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Organic Chemistry , 5 th Edition L. G. Wade, Jr. Chapter 17 Reactions of Aromatic Compounds. Jo Blackburn Richland College, Dallas, TX Dallas County Community College District ã 2003, Prentice Hall. =>. Electrophilic Aromatic Substitution.

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Chapter 17 Reactions of Aromatic Compounds

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Chapter 17 reactions of aromatic compounds l.jpg

Organic Chemistry, 5th EditionL. G. Wade, Jr.

Chapter 17Reactions of Aromatic Compounds

Jo Blackburn

Richland College, Dallas, TX

Dallas County Community College District

ã 2003,Prentice Hall


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Electrophilic Aromatic Substitution

Electrophile substitutes for a hydrogen on the benzene ring.

Chapter 17


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Mechanism

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Chapter 17


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Bromination of Benzene

  • Requires a stronger electrophile than Br2.

  • Use a strong Lewis acid catalyst, FeBr3.

Animation

Animation

Animation

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Chapter 17


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Energy Diagramfor Bromination

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Chapter 17


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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


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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


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Sulfonation

Sulfur trioxide, SO3, in fuming sulfuric acid is the electrophile.

Chapter 17


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Benzene-d6

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.

Chapter 17


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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.

Chapter 17


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Sigma Complex

Intermediate is more stable if nitration occurs at the orthoor para position.

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Chapter 17


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Energy Diagram

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Chapter 17


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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


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Nitration of Anisole

Ortho attack

Meta attack

Para attack

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Chapter 17


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The Amino Group

Aniline reacts with bromine water (without a catalyst) to yield the tribromide. Sodium bicarbonate is added to neutralize the HBr that’s also formed.

Chapter 17


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Summary ofActivators

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Chapter 17


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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 orthoand para isomers.

  • Meta-directors deactivate all positions on the ring, but the meta position is less deactivated. =>

Chapter 17


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Ortho Substitutionon Nitrobenzene

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Chapter 17


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Para Substitution on Nitrobenzene

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Chapter 17


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Meta Substitutionon Nitrobenzene

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Chapter 17


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Energy Diagram

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Chapter 17


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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


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Summary of Deactivators

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Chapter 17


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More Deactivators

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Chapter 17


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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


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Ortho and para attacks produce a bromonium ionand other resonance structures.

No bromonium ion possible with meta attack.

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Sigma Complexfor Bromobenzene

Chapter 17


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Energy Diagram

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Chapter 17


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Summary of Directing Effects

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Chapter 17


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Multiple Substituents

The most strongly activating substituent will determine the position of the next substitution. May have mixtures.

Chapter 17


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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


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Examples ofCarbocation Formation

Chapter 17


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+

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Formation of Alkyl Benzene

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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


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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


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Mechanism of Acylation

Chapter 17


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Clemmensen Reduction

Acylbenzenes can be converted to alkylbenzenes by treatment with aqueous HCl and amalgamated zinc.

Chapter 17


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Gatterman-KochFormylation

  • Formyl chloride is unstable. Use a high pressure mixture of CO, HCl, and catalyst.

  • Product is benzaldehyde.

Chapter 17


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NucleophilicAromatic Substitution

  • A nucleophile replaces a leaving group on the aromatic ring.

  • Electron-withdrawing substituents activate the ring for nucleophilic substitution. =>

Chapter 17


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Examples ofNucleophilic Substitution

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Chapter 17


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Addition-EliminationMechanism

Chapter 17


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Benzyne Mechanism

  • Reactant is halobenzene with no electron-withdrawing groups on the ring.

  • Use a very strong base like NaNH2. =>

Chapter 17


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Benzyne Intermediate

Chapter 17


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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.

Chapter 17


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Catalytic Hydrogenation

  • Elevated heat and pressure is required.

  • Possible catalysts: Pt, Pd, Ni, Ru, Rh.

  • Reduction cannot be stopped at an intermediate stage.

Chapter 17


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Birch Reduction: Regiospecific

  • A carbon with an e--withdrawing group

  • is reduced.

  • A carbon with an e--releasing group

  • is not reduced.

Chapter 17


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Birch Mechanism

Chapter 17


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Side-Chain Oxidation

Alkylbenzenes are oxidized to benzoic acid by hot KMnO4 or Na2Cr2O7/H2SO4.

Chapter 17


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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.

Chapter 17


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SN1 Reactions

  • Benzylic carbocations are resonance-stabilized, easily formed.

  • Benzyl halides undergo SN1 reactions.

Chapter 17


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SN2 Reactions

  • Benzylic halides are 100 times more reactive than primary halides via SN2.

  • Transition state is stabilized by ring.

Chapter 17


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Reactions of Phenols

  • Some reactions like aliphatic alcohols:

    • phenol + carboxylic acid  ester

    • phenol + aq. NaOH  phenoxide ion

  • Oxidation to quinones: 1,4-diketones.

Chapter 17


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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


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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.

Chapter 17


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End of Chapter 17

Chapter 17


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