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 potassium metal or sodium hydride with hydrogen evolution.

The acidity of an alcohol is further influenced by the inductive effect and the positions of its substituents, as seen here with fluoride. Trifluoro- and trichloroethanol are stronger acids than ethanol, since their conjugate bases are stabilized by the electron-withdrawing effect of the adjacent halogen atoms.

In phenols, while electron-withdrawing nitro and chloro substituents enhance the acidity, methoxy and alkyl substituents only change it marginally. Naphthols are slightly more acidic than unsubstituted phenol.

The relative gas-phase acidity of alcohols increases from methyl to tertiary because of the polarization effect of α-alkyl substituents.

This trend is reversed in the solution phase due to the solvation effect, which makes a branched tert-butanol molecule less acidic than unhindered ethanol.