Acidity and Basicity of Alcohols and Phenols

Alcohols are amphoteric because the oxygen atom polarizes both its adjacent bonds. The polarized O–H bond makes alcohol a weak acid, while lone pairs on oxygen make it both basic and nucleophilic. This dual nature allows alcohols to donate protons as acids or accept them as bases, similar to water.

Phenols are significantly more acidic than alcohols—approximately one million times stronger. The conjugate base of phenol, called a phenoxide or phenolate ion, is stabilized by resonance delocalization of the negative charge across the aromatic ring. This resonance stabilization, combined with the polar effect of benzene, enhances phenol acidity by eight orders of magnitude compared to cyclohexanol.

Alcohols require strong bases like sodium metal, potassium metal, or sodium hydride for deprotonation, which react violently with the alcohol. Phenols, however, can be deprotonated by weaker bases like hydroxide ion. This difference reflects the greater stability of the phenoxide ion compared to alkoxide ions due to resonance stabilization.

Electron-withdrawing groups like nitro and chloro substituents significantly enhance phenol acidity by stabilizing the phenoxide ion through delocalization of the negative charge. Ortho- and para-positioned nitro groups are most effective because they are directly conjugated to the phenoxide oxygen. Meta-positioned nitro groups have less stabilizing effect, making m-nitrophenol less acidic than its ortho or para isomers.

In the gas phase, alcohol acidity increases from methyl to tertiary alcohols due to polarization effects of alpha-alkyl substituents. However, this trend reverses in solution because solvation effects stabilize branched molecules less effectively. Tert-butanol becomes less acidic than ethanol in solution due to steric hindrance reducing solvation of the bulky alkoxide ion.

Phenols can be selectively extracted from an ether solution using dilute sodium hydroxide, which converts phenol to its sodium salt in the aqueous phase while alcohol remains in the ether layer. Conversely, dilute sodium bicarbonate extracts carboxylic acids and phenols together, leaving alcohols behind. These separations exploit the different acid-base properties of each compound class.

Halogen substituents enhance alcohol acidity by stabilizing the conjugate alkoxide ion through electron-withdrawing inductive effects. Trifluoroethanol and trichloroethanol are stronger acids than ethanol because their adjacent halogen atoms withdraw electron density, stabilizing the negative charge on the oxygen atom of the conjugate base.