Acid-Base Properties

The O-H bond is stronger (ΔH⁰ = 104 kcal/mol) than the C-H bond (ΔH⁰ = 98 kcal/mol) and yet it is much easier to break heterolytically. How come?

The heterolytic cleavage of an O-H bond produces an alkoxide where the negative charge is placed on an oxygen, much more electronegative than a carbon.

Dissociation equilibrium of an alcohol in water giving the alkoxide ion: Ka = [H₃O⁺][RO⁻]/[ROH] and pKa = −log Ka

The acidity of an alcohol can be qualitatively understood in terms of the relative stability of the resulting alkoxide.

Table of pKa values of alcohols ROH (water 15.7; methanol 15.5; ethanol 15.9; isopropanol 17.1; tert-butanol 18.0; 2-chloroethanol 14.3; 2,2,2-trifluoroethanol 12.4) compared with other acids, from H₂SO₄ (−5) to H₂O₂ (11.6)

Acidity decreases in the order methanol > primary > secondary > tertiary, because the alkoxides become less and less stable

Acidity decreases from methanol to primary, secondary and tertiary alcohols. The more alkyl groups surround the oxygen, the harder it is for solvent molecules to approach and stabilize (solvate) the negative charge of the alkoxide; moreover, alkyl groups donate some electron density and concentrate that charge even further. The tertiary alkoxide is the least stable and, therefore, the tertiary alcohol is the least acidic.

One needs relatively strong bases to quantitatively convert alcohols into their conjugate bases, the alkoxides:

Deprotonation of alcohols: hydroxide barely shifts the equilibrium (K ≈ 1), whereas amide NH₂⁻ shifts it completely (K = 10^19.5)

Alcohols are amphoteric because the lone electron pairs on the oxygen can be shared with an external acid, provided it is a strong one.

Alcohols are amphoteric: with a strong acid they give the alkyloxonium ion and with a strong base the alkoxide ion