Kinetic Study of the Reactions of Chlorine Atoms and Cl₂^•- Radical Anions in Aqueous Solutions. II. Toluene, Benzoic Acid, and Chlorobenzene

Abstract

Laser and conventional flash photolysis of Na2S2O8 aqueous solutions containing Cl- ions were employed to investigate the reactions of chlorine atoms and Cl2•- radical ions with toluene, benzoic acid, and chlorobenzene. A mechanism is proposed which accounts for the faster decay of Cl2•- in aqueous solutions containing increasing concentrations of the organic substrates. Interpretation of the experimental data is supported by kinetic computer simulations. Chlorine atoms react with the three substituted aromatics studied here almost with diffusion-controlled rate constants, k = (1.8 ± 0.3) × 1010 M-1 s-1. The high reactivity observed for Cl atoms contrasts with that of the Cl2•- radical ions, for which the rate constant for its reactions with the substituted benzenes is ≤ 1 × 106 M-1 s-1. The organic radicals produced from these reactions, as well as the nature of the reaction products are discussed. The observed results seem to support an addition mechanism yielding chlorocyclohexadienyl radicals (Cl−CHD) as the most significant reaction channel following reaction of Cl atoms and the organic compounds. In air-saturated solutions, subsequent thermal reactions of Cl−CHD radicals lead both to chlorination and oxidation of the aromatics.