Water-catalyzed O-H insertion/HI elimination reactions of isodihalomethanes (CH 2X-I, where X = Cl, Br, I) with water and the dehalogenation of dihalomethanes in water-solvated environments

Abstract

A combined experimental and theoretical investigation of the ultraviolet photolysis of CH 2XI (where X = Cl, Br, I) dihalomethanes in water is presented. Ultraviolet photolysis of low concentrations of CH 2XI (where X = Cl, Br, I) in water appears to lead to almost complete conversion into CH 2(OH) 2 and HX and HI products. Picosecond time-resolved resonance Raman (ps-TR 3) spectroscopy experiments revealed that noticeable amounts of CH 2X-I isodihalomethane intermediates were formed within several picoseconds after photolysis of the CH 2XI parent compound in mixed aqueous solutions. The ps-TR 3 experiments in mixed aqueous solutions revealed that the decay of the CH 2X-I isodihalomethane intermediates become significantly shorter as the water concentration increases, indicating that the CH 2X-I intermediates may be reacting with water. Ab initio calculations found that the CH 2X-I intermediates are able to react relatively easily with water via a water-catalyzed O-H insertion/HI elimination reaction to produce CH 2X(OH) and HI products, with the barrier for these reactions increasing as X changes from Cl to Br to I. The ab initio calculations also found that the CH 2X(OH) product can undergo a water-catalyzed HX elimination reaction to make H 2C=O and HX products, with the barrier to reaction decreasing as X changes from Cl to Br to I. The preceding two water-catalyzed reactions produce the HI and HX leaving groups observed experimentally, and the H 2C=O product further reacts with water to make the other CH 2(OH) 2 product observed in the photochemistry experiments. This suggests that that the CH 2X-I intermediates react with water to form the CH 2(OH) 2 and HI and HX products observed in the photochemistry experiments. Ultraviolet photolysis of CH 2XI (where X = Cl, Br, I) at low concentrations in water-solvated environments appears to lead to efficient dehalogenation and release of two strong acid leaving groups. We very briefly discuss the potential influence of this photochemistry in water on the decomposition of polyhalomethanes and halomethanols in aqueous environments.