chapter 17 reactions of aromatic compounds
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Organic Chemistry , 6 th Edition L. G. Wade, Jr. Chapter 17 Reactions of Aromatic Compounds. Jo Blackburn Richland College, Dallas, TX Dallas County Community College District ã 2006, Prentice Hall. =>. Electrophilic Aromatic Substitution. - PowerPoint PPT PresentationTRANSCRIPT
Chapter 17Reactions of
Aromatic Compounds
Jo BlackburnRichland College, Dallas, TX
Dallas County Community College District2006,Prentice Hall
Organic Chemistry, 6th EditionL. G. Wade, Jr.
Chapter 17 2
Electrophilic Aromatic Substitution
Electrophile substitutes for a hydrogen on the benzene ring.
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Chapter 17 3
Mechanism
Step 1: Attack on the electrophile forms the sigma complex.
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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
_+ -
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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 Diagramfor Bromination
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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.
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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+ toform nitrobenzene. =>
Chapter 17 9
SulfonationSulfur trioxide, SO3, in fuming sulfuric
acid is the electrophile.
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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
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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.
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Chapter 17 12
Sigma Complex
Intermediate is more stable if nitration occurs at the ortho or para position.
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Chapter 17 13
Energy Diagram
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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.
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Chapter 17 15
Substitution on Anisole
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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.
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Chapter 17 17
Summary ofActivators
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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 less deactivated. =>
Chapter 17 19
Ortho Substitutionon Nitrobenzene
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Chapter 17 20
Para Substitution on Nitrobenzene
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Chapter 17 21
Meta Substitutionon Nitrobenzene
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Chapter 17 22
Energy Diagram
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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.
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Chapter 17 24
Summary of Deactivators
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Chapter 17 25
More Deactivators
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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. =>
Chapter 17 27
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 28
Energy Diagram
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Chapter 17 29
Summary of Directing Effects
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Chapter 17 30
Multiple Substituents
The most strongly activating substituent will determine the position of the next substitution. May have mixtures.
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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.
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Chapter 17 32
Examples ofCarbocation Formation
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Chapter 17 33
Formation of Alkyl Benzene
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+
-
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 AlCl3 produces isopropylbenzene.
• The alkylbenzene product is more reactive than benzene, so polyalkylation occurs. =>
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
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Chapter 17 37
Clemmensen Reduction
Acylbenzenes can be converted to alkylbenzenes by treatment with aqueous HCl and amalgamated zinc.
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Chapter 17 38
Gatterman-KochFormylation
• Formyl chloride is unstable. Use a high pressure mixture of CO, HCl, and catalyst.
• Product is benzaldehyde.
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Chapter 17 39
NucleophilicAromatic 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 40
Examples ofNucleophilic Substitution
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Chapter 17 41
Addition-EliminationMechanism
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Chapter 17 42
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 43
Benzyne Intermediate
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Chapter 17 44
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 45
Catalytic Hydrogenation
• Elevated heat and pressure is required.
• Possible catalysts: Pt, Pd, Ni, Ru, Rh.
• Reduction cannot be stopped at an intermediate stage.
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Chapter 17 46
Birch Reduction: Regiospecific
• A carbon bearing an e--withdrawing groupis reduced.
• A carbon bearing an e--releasing groupis not reduced.
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Chapter 17 47
Birch Mechanism
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Chapter 17 48
Side-Chain Oxidation
Alkylbenzenes are oxidized to benzoic acid by hot KMnO4 or Na2Cr2O7/H2SO4.
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Chapter 17 49
Side-Chain Halogenation
• Benzylic position is the most reactive.
• Chlorination is not as selective as bromination, results in mixtures.
• Br2 reacts only at the benzylic position.
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Chapter 17 50
SN1 Reactions
• Benzylic carbocations are resonance-stabilized, easily formed.
• Benzyl halides undergo SN1 reactions.
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Chapter 17 51
SN2 Reactions
• Benzylic halides are 100 times more reactive than primary halides via SN2.
• Transition state is stabilized by ring.
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Chapter 17 52
Reactions of Phenols
• Some reactions like aliphatic alcohols:phenol + carboxylic acid esterphenol + aq. NaOH phenoxide ion
• Oxidation to quinones: 1,4-diketones.
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Chapter 17 53
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. =>
Chapter 17 54
ElectrophilicSubstitution 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.
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Chapter 17 55
End of Chapter 17