Kinetic Study of the Reactions of Chlorine Atoms and Cl₂•- Radical Anions in Aqueous Solutions. 1. Reaction with Benzene

The photolysis of Na 2 S 2 O8 aqueous solutions containing Cl - ions is a clean method for kinetic studies of the species Cl • / Cl 2 • - in the absence and presence of added aromatic substrates. Laser and conventional flash-photolysis techniques were employed to investigate the aqueous phase reactions of chlorine atoms and Cl 2 • - (340 nm) radical ions in the presence and absence of benzene. A mechanism is proposed which accounts for the decay of Cl 2 • - in aqueous solutions containing chloride ion concentrations in the range 1 × 10 -4 to 0.6 M, total radical (Cl • + Cl 2 • - ) concentrations in the range (0.1−1.5) × 10 -5 M, and pH in the range 2.5−3.0. Interpretation of the experimental data is supported by kinetic computer simulations. The rate constants 6 × 10 9 M -1 s -1 ≤ k ≤ 1.2 × 10 10 M -1 s -1 and < 1 × 10 5 M -1 s -1 were determined for the reactions of Cl • and Cl 2 • - with benzene, respectively, in the aqueous phase. The organic radicals produced from these reactions exhibit an absorption band with maximum at 300 nm, which was assigned to a Cl-cyclohexadienyl radical (Cl−CHD). The kinetic analysis of the traces supports a reversible reaction between O 2 and Cl−CHD. A reaction mechanism leading to the formation of chlorobenzene is proposed.