-
H
H
Br
FeBr3
+
H
_
+ FeBr4
H
H
H
Br
+
Chapter 17
HBr
4
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.
Chapter 17
5
Nitration of Benzene
Use sulfuric acid with nitric acid to form
the nitronium ion electrophile.
NO2+ then forms a sigma complex with
benzene, loses H+ to form nitrobenzene.
Chapter 17
6
Sulfonation
Sulfur trioxide, SO3, in fuming sulfuric
acid is the electrophile.
Chapter 17
7
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
8
Sigma Complex
Intermediate
is more
stable if
nitration
occurs at
the ortho
or para
position.
=>
Chapter 17
9
Activating, O-, PDirecting 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.
Chapter 17
10
Substitution on Anisole
Chapter 17
11
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.
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Chapter 17
12
Summary of
Activators
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Chapter 17
13
Deactivating MetaDirecting 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 less
deactivated.
=>
Chapter 17
14
Ortho Substitution
on Nitrobenzene
=>
Chapter 17
15
Para Substitution
on Nitrobenzene
=>
Chapter 17
16
Meta Substitution
on Nitrobenzene
=>
Chapter 17
17
Structure of MetaDirecting 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
18
Summary of Deactivators
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Chapter 17
19
More Deactivators
=>
Chapter 17
20
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.
=>
Chapter 17
21
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
22
Summary of
Directing Effects
Chapter 17
23
=>
Multiple Substituents
The most strongly activating substituent
will determine the position of the next
substitution. May have mixtures.
Chapter 17
24
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
25
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 reactive
than benzene, so polyalkylation occurs.
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Chapter 17
26
Friedel-Crafts
Acylation
• 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.
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Chapter 17
27
Mechanism of Acylation
Chapter 17
28
End of Chapter 17
Chapter 17
29