The reactivity of chlorine atoms in aqueous solution. Part III. The reactions of Cl• with solutes

Abstract

Laser flash photolysis of chloroacetone was used to measure the rate constants and activation energies for the reactions of the Cl^• atom with a number of oxygen-containing compounds and inorganic anions in aqueous solution. For the organic compounds there is a strong correlation at 25°C between k(Cl^•+RH) and k(^•OH+RH), where RH is CH₃OH, CH₃CH₂OH, CH₃CH(OH)CH₃, (CH₃)₃COH, HCHO, CH₃CHO, CH₃CO₂H, HCO₂H and HCO₂⁻, respectively. For CH₃CO₂⁻, k(Cl^•+RH)k(^•OH+RH), and for CH₃COCH₃ and CH₃COCH₂Cl, k(Cl^•+RH)k(^•OH+RH). Possible reasons for these differences are discussed in terms of preferential attack by Cl^• at O–H groups in the neutral molecules, rather than H-abstraction from C–H as with ^•OH, and electron transfer for the reactions of Cl^• with the anions. For the inorganic anions X=OCN⁻, NO₃⁻, SO₄²⁻, ClO₃⁻, SCN⁻, HCO₃⁻, N₃⁻, NO₂⁻, HSO₃, k(Cl^•+X) ranges from 1.0×10⁸ (NO₃⁻) to 5.3×10⁹ dm³ mol⁻¹ s⁻¹ (SCN⁻) but there is no strong correlation between k and the reduction potential of X. Comparison of the reactivity of Cl^• with reported rate constants for the reactions of Cl₂^•− indicates that, in many cases, these rate constants are largely accounted for by the fraction of Cl^• present in equilibrium with Cl₂^•−. The implications of these results for atmospheric chemistry are discussed.