The reaction of α-(alkylthio)alkyl radicals with tetranitromethane. Pulse radiolysis evidence for an intermediate radical adduct

Abstract

α-(Alkylthio)alkyl radicals, R¹ĊHSR², which are produced as intermediates in the ˙OH radical-induced oxidation of organic sulphides, exhibit reducing properties. The experimental data for their reaction with tetranitromethane are best described in terms of a two-step mechanism via an intermediate adduct: R¹ĊHSR²+ C(NO₂)₄→[R²SCHR¹⋯ O₂N ⋯ C(NO₂)₃]˙→(R¹CHSR²)⁺+ NO₂+ C(NO₂)₃^–. The bimolecular rate constants for formation of the adducts have been found to be in the range (2–4)× 10⁹mol^–1dm³ s^–1 for all the sulphides investigated and to be independent of temperature. The stability of the adduct depends significantly on the substituent R¹ and also on temperature. The rate constant, k₁, for its decay was found to be ca. 10⁵s^–1 for R¹= H, and (1–4)× 10⁶ s^–1for R¹= alkyl at 21 °C. Values for ΔH^‡ of 55.5 and 54.4 kJ mol^–1, and ΔS^‡ of +40.1 and +62.4 J mol^–1K^–1, have been determined for the activated transition state of the radical adducts derived from (CH₃)₂S and (C₂H₅)₂S, respectively. The positive change in activation entropy probably indicates that the final products are already more or less performed in the activated complex. The reaction of α-(alkylthio)alkyl radicals with tetranitromethane has been used to determine the site and yield of ˙OH radical attack on the sulphides. In the particular case of 2-(ethylthio)ethanol the kinetic data on the decay of the radical–tetranitromethane adduct have been used to characterize the identity of the various reducing radicals formed from this sulphide. Possible implications on the OH radical-induced oxidation of substituted aliphatic sulphides are discussed.