hydrodehalogenation

Hydrodehalogenation is a chemical transformation in which a carbon–halogen bond is replaced by a carbon–hydrogen bond, removing one or more halogen substituents from an organic molecule. The reaction is widely used in organic synthesis and environmental remediation, enabling conversion of halogenated substrates to non-halogenated hydrocarbons.

Most hydrodehalogenations are carried out with a hydrogen source and a metal catalyst. In catalytic hydrogenolysis, molecular hydrogen (H2) is activated on a metal surface (commonly palladium, nickel, platinum, or rhodium) and transfers hydride to the substrate, promoting cleavage of the C–X bond and formation of HX as a byproduct. Transfer hydrogenation uses hydrogen donors such as formic acid, ammonium formate, or alcohols, providing in situ hydrogen without gaseous H2. Substrates include alkyl, aryl, and vinyl halides; the reactivity typically decreases in the order RI > RBr > RCl, and aryl chlorides often require more active catalysts.

Mechanistically, the process can proceed via oxidative addition of C–X to the metal, followed by reductive elimination of C–H, or via surface hydrogenation for heterogeneous catalysts. Heterogeneous systems commonly use supported noble metals (Pd, Pt, Ru, Ni) or base metals (Ni, Fe, Cu) with various supports and modifiers.

Applications include synthesis steps to remove halogens, purification of pharmaceuticals, and environmental cleanup of chlorinated solvents in groundwater. In remediation, hydrodehalogenation is often coupled with iron- or palladium-based catalysts and may occur in aqueous media using hydrogen donors. Limitations include catalyst deactivation by halogens, incomplete dehalogenation, over-reduction of sensitive groups, and cost or scalability considerations for industrial use.