Evidence for the role of electron-withdrawing power of functional groups and [H⁺] for electron-transfer reaction in substituted alkyl sulfides

Abstract

The pulse radiolysis technique has been employed to demonstrate the effects of the electron-withdrawing power of functional groups and the H⁺ concentration on the nature of ^•OH radical reaction with substituted alkyl sulfides. The intermediate OH adduct and α-thioalkyl radical could be detected in substituted sulfides having a functional group of high electron-withdrawing power. The concentration of H⁺ required for the formation of solute radical cations appears to correlate with the electron-withdrawing power of the functional group. The reactivity of e_aq^- toward dialkyl sulfides increases upon the introduction of strongly electron-withdrawing groups which effectively reduce the electron density at sulfur. The transient absorption band (λ _max = 310 nm) observed from the reaction of ^•OH radicals with methyl thioacetic acid (MTA) is assigned to the a-thioalkyl radical formed via an intermediate OH adduct. In highly acidic solutions ([HClO₄] = 3 mol dm-3), ^•OH radicals are able to react with MTA to form dimer radical cations (λ _max = 490 nm). The specific one-electron oxidants (Cl₂^•-, Br₂^•-, and SO₄^•-) undergo electron-transfer reaction with the solute; however, the transient absorption band of the dimer radical cation at 490 nm could not be observed, which may be due to unstable nature of the transient species in neutral and slightly acidic solutions. The oxidation potential is determined to be 1.56 V. The decay kinetics of the solute dimer radical cation is discussed in detail, and deprotonation of the solute radical cation is found to be the rate-determining step. The stability constant for the dimer radical cation has been determined to be 10 dm³ mol^-1 at 25 ° C. The transient species (λ _max = 390 nm, k = 3.3 × 10⁹ dm³ mol^-1 s^-1), formed from the reaction of Br^• atom with the solute, is assigned to a three-electron-bonded Br^• adduct.