Acidity and Basicity of Alcohols and Phenols

While the neutral forms of alcohols and phenols are amphoteric in nature, their corresponding anions form weak bases.

The oxygen atom of an alcohol polarizes both its adjacent bonds. While the polarization of the O–H bond makes alcohol a weak acid, the lone pairs on oxygen make it both basic and nucleophilic.

The conjugate base of an aliphatic alcohol is an alkoxide ion, and that of a phenol is a phenoxide or phenolate ion. A negative charge is more stable on an oxygen atom than on nitrogen or carbon but most stable on a halogen atom.

Recall that conjugate base stability is a measure of acidity. Therefore, alcohols are more acidic than terminal alkynes, amines, and alkanes but less acidic than hydrogen halides.

Typical alcohols like methanol or ethanol are almost as acidic and basic as water. However, phenols are more acidic than alcohols but less acidic than carboxylic acids. Unlike alkoxide ions, the negative charge on the oxygen atom of phenoxide is stabilized by resonance and polarization via its aromatic ring.

Therefore, phenols can be deprotonated by a relatively weak base like hydroxide. However, alcohols can only be deprotonated by stronger bases like sodium metal or potass