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Phenols

Phenols. Infrared Spectrum of p -Cresol. H. H. H O. C H 3. H. H. 10.0. 9.0. 8.0. 7.0. 6.0. 5.0. 4.0. 3.0. 2.0. 1.0. 0. Proton NMR. Chemical shift (  , ppm). 13 C NMR. O H. O CH 3. 128.5. 155.1. 159.7. 115.5. 114.0. 129.8. 129.5. 121.1. 120.7.

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Phenols

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  1. Phenols

  2. Infrared Spectrum of p-Cresol

  3. H H HO CH3 H H 10.0 9.0 8.0 7.0 6.0 5.0 4.0 3.0 2.0 1.0 0 Proton NMR Chemical shift (, ppm)

  4. 13C NMR OH OCH3 128.5 155.1 159.7 115.5 114.0 129.8 129.5 121.1 120.7 Oxygen of hydroxyl group deshields carbonto which it is directly attached. The most shielded carbons of the ring are those thatare ortho and para to the oxygen.

  5. •+ OH •• Mass Spectrometry Prominent peak for molecular ion. Most intense peak in phenol is for molecular ion. m/z 94

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

  7. O O H Hydrogen Bonding in Phenols

  8. 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 Physical Properties C6H5CH3 C6H5OH C6H5F Molecular weight 92 94 96

  9. Their most characteristic property Acidity of Phenols

  10. – •• •• O O H •• •• •• pKa = 10 + H pKa = 16 – •• •• + CH3CH2O CH3CH2O H H •• •• •• Compare + +

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

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

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

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

  15. Substituent Effectson theAcidity of Phenols

  16. Question • Which one of the following has the lowest pKa? • A) B) • C) D)

  17. OH OH OH CH3 OCH3 Electron-releasing groups have little or no effect pKa: 10 10.3 10.2

  18. OH OH OH Cl NO2 Electron-withdrawing groups increase acidity pKa: 10 9.4 7.2

  19. OH OH OH NO2 NO2 NO2 Effect of electron-withdrawing groups is mostpronounced at ortho and para positions pKa: 7.2 8.4 7.2

  20. OH OH OH NO2 NO2 O2N NO2 NO2 NO2 Effect of strong electron-withdrawing groupsis cumulative pKa: 7.2 4.0 0.4 Picric acid

  21. – •• •• O O •• •• •• •• H H H H H H H H N N •• •• •• + + O O O O •• •• – – •• •• •• •• – •• •• Resonance

  22. Question • Which of the following compounds is more acidic? • A) o-Cresol • B) o-Chlorophenol • C) o-Methoxyphenol • D) o-nitrophenol • E) m-nitrophenol

  23. Preparation of Aryl Ethers

  24. ONa OR but the other combination + X RONa fails because aryl halides are normally unreactivetoward nucleophilic substitution Typical Preparation is by Williamson Synthesis SN2 + + NaX RX

  25. H2C CHCH2Br OH OCH2CH CH2 Example + K2CO3 acetone, heat (86%)

  26. F OCH3 NO2 NO2 Aryl Ethers from Aryl Halides • nucleophilic aromatic substitution is effective with nitro-substituted (ortho and/or para) aryl halides CH3OH + + KOCH3 KF 25°C (93%)

  27. Claisen Rearrangementof Allyl Aryl Ethers

  28. OCH2CH CH2 200°C OH CH2CH CH2 (73%) Allyl Aryl Ethers Rearrange on Heating • allyl group migrates to ortho position

  29. OCH2CH CH2 O keto-to-enolisomerization OH O H Mechanism rewrite as

  30. this  bond breaks O O H this  bond forms Sigmatropic Rearrangement Claisen rearrangement is an example of a sigmatropic rearrangement. A  bond migratesfrom one end of a conjugated  electron systemto the other. “conjugated  electron system” is the allyl group

  31. Question • What will be the Claisen rearrangement product of the carbon-14 labeled ether shown here? • A) B) • C) D)

  32. Oxidation of Phenols:Quinones

  33. OH O Na2Cr2O7, H2SO4 H2O OH O Quinones • The most common examples of phenol oxidationsare the oxidations of 1,2- and 1,4-benzenediolsto give quinones. (76-81%)

  34. OH OH CH3 CH3 Quinones • The most common examples of phenol oxidationsare the oxidations of 1,2- and 1,4-benzenediolsto give quinones. O O Ag2O diethyl ether (68%)

  35. O OH OH O Some quinones are dyes Alizarin(red pigment)

  36. OH UV-VIS Oxygen substitution on ring shifts max to longerwavelength; effect is greater in phenoxide ion. – O max204 nm256 nm max210 nm270 nm max235 nm287 nm

  37. O CH3 CH3 CH3 CH3 CH3 CH3 O Some quinones are important biomolecules Vitamin K (blood-clotting factor)

  38. Some quinone precursors are important in foods Antioxidants can protect against the cell-damaging effects of free radicals. Some dietary phenolics are oxidized to quinones as free radical-antioxidant traps. Eg. Reservatrol / Flavonoids

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