Protolysis

Protolysis is the chemical process in which a proton is transferred between two species. In Brønsted–Lowry terms, it encompasses both protonation (the gain of a proton by a species) and deprotonation (the loss of a proton by a species). Protolysis is a fundamental step in acid–base chemistry and in many catalytic and enzymatic reaction mechanisms, and it can occur in aqueous or non-aqueous media as part of broader chemical transformations.

In solution, protolysis reactions are governed by the relative strengths of acids and bases, which are often described by pKa values. A proton transfer from a stronger acid to a stronger base is favorable, and the position of equilibrium shifts toward the side containing the weaker acid and weaker base. Solvent effects are important: in water, for example, water itself participates in protolysis through autoprotolysis, 2 H2O ⇌ H3O+ + OH−, and many acid–base steps occur via hydronium or hydroxide as proton donors or acceptors. In non-aqueous solvents, protolysis can proceed through different protonation/deprotonation partners, such as alcohols, amines, or anions generated in situ.

Protolysis also plays a role in structural transformations that do not require bond-making or -breaking beyond proton transfer, such as acid-catalyzed activation of substrates (e.g., protonation of carbonyl oxygens to increase electrophilicity) and base-catalyzed deprotonation to generate reactive anions. In enzymology and catalysis, protolysis steps often occur in sequences that couple proton transfers to electron transfer or covalent changes, forming integral parts of reaction mechanisms.

Distinguishing protolysis from hydrolysis, protolysis refers specifically to proton transfer, while hydrolysis involves reaction with water that often accompanies bond cleavage and formation.