Chapter 24 Phenols

1. Chapter 24Phenols

2. 24.1Nomenclature

3. OH CH3 Cl Nomenclature • named on basis of phenol as parent • substituents listed in alphabetical order • lowest numerical sequence: first point ofdifference rule 5-Chloro-2-methylphenol

4. OH OH OH OH OH OH Nomenclature 1,2-Benzenediol 1,3-Benzenediol 1,4-Benzenediol (common name:pyrocatechol) (common name:resorcinol) (common name:hydroquinone)

5. OH CO2H Nomenclature • name on basis of benzoic acid as parent • higher oxidation states of carbon outrankhydroxyl group p-Hydroxybenzoic acid

6. 24.2Structure and Bonding

7. Structure of Phenol • phenol is planar • C—O bond distance is 136 pm, which isslightly shorter than that of CH3OH (142 pm)

8. 24.3Physical Properties The OH group of phenols allows hydrogen bondingto other phenol molecules and to water.

9. O O H Hydrogen Bonding in Phenols

10. Physical Properties (Table 24.1) Compared to compounds of similar size andmolecular weight, hydrogen bonding in phenol raises its melting point, boiling point, andsolubility in water.

11. Physical Properties (Table 24.1) C6H5CH3 C6H5OH C6H5F Molecular weight 92 94 96 Melting point (°C) –95 43 –41 Boilingpoint (°C,1 atm) 111 132 85 Solubility inH2O (g/100 mL,25°C) 0.05 8.2 0.2

12. most characteristic property of phenols is their acidity 24.4Acidity of Phenols

13. – •• •• O O H •• •• •• Ka = 10-10 + H Ka = 10-16 – •• •• + CH3CH2O CH3CH2O H H •• •• •• Compare + +

14. Delocalized negative charge in phenoxide ion – •• O •• •• H H H H H

15. •• O •• H H – •• H H H Delocalized negative charge in phenoxide ion – •• O •• •• H H H H H

16. •• O •• H H – •• H H H Delocalized negative charge in phenoxide ion

17. •• •• O O •• •• H H H H – •• •• H H H H – H H Delocalized negative charge in phenoxide ion

18. •• O •• H H •• H H – H Delocalized negative charge in phenoxide ion

19. •• O •• H H •• H H – H Delocalized negative charge in phenoxide ion •• O •• – H H •• H H H

20. •• – •• O H O •• •• •• – HO Phenols are converted to phenoxide ionsin aqueous base + + H2O stronger acid weaker acid

21. 24.5Substituent Effectson theAcidity of Phenols

22. OH OH OH CH3 OCH3 Electron-releasing groups have little or no effect Ka: 1 x 10-10 5 x 10-11 6 x 10-11

23. OH OH OH Cl NO2 Electron-withdrawing groups increase acidity Ka: 1 x 10-10 4 x 10-9 7 x 10-8

24. OH OH OH NO2 NO2 NO2 Effect of electron-withdrawing groups is mostpronounced at ortho and para positions Ka: 6 x 10-8 4 x 10-9 7 x 10-8

25. OH OH OH NO2 NO2 O2N NO2 NO2 NO2 Effect of strong electron-withdrawing groupsis cumulative Ka: 7 x 10-8 1 x 10-4 4 x 10-1

26. – •• •• O O •• •• •• •• H H H H H H H H N N •• •• •• + + O O O O •• •• – – •• •• •• •• – •• •• Resonance Depiction

27. 24.6Sources of Phenols • Phenol is an important industrial chemical. • Major use is in phenolic resins for adhesives and plastics. • Annual U.S. production is about 4 billion pounds per year.

28. CH(CH3)2 Cl SO3H OH IndustrialPreparationsof Phenol 1. NaOH heat 2. H+ 1. O2 1. NaOH heat 2. H2O H2SO4 2. H+

29. 1. NaNO2, H2SO4, H2O O2N O2N NH2 OH 2. H2O, heat Laboratory Synthesis of Phenols • from arylamines via diazonium ions (81-86%)

30. 24.7Naturally Occurring Phenols • Many phenols occur naturally

31. Example: Thymol OH CH3 CH(CH3)2 Thymol(major constituent of oil of thyme)

32. Example: 2,5-Dichlorophenol OH Cl Cl 2,5-Dichlorophenol(from defensive secretion ofa species of grasshopper)

33. 24.8Reactions of Phenols:Electrophilic Aromatic Substitution Hydroxyl group strongly activates the ringtoward electrophilic aromatic substitution

34. Electrophilic Aromatic Substitution in Phenols • Halogenation • Nitration • Nitrosation • Sulfonation • Friedel-Crafts Alkylation • Friedel-Crafts Acylation

35. OH OH ClCH2CH2Cl 0°C Br Halogenation • monohalogenation in nonpolar solvent(1,2-dichloroethane) + Br2 (93%)

36. OH OH Br Br F F Br Halogenation • multiple halogenation in polar solvent(water) H2O + 3Br2 25°C (95%)

37. Electrophilic Aromatic Substitution in Phenols • Halogenation • Nitration • Nitrosation • Sulfonation • Friedel-Crafts Alkylation • Friedel-Crafts Acylation

38. OH OH NO2 HNO3 acetic acid5°C CH3 CH3 Nitration • OH group controls regiochemistry (73-77%)

39. Electrophilic Aromatic Substitution in Phenols • Halogenation • Nitration • Nitrosation • Sulfonation • Friedel-Crafts Alkylation • Friedel-Crafts Acylation

40. OH NaNO2 H2SO4, H2O0°C Nitrosation NO • only strongly activated rings undergo nitrosation when treated with nitrous acid OH (99%)

41. Electrophilic Aromatic Substitution in Phenols • Halogenation • Nitration • Nitrosation • Sulfonation • Friedel-Crafts Alkylation • Friedel-Crafts Acylation

42. OH OH H3C CH3 H3C CH3 Sulfonation • OH group controls regiochemistry H2SO4 100°C SO3H (69%)

43. Electrophilic Aromatic Substitution in Phenols • Halogenation • Nitration • Nitrosation • Sulfonation • Friedel-Crafts Alkylation • Friedel-Crafts Acylation

44. OH OH CH3 CH3 (CH3)3COH H3PO460°C CH3 C H3C CH3 Friedel-Crafts Alkylation • (CH3)3COH reacts with H3PO4 to give (CH3)3C+ (63%)

45. Electrophilic Aromatic Substitution in Phenols • Halogenation • Nitration • Nitrosation • Sulfonation • Friedel-Crafts Alkylation • Friedel-Crafts Acylation

46. 24.9Acylation of Phenols Acylation can take place either on the ringby electrophilic aromatic substitution or onoxygen by nucleophilic acyl substitution

47. OH OH O CH3CCl C CH3 O Friedel-Crafts Acylation • under Friedel-Crafts conditions, acylation of the ring occurs(C-acylation) + ortho isomer AlCl3 (74%) (16%)

48. O OH OC(CH2)6CH3 O + CH3(CH2)6CCl O-Acylation • in the absence of AlCl3, acylation of the hydroxyl group occurs (O-acylation) (95%)

49. O OH OC(CH2)6CH3 C (CH2)6CH3 O O- versus C-Acylation • O-Acylation is kinetically controlled process; C-acylation is thermodynamically controlled • AlCl3 catalyzes the conversion of the aryl ester to the aryl alkyl ketones; this is called the Fries rearrangement AlCl3 formed faster more stable