Effect of substituents on different channels of ^•OH radical reaction with substituted organic sulfides

Abstract

Pulse radiolysis technique has been employed to study the nature of ^•OH radical reaction in aqueous solutions of substituted organic sulfides. The transient absorption band at 345 nm observed on reaction of ^•OH radicals in neutral aqueous solution of 3,3'-thiodipropionitrile is assigned to OH-adduct at sulfur. OH-adduct is observed to have high reactivity with oxygen (k=8.8 × 10⁸ dm³ mol^-1 s^-1). The reaction of ^•OH radicals in neutral aqueous solution of methyl propyl sulfide has shown the formation of sulfur-centered dimer radical cation with a small fraction (~10%) of α-(alkylthio)alkyl radicals. The reaction of ^•OH radicals with thiodiglycolic acid showed an absorption band at 285 nm, which is assigned to α-(alkylthio)alkyl radicals. The reaction of ^•OH radicals with dimethyl 2,2'-thiodiethanoic acid has been assigned to OH-adduct at sulfur, whereas the transient absorption band at 390 observed with 3,3'-thiodipropionic acid is assigned to intra-molecular radical cation formed on p-orbital overlap of oxidized sulfur with oxygen. In acidic solutions, sulfur-centered dimer radical cation is the only transient species observed with substituted alkyl sulfides. The concentration of acid required to observe the formation of dimer radical cation is found to depend on the electron-withdrawing power of the substituted group. The reaction of ^•OH radicals in neutral aqueous solution of substituted aryl sulfides has shown the formation of monomer radical cation and OH-adduct at benzene ring. Sulfur-centered dimer radical cations are not observed even in acidic conditions.