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

The pulse radiolysis technique has been employed to demonstrate the effects of the electron-withdrawing power of functional groups and the H+ concentr ation on the nature of (OH)-O-. radical reaction with substituted alkyl sul fides. The intermediate OH adduct and alpha -thioalkyl radical could be det ected in substituted sulfides having a functional group of high electron-wi thdrawing power. The concentration of H+ required for the formation of solu te radical cations appears to correlate with the electron-withdrawing power of the functional group. The reactivity of e(aq)(-) toward dialkyl sulfide s increases upon the introduction of strongly electron-withdrawing groups w hich effectively reduce the electron density at sulfur. The transient absor ption band (lambda (max) = 310 nm) observed from the reaction of (OH)-O-. r adicals with methyl thioacetic acid (MTA) is assigned to the alpha -thioalk yl radical formed via an intermediate OH adduct. In highly acidic solutions ([HClO4] greater than or equal to 3 mol dm(-3)), (OH)-O-. radicals are abl e to react with MTA to form dimer radical cations (lambda (max) = 490 nm). The specific one-electron oxidants (Cl-2(.-), Br-2(.-), and SO4.-) undergo electron-transfer reaction with the solute; however, the transient absorpti on 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 sligh tly acidic solutions. The oxidation potential is determined to be 1.56 V. T he 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-de termining step. The stability constant for the dimer radical cation has bee n determined to be 10 dm(3) mol(-1) at 25 degreesC. The transient species ( lambda (max) = 390 nm, k = 3.3 x 10(9) dm(3) mol(-1) s(-1)), formed from th e reaction of Br-. atom with the solute, is assigned to a three-electron-bo nded Br-. adduct.