protonating

Protonating is the chemical process of adding a proton (H+) to a molecule, yielding a positively charged conjugate acid. In acid–base chemistry, protonation is a common form of electrophilic attack in which a base accepts a proton from an acid. The site and extent of protonation depend on the molecule’s basicity and the solvent; typical targets include lone-pair sites on nitrogen, oxygen, or sulfur, and pi systems in alkenes or carbonyls.

In organic synthesis, protonation is used to activate substrates or to form stable salts. Protonation of amines gives ammonium salts; protonation of carbonyls increases electrophilicity; protonation of imines yields iminium ions. The reaction is usually reversible, with deprotonation regenerating the original species.

The degree of protonation is governed by pH and the acidity of the medium, often described by pKa values of the conjugate acid–base pair. In water, protonation is favorable for species with conjugate acids stronger than hydronium; in nonpolar solvents, protonation is less favorable and more dependent on specific interactions.

In biology, protonation states of ionizable groups influence protein structure and function. Amino-acid side chains such as histidine, aspartate, glutamate, lysine, and cysteine can change protonation state near physiological pH, affecting charge, hydrogen bonding, and catalytic activity.