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Chapter 17 Reactions of Aromatic Compounds

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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Chapter 17 Reactions of Aromatic Compounds

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  1. 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

  2. Part 1 - Electrophilic Aromatic Substitution Electrophile substitutes for a hydrogen on the benzene ring. Chapter 17

  3. Mechanism Chapter 17

  4. E +E E Energy Diagramfor aromatic electrophilic addition Chapter 17

  5. 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

  6. 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

  7. 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

  8. Sigma Complex Intermediate is more stable if nitration occurs at the orthoor para position. Chapter 17

  9. Energy Diagram Chapter 17

  10. 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

  11. 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

  12. Summary ofActivators Chapter 17

  13. Ortho Substitutionon Nitrobenzene Chapter 17

  14. Para Substitution on Nitrobenzene Chapter 17

  15. Meta Substitutionon Nitrobenzene Chapter 17

  16. 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

  17. Energy Diagram 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. Chapter 17

  19. Summary of Deactivators Chapter 17

  20. More Deactivators Chapter 17

  21. Summary of Directing Effects Chapter 17

  22. Multiple Substituents The most strongly activating substituent will determine the position of the next substitution. May have mixtures. Chapter 17

  23. Bromination of Benzene • Requires a stronger electrophile than Br2. • Use a strong Lewis acid catalyst, FeBr3. Chapter 17

  24. 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

  25. 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

  26. Ortho and para attacks produce a bromonium ionand other resonance structures. No bromonium ion possible with meta attack. Sigma Complexfor Bromobenzene Chapter 17

  27. Energy Diagram Chapter 17

  28. Sulfonation Sulfur trioxide, SO3, in fuming sulfuric acid is the electrophile. Chapter 17

  29. 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

  30. Friedel-Crafts Alkylation Chapter 17

  31. - + Formation of Alkyl Benzene Chapter 17

  32. 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

  33. Friedel-Crafts Acylation Chapter 17

  34. Gatterman-KochFormylation • Formyl chloride is unstable. Use a high pressure mixture of CO, HCl, and catalyst. • Product is benzaldehyde. Chapter 17

  35. Clemmensen Reduction Acylbenzenes can be converted to alkylbenzenes by treatment with aqueous HCl and amalgamated zinc. Chapter 17

  36. 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

  37. Examples ofNucleophilic Substitution Chapter 17

  38. Addition-EliminationMechanism Chapter 17

  39. Benzyne Mechanism • Reactant is halobenzene with no electron-withdrawing groups on the ring. • Use a very strong base like NaNH2. Chapter 17

  40. Benzyne Intermediate Chapter 17

  41. 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

  42. 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

  43. 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

  44. Birch Mechanism Chapter 17

  45. Side-Chain Oxidation Alkylbenzenes are oxidized to benzoic acid by hot KMnO4 or Na2Cr2O7/H2SO4. Chapter 17

  46. 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

  47. SN1 Reactions • Benzylic carbocations are resonance-stabilized, easily formed. • Benzyl halides undergo SN1 reactions. Chapter 17

  48. SN2 Reactions • Benzylic halides are 100 times more reactive than primary halides via SN2. • Transition state is stabilized by ring. Chapter 17

  49. 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

  50. 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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