deprotonation

Deprotonation is a chemical process in which a molecule loses a proton (H+). In most contexts it accompanies acid-base reactions and results in the formation of a conjugate base. The reverse process, protonation, adds a proton to a base.

In aqueous solution, deprotonation is described by acid dissociation equilibria: HA <=> A- + H+. The tendency of HA to lose a proton is quantified by its acid dissociation constant Ka, or equivalently its pKa. At a given pH, the fraction of deprotonated species is determined by pH and pKa via Henderson-Hasselbalch. Stronger acids have larger Ka and lower pKa; weaker acids have smaller Ka and higher pKa.

Common deprotonation examples include carboxylic acids to form carboxylate (RCOO-), and phenols to form phenoxides (ArO-). Alcohols and water also deprotonate under strongly basic conditions, but their pKa values are high (roughly 15-20 for alcohols, 14 for water), so deprotonation requires stronger bases. Amines are typically protonated (forming ammonium) rather than deprotonated in neutral or acidic conditions, but their conjugate acids can deprotonate to yield the neutral amine under basic conditions. In nonaqueous solvents the same concepts apply but with solvent-specific acidities and pKa values.

Deprotonation is fundamental in organic synthesis, catalysis, and biochemistry. It affects solubility, reactivity, and the charge state of molecules, influencing enzyme activity, metal complex formation, and receptor binding. In gas phase, deprotonation can occur without solvent stabilization and is governed by intrinsic gas-phase acidities.