HAtomTransfer

HAtomTransfer, also known as hydrogen atom transfer (HAT), is a fundamental reaction in chemistry involving the migration of a hydrogen atom, comprising one proton and one electron, between two chemical species. In a typical HAT event, a hydrogen atom is abstracted from a donor molecule by a radical reagent, yielding a new radical at the donor site and a reduced radical at the acceptor. The reaction can be represented generally as R–H + X• → R• + X–H, where R–H is the hydrogen donor and X• is the hydrogen acceptor radical. HAT processes are reversible and often play a key role in radical chain reactions, photochemical transformations, and catalytic cycles.

Mechanistically, HAT is driven by the relative bond dissociation energies (BDEs) of the donor and acceptor bonds. The reaction proceeds more favorably when the BDE of the bond being formed is lower than that of the bond being broken. Transition state theory suggests that HAT involves a synchronous transfer of the proton and electron, leading to a highly localized transition state that can be described by radical intermediates. The kinetics of HAT are sensitive to steric and electronic factors; electron‑rich donors generally undergo HAT more readily than electron‑poor ones, while radical stabilisation enhances the selectivity and rate of the transfer.

HAtomTransfer reactions are central to many areas of chemistry. In organic synthesis, they enable the functionalisation of alkanes, the construction of complex molecules, and the development of catalytic hydrogen‑atom sources such as transition‑metal hydrides and photoredox catalysts. In biochemistry, HAT underlies processes like oxidative DNA damage, enzymatic reactions involving iron‑sulfur clusters, and the activity of enzymes such as oxygenases. Environmental chemistry also exploits HAT in the degradation of pollutants and in atmospheric oxidation processes. The growing interest in "metal‑free" HAT catalysis has spurred the discovery of new organocatalysts and photoinduced HAT pathways that often improve reaction efficiency and selectivity.

Overall, HAtomTransfer offers a versatile and environmentally friendly strategy for manipulating hydrogen atoms in synthetic and biological systems. Its mechanistic simplicity, combined with the breadth of its applications, continues to make HAT a focus of intense research across multiple disciplines.