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Learn about the unique acidity of phenols, the delocalized negative charge in phenoxide ions, substituent effects on acidity, and the resonance depiction of phenols. Discover how electron-releasing and -withdrawing groups impact acidity. Phenols convert to phenoxide ions in aqueous base.
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most characteristic property of phenols is their acidity 24.4Acidity of Phenols
– •• •• O O H •• •• •• Ka = 10-10 + H Ka = 10-16 – •• •• + CH3CH2O CH3CH2O H H •• •• •• Compare + +
Delocalized negative charge in phenoxide ion – •• O •• •• H H H H H
•• O •• H H – •• H H H Delocalized negative charge in phenoxide ion – •• O •• •• H H H H H
•• O •• H H – •• H H H Delocalized negative charge in phenoxide ion
•• •• O O •• •• H H H H – •• •• H H H H – H H Delocalized negative charge in phenoxide ion
•• O •• H H •• H H – H Delocalized negative charge in phenoxide ion
•• O •• H H •• H H – H Delocalized negative charge in phenoxide ion •• O •• – H H •• H H H
•• – •• O H O •• •• •• – HO Phenols are converted to phenoxide ionsin aqueous base + + H2O stronger acid weaker acid
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
OH OH OH Cl NO2 Electron-withdrawing groups increase acidity Ka: 1 x 10-10 4 x 10-9 7 x 10-8
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
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
– •• •• O O •• •• •• •• H H H H H H H H N N •• •• •• + + O O O O •• •• – – •• •• •• •• – •• •• Resonance Depiction
