hydrogenatomtransfer

Hydrogen atom transfer (HAT) is a reaction mechanism in radical chemistry in which a hydrogen atom, consisting of a proton and its electron, is transferred between two species. In typical HAT events, a radical abstracts a hydrogen from a donor substrate, generating a new radical on the substrate and a neutral product. The process can occur intramolecularly or between separate molecules and is a common pathway in oxidation and hydrofunctionalization reactions.

Mechanistic features of HAT include its characterization as a concerted transfer of a hydrogen atom. The reaction rate is often correlated with the bond dissociation energies (BDE) of the donor X–H bond and the ability of the abstracting radical to accommodate the resulting radical center. Factors such as polarity, steric accessibility, and solvent environment influence the rate and outcome. Kinetic isotope effects, observed when hydrogen is replaced by deuterium, are frequently used to probe HAT mechanisms.

HAT is distinct from hydride transfer, which involves two electrons and a formal H− ion, and from pure electron or proton transfer steps. It is related to proton-coupled electron transfer (PCET) in that a hydrogen atom transfer can be viewed as a concerted transfer of a proton and an electron, though HAT emphasizes the neutral hydrogen atom as the transported species.

Applications of HAT appear across organic synthesis, combustion, atmospheric and photochemical processes, and enzymatic radical relay pathways. It enables selective C–H functionalization and controlled radical generation, with practical examples including abstraction by alkoxyl or hydroxyl radicals and by photogenerated radicals in oxidation catalysis.