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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. Part 1 - Electrophilic Aromatic Substitution.
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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
Part 1 - Electrophilic Aromatic Substitution Electrophile substitutes for a hydrogen on the benzene ring. Chapter 17
Mechanism Chapter 17
E +E E Energy Diagramfor aromatic electrophilic addition Chapter 17
Reactions • Nitration HNO3 and H2SO4 • Bromination Br2 and FeBr3 • Chlorination Cl2 and AlCl3 • Iodination I2 and HNO3 • Sulfonation SO3 (fuming sulfuric acid) • Alkylation R-Cl and AlCl3 • Acylation RCOCl and AlCl3 Chapter 17
Nitration of Benzene Use sulfuric acid with nitric acid to form the nitronium ion electrophile. NO2+ then forms a sigma complex with benzene Chapter 17
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 so methyl group is ortho, para directing. Chapter 17
Sigma Complex Intermediate is more stable if nitration occurs at the orthoor para position. Chapter 17
Energy Diagram Chapter 17
Activating, O-, P-Directing Substituents • Alkyl groups stabilize the sigma complex by induction, donating electron density through the sigma bond. • Other kinds of groups with a lone pair of electrons stabilize the sigma complex by resonance. Chapter 17
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
Summary ofActivators Chapter 17
Ortho Substitutionon Nitrobenzene Chapter 17
Para Substitution on Nitrobenzene Chapter 17
Meta Substitutionon Nitrobenzene Chapter 17
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
Energy Diagram Chapter 17
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
Summary of Deactivators Chapter 17
More Deactivators Chapter 17
Summary of Directing Effects Chapter 17
Multiple Substituents The most strongly activating substituent will determine the position of the next substitution. May have mixtures. Chapter 17
Bromination of Benzene • Requires a stronger electrophile than Br2. • Use a strong Lewis acid catalyst, FeBr3. Chapter 17
Chlorination and Iodination • Chlorination uses 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
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
Ortho and para attacks produce a bromonium ionand other resonance structures. No bromonium ion possible with meta attack. Sigma Complexfor Bromobenzene Chapter 17
Energy Diagram Chapter 17
Sulfonation Sulfur trioxide, SO3, in fuming sulfuric acid is the electrophile. Chapter 17
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
Friedel-Crafts Alkylation Chapter 17
- + Formation of Alkyl Benzene Chapter 17
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
Friedel-Crafts Acylation Chapter 17
Gatterman-KochFormylation • Formyl chloride is unstable. Use a high pressure mixture of CO, HCl, and catalyst. • Product is benzaldehyde. Chapter 17
Clemmensen Reduction Acylbenzenes can be converted to alkylbenzenes by treatment with aqueous HCl and amalgamated zinc. Chapter 17
Part 2- NucleophilicAromatic Substitution • A nucleophile replaces a leaving group on the aromatic ring. • Electron-withdrawing substituents activate the ring for nucleophilic substitution. Chapter 17
Examples ofNucleophilic Substitution Chapter 17
Addition-EliminationMechanism Chapter 17
Benzyne Mechanism • Reactant is halobenzene with no electron-withdrawing groups on the ring. • Use a very strong base like NaNH2. Chapter 17
Benzyne Intermediate Chapter 17
Part 3- Addition ReactionsChlorination 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
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
Part 4 – Side Chain ReactionsBirch Reduction: Regiospecific • A carbon with an e--withdrawing group • is reduced. • A carbon with an e--releasing group • is not reduced. Chapter 17
Birch Mechanism Chapter 17
Side-Chain Oxidation Alkylbenzenes are oxidized to benzoic acid by hot KMnO4 or Na2Cr2O7/H2SO4. Chapter 17
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
SN1 Reactions • Benzylic carbocations are resonance-stabilized, easily formed. • Benzyl halides undergo SN1 reactions. Chapter 17
SN2 Reactions • Benzylic halides are 100 times more reactive than primary halides via SN2. • Transition state is stabilized by ring. Chapter 17
Part 5 - 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
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