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Chapter 21

Chapter 21. Phenols and Aryl Halides Nucleophilic Aromatic Substitution. Structure and Nomenclature of Phenols. 1A . Nomenclature of Phenols. The benzenediols also have common names:. Naturally Occurring Phenols. Physical Properties of Phenols. Synthesis of Phenols.

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Chapter 21

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  1. Chapter 21 Phenols and Aryl Halides Nucleophilic Aromatic Substitution

  2. Structure and Nomenclatureof Phenols

  3. 1A. Nomenclature of Phenols

  4. The benzenediols also have common names:

  5. Naturally Occurring Phenols

  6. Physical Properties of Phenols

  7. Synthesis of Phenols 4A. Laboratory Synthesis:

  8. 4B. Industrial Syntheses • Hydrolysis of chlorobenzene (Dow Process).

  9. From cumene hydroperoxide:

  10. Mechanism: • Chain initiation:

  11. Chain propagation:

  12. Chain propagation:

  13. Reactions of Phenols as Acids Note the electron withdrawing power of NO2.

  14. (NO resonance stabilization) with the saturated ring.

  15. Resonance stabilization of the phenolate anion.

  16. 5B. Distinguishing and SeparatingPhenols from Alcohols andCarboxylic Acids Question • If you are given three unknown samples: one is benzoic acid; one is phenol; and one is cyclohexyl alcohol; how would you distinguish them by simple chemical tests ? • Recall acidity of:

  17. Reaction with NaOH (strong base).

  18. Reaction with NaHCO3 (weaker base).

  19. Other Reactions of the O–H Group of Phenols

  20. 6A. Phenols in the Williamson Synthesis

  21. Cleavage of Alkyl Aryl Ethers Always get phenol as a product of aryl ether cleavage.

  22. Reactions of the Benzene Ring of Phenols • Bromination: (NO Lewis acid required for the bromination)

  23. Nitration: Mild conditions for EAS are due to the fact that the OH of phenol is a strong ring activator.

  24. Sulfonation:

  25. Kolbe reaction:

  26. Mechanism:

  27. Esterification of salicylic acid.

  28. The Claisen Rearrangement A sigmatropic rearrangement.

  29. Via a [3,3] sigmatropic rearrangement: Intermediate or

  30. Quinones

  31. Aryl Halides and Nucleophilic Aromatic Substitution (NAS) X

  32. Resonance structures of halobenzene.

  33. 11A. Nucleophilic Aromatic SubstitutionMethod 1: Addition–EliminationThe SNAr Mechanism. • Nucleophilic aromatic substitution can occur when strong electron-withdrawing groups are ortho or para to the halogen atom.

  34. Additional NO2 groups enhance NAS.

  35. The mechanism that operates in these reactions is an addition–elimination mechanism involving the formation of a carbanion with delocalized electrons, called a Meisenheimer intermediate. The process is called nucleophilic aromatic substitution (SNAr or NAS).

  36. The NAS mechanism:

  37. Resonance stabilization of the NAS intermediate.

  38. 11B. Nucleophilic Aromatic Substitution Method 2: Elimination–Addition The Benzyne Mechanism.

  39. The benzyne elimination–addition mechanism: Note reversal of steps.

  40. Tracking NH2 by labeling the carbon bonded to Cl. • NH2 can atack either C of the benzyne.

  41. Only one product, see next slide.

  42. X

  43. Benzyne intermediates have been “trapped” through the use of Diels–Alder reactions

  44. 11C. Phenylation

  45. Spectroscopic Analysis of Phenols and Aryl Halides • Infrared spectra (IR)

  46. 1H NMR spectra d 0.5 – 1.0 ppm intramolecular hydrogen bonding

  47. d 7 – 9 ppm

  48. 13C NMR spectra d 135 – 170 ppm

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