Structure-reactivity studies on OH radical reaction with substituted dialkyl sulfides

Abstract

The results of the reaction of an OH radical with a number of functionalized organic sulfides reported here demonstrate that the pH, the nature of the functional group and the chain length affect the nature of the OH radical reaction with sulfides. The transient absorption spectrum (λ_max = 295 nm), formed on reaction of H atoms and OH radicals with an aqueous solution of 2-(methylthio)ethanol and 2-(ethylthio)ethanol is assigned to the a-thio radical. It decayed by second order kinetics with 2k = 5.2 × 10⁹ dm³ mol^-1 s^-1 and was quenched by oxygen. The transient absorption spectra (λ_max = 480 and 500 nm respectively), obtained on reaction with OH radicals (pH = 1), are assigned to a sulfur centered dimer radical cation. The variation of the transient absorbance with pH showed an inflection point at pH = 2.1. The reaction of OH radicals (pH = 6) with 2,2'-thiodiethanoyl chloride showed the formation of a-thio radicals (λ_max = 300 nm) and an OH-adduct (λ = 350-380 nm), whereas in acidic solutions, the transient spectrum (λ_max = 340 nm,Τ = 0.8 μs) is assigned to an intra-molecular radical cation with a 4-membered ring configuration. The transient species (λ_max = 370 nm, Τ = 17 μs) formed on reaction of OH radicals with a 3,3'-thiodipropionyl chloride is assigned to a 5-membered intra-molecular radical cation and remained independent of pH. The OH-adduct (λ_max = 350 nm, pH = 11) of 3,3'-thiodipropionamide is observed to undergo transformation to an intra-molecular radical cation (λ_max = 370 nm). The transformation is not observed in acidic solutions and only an intra-molecular radical cation (λ_max = 370 nm) is observed immediately after the pulse. The contribution of α-thio radicals of 2,2'-thiodiethanamide decreased with pH, and in neutral solutions OH radicals are observed to react mainly by OH-adduct formation whereas in acidic solutions (pH = 1), as intra-molecular radical cation (λ_max = 335 nm) with a 4-membered ring configuration is inferred to be the transient species.