Acidity of alcohols and phenols

Alcohols and phenols release hydrogen gas with reactive metals such as sodium, potassium and aluminium, forming alkoxides and phenoxides. Phenols also react with aqueous sodium hydroxide; alcohols do not.

The polar O–H bond makes alcohols weak Brönsted acids. An alkyl group pushes electron density towards oxygen (+I), which strengthens the O–H bond and destabilises the alkoxide, so acidity runs primary > secondary > tertiary.

Alcohols are weaker acids than water: water displaces the alcohol from an alkoxide, and the alkoxide is the stronger base. Sodium ethoxide is a stronger base than sodium hydroxide.

Through the lone pairs on oxygen, alcohols also act as Brönsted bases and accept a proton from strong acids.

Phenol is more acidic than an alcohol. The –OH is on an electron-withdrawing sp² carbon, and the negative charge of phenoxide is spread over the ring by resonance, which makes the ion more stable than phenol itself.

Table values of pKa: o-nitrophenol 7.2, m-nitrophenol 8.3, p-nitrophenol 7.1, phenol 10.0, o-cresol 10.2, m-cresol 10.1, p-cresol 10.2, ethanol 15.9. A larger pKa means a weaker acid.

Phenol (pKa 10.0) is roughly a million times as acidic as ethanol (pKa 15.9): the pKa gap of 5.9 is a factor of about 7.9 × 10⁵ in Ka.

An electron-withdrawing group such as –NO₂ raises the acidity of phenol, most strongly from the ortho and para positions where it delocalises the charge. An electron-releasing alkyl group, as in the cresols, lowers it.

Worked order, most acidic first: 2,4,6-trinitrophenol > 3,5-dinitrophenol > 3-nitrophenol > phenol > 4-methylphenol > propan-1-ol.