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

Abstract

The photolysis of Na2S2O8 aqueous solutions containing Cl- ions is a clean method for kinetic studies of the species Cl•/ Cl2•- 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 Cl2•- (340 nm) radical ions in the presence and absence of benzene. A mechanism is proposed which accounts for the decay of Cl2•- in aqueous solutions containing chloride ion concentrations in the range 1 × 10-4 to 0.6 M, total radical (Cl• + Cl2•-) 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 × 109 M-1 s-1 ≤ k ≤ 1.2 × 1010 M-1 s-1 and < 1 × 105 M-1 s-1 were determined for the reactions of Cl• and Cl2•- 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 O2 and Cl−CHD. A reaction mechanism leading to the formation of chlorobenzene is proposed.