protondonation

Protondonation refers to the process by which a molecule or ion donates a proton (H+) to another species in a chemical reaction. In Brønsted–Lowry acid–base theory, proton donors are called acids and the species that receive the proton are bases. Protondonation is a central aspect of acid–base chemistry, governing many reactions in solution, catalysis, and biochemistry. After donation, the original donor forms its conjugate base, while the recipient becomes protonated.

The tendency of a substance to donate a proton is described by its acidity constant (Ka) or, more commonly, by its pKa. Lower pKa values indicate stronger proton donation. In aqueous solutions, donated protons typically form hydronium (H3O+), with the equilibrium shaped by solvent effects and temperature. In non-aqueous solvents, pKa values are solvent-dependent and the identities of the conjugate bases influence the equilibrium. In the gas phase, proton donation is discussed in terms of gas-phase acidity and the proton affinity of the resulting base.

Common examples include hydrochloric acid donating a proton to water to produce chloride and hydronium, and acetic acid donating a proton to water to produce acetate and hydronium. Biologically, protondonation underlies many enzymatic acid–base catalysis steps, such as general acid catalysis in active sites, and supports proton relay networks that regulate the protonation state of biomolecules. Protondonation thus plays a fundamental role across chemistry, biology, and materials science.