Spectral, kinetic, and redox studies on the transients formed on reaction of hydroxyl radicals with thioanisole

Abstract

The hydroxyl radicals are observed to react with thioanisole (TA) with a bimolecular rate constant of 3.5 × 10⁹ dm³ mol^-1 s^-1 and form transient optical absorption bands with λ _max = 310, 365, 530, and 740 nm. The transient absorption bands at 310, 530, and 740 nm are also formed on reaction of specific one-electron oxidants such as CCl₃OO^•, Br₂^•¯, and Cl₂^•¯ with thioanisole and are assigned to solute radical cations with positive charge on the benzene ring. Based on the studies with specific one-electron oxidants, ≈73% of ^•OH radicals are inferred to react with thioanisole by electron transfer and 27% by OH-adduct formation. The oxidation potential for the TA/TA^•+ couple, based on the equilibrium established with the Br₂^•¯ / 2Br^- couple, is determined to be 1.53 V vs NHE. The OH-adduct (λ _max = 365 nm) is observed to undergo acid-catalyzed dehydration and form a solute radical cation. A small fraction of it is converted to sulfur-centered dimer radical cations and the major fraction into the monomer radical cations with the positive charge on the benzene ring. The monomer radical cations, formed in neutral aqueous solutions, are a strong one-electron oxidant and are able to oxidize I^- with a bimolecular rate constant value of 6.4 × 10⁹ dm³ mol^-1 s^-1.