24.4 Acidity of Phenols

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

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

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

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

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

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

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

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

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

10. 24.5Substituent Effectson theAcidity of Phenols

11. 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

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

13. 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

14. 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

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