Chapter15苯和芳香性

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Chapter15苯和芳香性

  1. 1. 1825 Faraday discovers benzene by pyrolysis of whale oil: Colorless liquid bp ~80°C. Very unreactive. Analysis: :C H :1 1 Synthesis: Mitscherlich by chemical degradation of benzoic acid: C6H6 Benzene and Aromaticity COOH Cyclic structure: Kekulé 1865 Michael Faraday 1791-1867
  2. 2. Nomenclature Many common names because many benzene derivatives occur in nature. Functional groups have priority, but others are substituents, hence alkylbenzene, halobenzene, nitrobenzene. Disubstituted: ortho meta para
  3. 3. Functional groups take over: General term for benzene-containing compounds: Arene. C6H5- is phenyl; general aryl. C6H5CH2- is phenylmethyl or benzyl.
  4. 4. Because many benzene derivatives exhibit a nice smell, compounds containing the benzene ring were called historically “aromatic”.
  5. 5. Structure: Regular Hexagon Both the π and ζ frame symmetrize the structure
  6. 6. Benzene is unusually unreactive. Does this mean that it is also especially stable thermodynamically? Look at ΔH hydrogenation: Special stability is now called aromaticity. All cyclic 6e arrangements are aromatic, including transition states. Cyclohexane
  7. 7. Aromatic Transition States All 6 e TSs
  8. 8. π Molecular Orbitals
  9. 9. Spectra UV-Visible: Complex set of peaks at 250– 290 nm, which shift to visible range with resonating substituents: Dyes. λmax (ε ) = 289 (18,600) nm: suntan lotions.
  10. 10. IR:
  11. 11. 1H NMR δ ~ 6.5–8.5 ppm; benzene 7.27 (s) ppm: Low field! Why? Ring current!
  12. 12. Substitued Benzenes Can Show Complex NMR Patterns
  13. 13. Strong push-pull effects especially on ortho-H: Pull Push Aldehydes deshielded Jortho = 9 Hz
  14. 14. 13C NMR: Alkene-like Benzene 128.7 ppm.
  15. 15. Polycyclic Benzenoid Hydrocarbons Fusion: Linear and angular Naphthalene: Aromatic
  16. 16. UV
  17. 17. Most fused benzenoids are aromatic: Resonance forms with full benzenoid e-sextets. The number of resonance forms of isomers often indicative of relative stability: Compare anthracene with phenanthrene. More resonance forms: More stable (by 5.7 kcal mol-1).
  18. 18. Fullerenes: “Curved Graphite”
  19. 19. Conjugated Cyclopolyenes: Annulenes Hückel’s rule: Cyclically delocalized polyenes with [4n +2] π electrons are aromatic (stabilized relative to acyclic reference), those with [4n] π electrons are antiaromatic (destabilized). Interruption of cyclic conjugation: Nonaromatic. Easiest detection by 1H NMR: Aromatic systems show deshielded outer Hs (shielded inner Hs). Antiaromatic systems show the reverse: shielded outer, deshielded inner Hs. Erich Hückel 1896-1980
  20. 20. Recall: Examples of cyclic, delocalized polyenes: Deshiel- ding zone Deshiel- ding zone Shielding zone Shielding zone
  21. 21. Cyclobutadiene ([4]Annulene) Unstable, very reactive (Diels-Alder dimerization), rectangular (not square). Stabilized by bulky substituents. 1H NMR of tri(tert-butyl)cyclobutadiene: δ = 5.38 ppm: High field! Tetra(tert-butyl)cyclobutadiene X-ray structure:
  22. 22. Cyclooctatetraene ([8]Annulene) Nonplanar, which inhibits delocalization: Nonaromatic (like a polyene). Barrier to ring flip 11 kcal mol-1
  23. 23. [10]Annulene H H Planar structure too crowded by the inside hydrogens. Other isomers suffer bond angle strain: Undergo electrocyclic ring closures!trans,trans Isomer all-cis Isomer 154º con dis
  24. 24. 7.10 ppm - 0.50 ppm Solution to this problem: Bridged annulenes. Shielded by ring current
  25. 25. 9.28 ppm deshielded -2.99 ppm shielded [18]Annulene Franz Sondheimer
  26. 26. [18]Annulene -3 ppm010 Rotation equilibrates inside and outside hydrogens 5δ
  27. 27. [16]Annulene 10.43 ppm 5.40 ppm
  28. 28. Charged Annulenes 5.57 ppm Corr. for charge: ~7.5 ppm But, the cyclopentadienyl cation is antiaromatic, very unstable, much worse than allyl cation: 4 electrons! Six e Bromocycloheptatriene spontaneously dissociates: Six e. But, the cycloheptatrienyl anion is unstable: pKa of cycloheptatriene ~ 39!, 8 electrons. (propene = 40) Six e 9.17 ppm
  29. 29. Hydride shift in carbocations: 2 e TS [4n +2], n = 0. Hückel’s Rule in TSs
  30. 30. Hückel’s Rule in Polycycles Aromatic Periphery aromatic Isolated 3 Benzenes
  31. 31. Antiaromatic But: Base 10 e What are these? Aromatic Aromaticity is an important concept, includes heterocycles (Chapter 25). 300,000 papers since 1981!
  32. 32. Electrophilic Aromatic Substitution, EAS (Not addition) Mechanism:
  33. 33. The Intermediate Cation in EAS X-ray structure of [C6H7][HCB11Me5Br6] D. Stasko, C. A. Reed, JACS 2002, 124, 1148 . .
  34. 34. 1. Halogenation: F2 violent; Cl2, Br2 need catalyst; I2 endothermic -10 kcal mol-1 Electron poor Other Lewis acids: BF3, AlCl3, etc. Stops. Br is e-withdrawing, deactivates ring
  35. 35. 2. Nitration with nitric acid HNO3 = HO-NO2 Mechanism: HNO3, H2SO4 N+ O– O
  36. 36. Unified Mechanistic Concept of Electrophilic Aromatic Nitration: Convergence of Computational Results and Experimental Data, Pierre M. Esteves, George A. Olah, G. K. Surya Prakash et al., J. Am. Chem. Soc. 2003, 125, 4836. Recent stuff!!
  37. 37. 3. Sulfonation: Reversible. Reagent is fuming sulfuric acid: H2SO4 + SO3 SO3, H2SO4 S OH O O Sulfonation
  38. 38. Driven by large heat of hydration of SO3 (will become important in Chapter 16) Applications of sulfonic acids: Benzenesulfonyl chloride
  39. 39. Sulfa drugs: Antibacterial (urinary, malaria, leprosy) S Cl O O ROH S OR O O RNH2 S NHR O O Recall: Nu Sulfonamides Sulfonic esters Hoffmann La Roche Kidney Model
  40. 40. 4. Friedel-Crafts reactions: Alkylation and alkanoylation A. Alkylation Mechanism: Charles Friedel 1832-1899 James Craft 1839-1917
  41. 41. Often low yields. Problems: 1. Product contains an e-pushing alkyl group, making the benzene more reactive, causing overalkylation; 2. Lewis acid activation makes carbocationic alkyl, which is prone to rearrangements and polymerization.
  42. 42. B. Alkanoylation - selective
  43. 43. Mechanism:
  44. 44. Lewis acid needed in equimolar amounts, because it binds to product: EAS: Immediate synthetic take home lessons 1. Halobenzenes make Grignards, lithium reagents 2. Alkanoylbenzenes have carbonyl function We shall see next (Chapter 16) how to use the nitro and sulfonyl functions.

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