Differences in the Reaction Mechanisms of Chlorine Atom and Hydroxyl Radical with Organic Compounds: From Thermodynamics to Kinetics

Hydroxyl radical (HO^•) and chlorine atom (Cl^•) are common reactive species in aqueous environments. However, the intrinsic difference in their reactions with organic compounds has not been revealed. This study compared the reaction mechanisms of HO^• and Cl^• with 13 aromatic and 11 aliphatic compounds by quantum chemical calculation and laser flash photolysis. Both HO^• and Cl^• can spontaneously react with aromatic compounds via radical adduct formation (RAF), hydrogen atom transfer (HAT), and single electron transfer (SET) pathways. The SET reactions of Cl^• were more thermodynamically favorable than HO^•, but contrary results were obtained for HAT reactions. According to the free energy of activation (Δ<i>G</i>_aq^‡), the dominant oxidation mechanisms of aromatic compounds were RAF and SET by HO^• and SET by Cl^•. The important role of SET in the HO^• reactions with aromatic compounds was further verified by accurately calculating the solvation free energy of HO^•/HO^- and experimentally tracking the radical cations, which were generally neglected in previous studies. Meanwhile, the Δ<i>G</i>_aq^‡ value of each reaction pathway of Cl^• was lower than that of HO^•, resulting in higher rate constants of Cl^• with aromatic compounds than HO^•. For saturated aliphatic compounds, HAT was found to be the only mechanism accounting for their transformation by HO^• and Cl^•. This study proposed general rules for the reaction mechanisms of HO^• and Cl^• and unraveled their differences in the aspects of thermodynamics and kinetics, providing fundamental information for understanding contaminant transformation in processes involving HO^• and Cl^•.