Aqueous Chlorination Kinetics of Cyclic Alkenes-Is HOCI the Only Chlorinating Agent that Matters?

Although Cl-2 and Cl2O have been recognized as highly reactive constituents of free available chlorine (FAC), robust rate constants for Cl-2 and Cl2O remain scarce in the environmental literature. In this work, we explored the chlorination kinetics of three structurally related alkenes (alpha-ionone, beta-ionone, and dehydro-beta-ionone), a class of compounds whose reactivities with Cl-2 and Cl2O have not been previously investigated. Second-order rate constants for Cl-2, Cl2O and HOCl were computed from experimental rate constants obtained at various pH values, [Cl-], and [FAC]. Our results show that while HOCl is the predominant chlorinating agent for the most reactive alkene, Cl-2 and Cl2O can dominate the chlorination kinetics of the less reactive alkenes at high [Cl-] and high [FAC], respectively. The tradeoff between overall reactivity with FAC and selectivity for Cl-2 and Cl2O previously observed for aromatic compounds also applies to the alkenes examined. In laboratory experiments in which high [FAC] may be used, omission of Cl2O in data modeling could yield second-order rate constants of dubious validity. In chlorinating real waters with elevated [Cl-], formation of Cl-2 may enhance the formation kinetics of chlorinated disinfection byproducts (DBPs) and exacerbate the burden of DBP control for water utilities.