Unveiling the reaction chemistry of sulfoxides during water chlorination

Species-specific second-order rate constants for the reactions of eight model sulfoxides with hypochlorous acid (k_HOCl) were determined to be in the range of 2.7 M^-1 s^-1 to 5.8 × 10³M^-1 s^-1. A quantitative structure-activity relationships (QSAR) with Taft σ* constants was developed based on eight measured k_HOCl-values, showing a good linear correlation (R² = 0.89) with a negative slope ρ = -1.5 typical for electrophilic reactions. The reaction is mainly controlled by HOCl, with a minor contribution of OCl^-. The contributions of other reactive chlorine species (e.g., Cl₂ and Cl₂O) to the overall kinetics are only 7 % for Cl₂O and 5 % for Cl₂ under typical drinking water treatment conditions. A combination of several analytical methods (HPLC-MS/MS, HPLC-ICP-MS/MS, and NMR) was applied for the identification of transformation products. Major transformation products from the reactions of chlorine with sulfoxides are sulfones, Cl-substituted sulfoxides, aldehydes, and sulfonic acids potentially formed via a transient chlorosulfonium cation. In general, sulfoxides react more readily with chlorine compared to bromine. This might be caused by a partial positive charge on the sulfur which leads to a stronger interaction with Cl in HOCl having a smaller partial positive charge than Br in HOBr. The ratios of the species-specific second-order rate constants for the reactions of the selected sulfoxides with chlorine or bromine (k_HOCl/k_HOBr) range from 6 to 480. For sulfoxide compounds with strong electron-withdrawing substituents the reaction occurs most likely via a carbanion intermediate for which the reaction with HOBr is preferred, resulting in a k_HOCl/k_HOBr = 0.8.