reprotonate

Reprotonate is a term used in chemistry to describe the act of adding a proton (H+) to a molecule, restoring its protonated form after deprotonation. It is the reverse process of deprotonation and is governed by acid–base equilibria, often described by pKa values of the species involved.

In solution, reprotonation depends on the availability of a proton donor, commonly hydronium (H3O+) in water. The extent of reprotonation is determined by the pH relative to the conjugate acid’s pKa; protonation is favored when the environment is more acidic (lower pH) than the pKa of the site being protonated.

Typical examples include: R-NH2 + H+ ⇌ R-NH3+ (amines becoming protonated to form ammonium ions) and RO− + H+ ⇌ ROH (alkoxides becoming alcohols). Reprotonation can occur at various sites, such as nitrogen, oxygen, sulfur, or carbon centers, depending on the molecule’s basicity.

Reprotonation plays a central role in many reaction mechanisms and catalytic cycles. After a base abstracts a proton to generate an anionic or neutral intermediate, reprotonation by an acid can restore a neutral or differently charged species. In biological systems, protonation states influence the charge, structure, and activity of proteins and enzymes, and they shift with cellular pH.

Isotopic techniques sometimes use deuterons to study proton transfer and reprotonation dynamics, providing insight into reaction mechanisms and kinetics. Overall, reprotonation is a fundamental, reversible aspect of chemical and biological proton transfer.