Protondonatorn

Protondonatorn, or the proton donor, is a substance that donates a proton (H+) to another chemical species during a reaction. In Brønsted-Lowry acid-base theory, proton donors are acids; the recipient is the base. The term is used across languages; in Swedish, protondonatorn is used as the definite form.

In solution, donation typically occurs as AH + H2O → A− + H3O+. The strength of a proton donor is described by its acid strength, commonly expressed as the acid dissociation constant Ka or its negative log pKa. Strong proton donors (strong acids) dissociate largely in water, producing a high concentration of hydronium ions; weak donors dissociate only slightly, leaving most of the donor intact. Solvent effects are important: in non-aqueous media, acidity scales differ, and the same substance can behave as a stronger or weaker donor.

Examples include hydrochloric acid (HCl), a conventional strong proton donor in water, and acetic acid (CH3COOH), a comparatively weak donor. The ammonium ion (NH4+) donates a proton to form ammonia (NH3) and is also a proton donor. Some substances can donate more than one proton (polyprotic donors), each step with its own pKa.

Applications of the concept include understanding buffer design, acid-base catalysis, and many organic synthesis reactions. The term distinguishes Brønsted-Lowry acids from bases and contrasts with the Lewis concept of acids, which focuses on electron-pair acceptors.