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 x 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.

Water‐Catalyzed Dehalogenation Reactions of the Isomer of CBr4 and Its Reaction Products and a Comparison to Analogous Reactions of the Isomers of Di‐ and Trihalomethanes

“…The measured molar extinction coefficients for CBr 4 and Br À were employed to determine the concentrations of these species for each photolysis time from the UV/Vis spectra of Figure 1 (Figure 1 c) show a linear correlation with a slope of about 0.9, which is close to 1 and suggests that each Br À ion is accompanied by formation of an H + ion, similar to previous photochemical results for CHBr 3 . [28,29] The UV/Vis and pH photochemical data in Figure 1 suggest that UV photolysis of low concentrations of CBr 4 (Figure 2 a). The small amount of CD 3 CN in the solvent system provides convenient reference bands during the experiments.…”

“…We recently observed that photolysis of polyhalomethanes such as CH 2 I 2 and CHBr 3 in aqueous environments can form appreciable amounts of isopolyhalomethanes. [28][29][30]33] In the case of CHBr 3 we were able to directly observe the OÀH insertion reaction of the isobromoform species with water to produce a CHBr 2 OH product that undergoes further water-catalyzed decomposition reactions. [28,29] In the next section, we present picosecond timeresolved resonance Raman experiments performed to examine what photoproducts are produced shortly after photolysis of CBr 4 in the presence of water and whether they can react with water.…”

“…[24] We recently used a combination of photochemical experiments and theoretical calculations to help elucidate the reaction mechanism for photochemical conversion of low concentrations of CHBr 3 in water to three bromide ions. [28,29] Ultraviolet excitation of CHBr 3 (9 10 À5 m) in water gave almost complete conversion to three HBr leaving groups and CO (major product) and HCOOH (minor product). [29] Picosecond time-resolved resonance Raman (ps-TR 3 ) experiments and ab initio calcu-Abstract: A combined experimental and theoretical study of the UV photolysis of a typical tetrahalomethane, CBr 4 , in water and acetonitrile/water was performed.…”

“…[28,29] Ultraviolet excitation of CHBr 3 (9 10 À5 m) in water gave almost complete conversion to three HBr leaving groups and CO (major product) and HCOOH (minor product). [29] Picosecond time-resolved resonance Raman (ps-TR 3 ) experiments and ab initio calcu-Abstract: A combined experimental and theoretical study of the UV photolysis of a typical tetrahalomethane, CBr 4 , in water and acetonitrile/water was performed. Ultraviolet photolysis of low concentrations of CBr 4 in water mostly leads to the production of four HBr leaving groups and CO 2 .…”

“…Keywords: ab initio calculations · dehalogenation · elimination · reaction mechanisms · water chemistry lations indicate that the water-catalyzed OÀH insertion/HBr elimination reaction of isobromoform and subsequent reactions of its products are responsible for the formation of the final products observed after UV photolysis of CHBr 3 in water. [29] Ultraviolet photolysis of low concentrations of CH 2 I 2 in water led to production of methanediol (CH 2 (OH) 2 ) and two HI leaving groups as products. [30] Ps-TR 3 experiments and ab initio calculations indicate that water-catalyzed OÀH insertion/HI elimination of isodiiodomethane and subsequent reactions of its products are responsible for the formation of methanediol and two HI leaving groups.…”