PHENOL

Phenol has a limited solubility in water (8.3 g/100 ml). It is slightly acidic: the phenol molecule has weak tendencies to lose the H+ ion from the hydroxyl group, resulting in the highly water-soluble phenolate anion C6H5O−, also called phenoxide anion.

Compared to aliphatic alcohols, phenol shows much higher acidity; it even reacts with aqueous NaOH to lose H+, whereas aliphatic alcohols do not. However, many carboxylic acids are more acidic than phenol. One explanation for the increased acidity over alcohols is resonance stabilization of the phenoxide anion by the aromatic ring. In this way, the negative charge on oxygen is shared by the ortho and para carbon atoms. In another explanation, increased acidity is the result of orbital overlap between the oxygen's lone pairs and the aromatic system.

In a third, the dominant effect is the induction from the sp2 hybridised carbons]; the comparatively more powerful inductive withdrawal of electron density that is provided by the sp2 system compared to an sp3 system allows for great stabilization of the oxyanion. In making this conclusion, one can examine the pKa of the enol of acetone, which is 10.9 in comparison to phenol with a pKa of 10.0.

However, this similarity of acidities of phenol and acetone enol is not observed in the gas phase, and is due to the fact that the difference of solvation energies of the deprotonated acetone enol and phenoxide almost exactly offsets the experimentally observed gas phase acidity difference. It has been recently shown that only about 1/3 of the increased acidity of phenol is due to inductive effects, with resonance accounting for the rest.