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

Although Cl₂ and Cl₂O have been recognized as highly reactive constituents of free available chlorine (FAC), robust rate constants for Cl₂ and Cl₂O remain scarce in the environmental literature. In this work, we explored the chlorination kinetics of three structurally related alkenes (α-ionone, β-ionone, and dehydro-β-ionone), a class of compounds whose reactivities with Cl₂ and Cl₂O have not been previously investigated. Second-order rate constants for Cl₂, 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, Cl₂ 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 Cl₂ 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 Cl₂ may enhance the formation kinetics of chlorinated disinfection byproducts (DBPs) and exacerbate the burden of DBP control for water utilities.