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Chapter 17 Reactions of Aromatic Compounds Organic Chemistry, 6 th Edition L. G. Wade, Jr.
Chapter 17Reactions of
Aromatic Compounds
Organic Chemistry, 6th EditionL. G. Wade, Jr.
Chapter 17 2
Electrophilic Aromatic Substitution
Electrophile substitutes for a hydrogen on the benzene ring.
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
_+ -
Chapter 17 5
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 6
Nitration of BenzeneUse 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 7
SulfonationSulfur trioxide, SO3, in fuming sulfuric
acid is the electrophile.
Chapter 17 8
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.
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Chapter 17 9
Sigma Complex
Intermediate is more stable if nitration occurs at the ortho or para position.
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Chapter 17 10
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.
Chapter 17 11
Substitution on Anisole
Chapter 17 12
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 13
Summary ofActivators
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Chapter 17 14
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 less deactivated. =>
Chapter 17 15
Ortho Substitutionon Nitrobenzene
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Chapter 17 16
Para Substitution on Nitrobenzene
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Chapter 17 17
Meta Substitutionon Nitrobenzene
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Chapter 17 18
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 19
Summary of Deactivators
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Chapter 17 20
More Deactivators
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Chapter 17 21
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 22
Sigma Complexfor Bromobenzene
Ortho and para attacks produce a bromonium ionand other resonance structures.
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No bromonium ion possible with meta attack.
Chapter 17 23
Summary of Directing Effects
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Chapter 17 24
Multiple Substituents
The most strongly activating substituent will determine the position of the next substitution. May have mixtures.
Chapter 17 25
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 26
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 AlCl3 produces isopropylbenzene.
• The alkylbenzene product is more reactive than benzene, so polyalkylation occurs. =>
Chapter 17 27
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 28
Mechanism of Acylation
Chapter 17 29
End of Chapter 17
