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Chapter 17 Reactions of Aromatic Compounds

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.

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Chapter 17 Reactions of Aromatic Compounds

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  1. Organic Chemistry, 6th EditionL. G. Wade, Jr. Chapter 17Reactions of Aromatic Compounds Jo Blackburn Richland College, Dallas, TX Dallas County Community College District ã 2006,Prentice Hall

  2. => Electrophilic Aromatic Substitution Electrophile substitutes for a hydrogen on the benzene ring. Chapter 17

  3. Step 1: Attack on the electrophile forms the sigma complex. Step 2: Loss of a proton gives the substitution product. => Mechanism Chapter 17

  4. - + - + Bromination of Benzene • Requires a stronger electrophile than Br2. • Use a strong Lewis acid catalyst, FeBr3. => 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 => 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. Chapter 17

  8. 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

  9. => Sulfonation Sulfur trioxide, SO3, in fuming sulfuric acid is the electrophile. Chapter 17

  10. => 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

  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. Chapter 17

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

  13. Energy Diagram => 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. Chapter 17

  15. Substitution on Anisole => 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. Chapter 17

  17. Summary ofActivators => 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 orthoand para isomers. • Meta-directors deactivate all positions on the ring, but the meta position is less deactivated. => Chapter 17

  19. Ortho Substitutionon Nitrobenzene => Chapter 17

  20. Para Substitution on Nitrobenzene => Chapter 17

  21. Meta Substitutionon Nitrobenzene => Chapter 17

  22. Energy Diagram => 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. => Chapter 17

  24. Summary of Deactivators => Chapter 17

  25. More Deactivators => 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. Ortho and para attacks produce a bromonium ionand other resonance structures. No bromonium ion possible with meta attack. => Sigma Complexfor Bromobenzene Chapter 17

  28. Energy Diagram => Chapter 17

  29. Summary of Directing Effects => Chapter 17

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

  32. => Examples ofCarbocation Formation Chapter 17

  33. - + => Formation of Alkyl Benzene 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 Chapter 17

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

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

  39. NucleophilicAromatic Substitution • A nucleophile replaces a leaving group on the aromatic ring. • Electron-withdrawing substituents activate the ring for nucleophilic substitution. => Chapter 17

  40. Examples ofNucleophilic Substitution => Chapter 17

  41. => Addition-EliminationMechanism Chapter 17

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

  43. => Benzyne Intermediate 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. 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. Chapter 17

  46. => Birch Reduction: Regiospecific • A carbon bearing an e--withdrawing group • is reduced. • A carbon bearing an e--releasing group • is not reduced. Chapter 17

  47. => Birch Mechanism Chapter 17

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

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

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