Efficient dehalogenation of polyhalomethanes and production of strong acids in aqueous environments: Water-catalyzed O–H-insertion and HI-elimination reactions of isodiiodomethane (CH2I–I) with water

Abstract

A combined experimental and theoretical study of the ultraviolet photolysis of CH2I2 in water is reported. Ultraviolet photolysis of low concentrations of CH2I2 in water was experimentally observed to lead to almost complete conversion into CH2(OH)2 and 2HI products. Picosecond time-resolved resonance Raman spectroscopy experiments in mixed water/acetonitrile solvents (25%-75% water) showed that appreciable amounts of isodiiodomethane (CH2I-I) were formed within several picoseconds and the decay of the CH2I-I species became substantially shorter with increasing water concentration, suggesting that CH2I-I may be reacting with water. Ab initio calculations demonstrate the CH2I-I species is able to react readily with water via a water-catalyzed O--H-insertion and HI-elimination reaction followed by its CH2I(OH) product undergoing a further water-catalyzed HI-elimination reaction to make a H2C=O product. These HI-elimination reactions produce the two HI leaving groups observed experimentally and the H2C=O product further reacts with water to produce the other final CH2(OH)2 product observed in the photochemistry experiments. These results suggest that CH2I-I is the species that reacts with water to produce the CH2(OH)2 and 2HI products seen in the photochemistry experiments. The present study demonstrates that ultraviolet photolysis of CH2I2 at low concentration leads to efficient dehalogenation and release of multiple strong acid (HI) leaving groups. Some possible ramifications for the decomposition of polyhalomethanes and halomethanols in aqueous environments as well as the photochemistry of polyhalomethanes in the natural environment are briefly discussed.

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

“…Previous photochemical experiments on trihalomethanes such as CHBr 3 , CHBr 2 Cl, and CHCl 2 Br found that UV excitation (253.7 nm light from a Hg lamp) at low concentrations (< 10 À6 m) in water led to complete conversion of the organic halogen of the polyhalomethanes to halide ions (Br À and/or Cl À ) with a similar photoquantum yield of 0.43. [24] Our present results for CBr 4 and the dihalomethane CH 2 I 2 at relatively low concentrations [30] are consistent with the results for trihalomethanes, that is, conversion of organic halogens to halide ions with an appreciable photoquantum yield. This suggests that it is probably fairly common for UV excitation of many polyhalomethanes at low concentrations in water to lead to almost quantitative conversion to halide ions.…”

“…This behavior is very similar to previous ps-TR 3 spectra observed for formation of other related isopolyhalomethanes such as isobromoform and isodiiodomethane following UV photolysis in acetonitrile and acetonitrile/ water. [28][29][30]33] The prominent n 1 CÀBr stretching fundamental band in the spectra is in excellent agreement with the observation of the same band for isotetrabromomethane previously observed by ns-TR 3 spectroscopy in cyclohexane and clearly identifies this species, since the CÀBr stretch shifts significantly in other likely photoproducts such as CBr 4 + , CBr 3 radical, and CBr 3 + . [34] The different probe wavelength (400 nm) and/or polar solvents used in the ps-TR 3 experiments reported here probably account for why the n 3 mode is more clearly observed in these spectra than in the previous ns-TR 3 spectra obtained with a probe wavelength of 436 nm in cyclohexane.…”

“…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.…”

“…[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. [30] We note that UV photolysis of dihalomethanes (e.g., CH 2 I 2 ) and trihalomethanes (e.g., CHBr 3 ) led to the formation of different products from the carbon atom of the polyhalomethane.…”

“…[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. [30] We note that UV photolysis of dihalomethanes (e.g., CH 2 I 2 ) and trihalomethanes (e.g., CHBr 3 ) led to the formation of different products from the carbon atom of the polyhalomethane. Thus, it is not clear what products will be formed from decomposition of tetrahalomethanes such as CBr 4 , CCl 4 , and CFCl 3 .…”