Water-Catalyzed Dehalogenation Reactions of Isobromoform and Its Reaction Products

Abstract

A combined experimental and theoretical study of the photochemistry of CHBr 3 in pure water and in acetonitrile/water mixed solvents is reported that elucidates the reactions and mechanisms responsible for the photochemical conversion of the halogen atoms in CHBr 3 into three bromide ions in water solution. Ultraviolet excitation at 240 nm of CHBr 3 (9 × 10 -5 M) in water resulted in almost complete conversion into 3HBr leaving groups and CO (major product) and HCOOH (minor product) molecules. Picosecond time-resolved resonance Raman (ps-TR 3) experiments and ab initio calculations indicate that the water-catalyzed O-H insertion/HBr elimination reaction of isobromoform and subsequent reactions of its products are responsible for the production of the final products observed following ultraviolet excitation of CHBr 3 in water. These results have important implications for the phase-dependent behavior of polyhalomethane photochemistry and chemistry in water-solvated environments as compared to gas-phase reactions. The dissociation reaction of HBr into H + and Br - ions is the driving force for several O-H insertion and HBr elimination reactions and allows O-H and C-H bonds to be cleaved more easily than in the absence of water molecules. This water-catalysis by solvation of a leaving group and its dissociation into ions (e.g., H + and Br - in the examples investigated here) may occur for a wide range of chemical reactions taking place in water-solvated environments.