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 α-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 α-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.