63524-79-8Relevant academic research and scientific papers
One-electron redox potential of thiobenzoic acid. Kinetic characteristics of benzoylthiyl radical β-fragmentation
Zhao, Rong,Lind, Johan,Merényi, Gábor,Eriksen, Trygve E.
, p. 2811 - 2816 (1998)
By means of pulse radiolysis, one-electron oxidation of benzoylthiolate (PhCOS- was achieved by the azide radical (N3*) in aqueous solution. The spectrum of the resulting benzoylthiyl radical (PhCOS*) shows a broad absorption in the wavelength region from 350 to 500 nm. With N3*/N3- as reference couple, the reduction potential E°(PhCOS*/PhCOS-) was measured to be 1.21 V vs NHE. Using the pK(a) of 2.48 for thiobenzoic acid (PhCOSH), we derive the standard reduction potential E°(PhCOS*, H+/PhCOSH) to be 1.36 V vs NHE. This reduction potential implies an S-H bond energy of PhCOSH of 87 kcal/mol, which is very close to the bond energy of an alkylthiol S-H bond (87.4 kcal/mol). At 22 °C, the PhCOS* radicals decay with a rate constant of 8.5 x 103 s-1 to form colorless species. This process is presumed to be the β-fragmentation of the PhCOS* radical, PhCOS* → Ph* + COS. The kinetics of the β-fragmentation of the PhCOS* radical was found to follow the Arrhenius equation, log(k2/s-1) = (12.3 ± 0.1) - (11.4 ± 0.2)/θ, where θ = 2.3RT kcal/mol. Strong evidence for this process was provided byproduct identification by GCMS, where the main products of the γ- irradiation-induced N3* radical oxidation of PhCOS- in 0.1 M NaN3 solution were found to be phenyl azide (PhN3) and aniline (PhNH2). These products were formed via addition of the Ph* radical to N3- to form the PhN3(-*) radical anion, which subsequently disproportionated. In the Discussion, we summarize the reduction potentials and the bond dissociation energies of related thiols and their oxygen couterparts. The β-fragmentation of PhCOS* radicals resembles that of the oxygen counterpart, benzoyloxyl radical (PhCOO*), and their thermochemical properties are also compared.
