Phenol
Acidity

Phenol is a weak acid (pH 6.6). In aqueous solution in the pH range ca. 8 - 12 it is in equilibrium with the phenolate anion C H O (also called phenoxide or carbolate):

Phenol is more acidic than aliphatic alcohols. Its enhanced acidity is attributed to resonance stabilization of phenolate anion. In this way, the negative charge on oxygen is delocalized on to the ortho and para carbon atoms through the pi system. An alternative explanation involves the sigma framework, postulating that the dominant effect is the induction from the more electronegative sp hybridised carbons; the comparatively more powerful inductive withdrawal of electron density that is provided by the sp system compared to an sp system allows for great stabilization of the oxyanion. In support of the second explanation, the p K of the enol of acetone in water is 10.9, making it only slightly less acidic than phenol (p K 10.0). Thus, the greater number of resonance structures available to phenoxide compared to acetone enolate seems to contribute little to its stabilization. However, the situation changes when solvation effects are excluded.
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