Chlorination and bromination of olefins: Kinetic and mechanistic aspects.

Hypochlorous acid (HOCl) is typically assumed to be the primary reactive species in free available chlorine (FAC) solutions. Lately, it has been shown that less abundant chlorine species such as chlorine monoxide (ClO) and chlorine (Cl) can also influence the kinetics of the abatement of certain organic compounds during chlorination. In this study, the chlorination as well as bromination kinetics and mechanisms of 12 olefins (including 3 aliphatic and 9 aromatic olefins) with different structures were explored. HOCl shows a low reactivity towards the selected olefins with species-specific second-order rate constants <1.0 Ms, about 4-6 orders of magnitude lower than those of ClO and Cl. HOCl is the dominant chlorine species during chlorination of olefins under typical drinking water conditions, while ClO and Cl are likely to play important roles at high FAC concentration near circum-neutral pH (for ClO) or at high Cl concentration under acidic conditions (for Cl). Bromination of the 12 olefins suggests that HOBr and BrO are the major reactive species at pH 7.5 with species-specific second-order rate constants of BrO nearly 3-4 orders of magnitude higher than of HOBr (ranging from <0.01 to >10 Ms). The reactivities of chlorine and bromine species towards olefins follow the order of HOCl < HOBr < BrO < ClO ≈ Cl. Generally, electron-donating groups (e.g., CHOH- and CH-) enhances the reactivities of olefins towards chlorine and bromine species by a factor of 3-10, while electron-withdrawing groups (e.g., Cl-, Br-, NO-, COOH-, CHO-, -COOR, and CN-) reduce the reactivities by a factor of 3-10. A reasonable linear free energy relationship (LFER) between the species-specific second-order rate constants of BrO or ClO reactions with aromatic olefins and their Hammett σ was established with a more negative ρ value for BrO than for ClO, indicating that BrO is more sensitive to substitution effects. Chlorinated products including HOCl-adducts and decarboxylated Cl-adduct were identified during chlorination of cinnamic acid by high-performance liquid chromatography/high resolution mass spectrometry (HPLC/HRMS).