ONE-ELECTRON REDOX POTENTIAL OF THIOBENZOIC ACID - KINETIC CHARACTERISTICS OF BENZOYLTHIYL RADICAL BETA-FRAGMENTATION

By means of pulse radiolysis, one-electron oxidation of benzoylthiolat e (PhCOS-) was achieved by the azide radical (N-3(.)) in aqueous solut ion. The spectrum of the resulting benzoylthiyl radical (PhCOS.) shows a broad absorption in the wavelength region from 350 to 500 nm. With N-3 ./N-3(-) as reference couple, the reduction potential E degrees(Ph COS ./PhCOS-) was measured to be 1.21 V vs NHE. Using the pK(a) of 2.4 8 for thiobenzoic acid (PhCOSH), we derive the standard reduction pote ntial E degrees(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 degrees C, the PhCOS. radicals decay with a rate constant of 8.5 x 10(3) s(-1) to form colorless species. This process is presum ed to be the beta-fragmentation of the PhCOS. radical, PhCOS. --> Ph-. + COS. The kinetics of the beta-fragmentation of the PhCOS. radical w as found to follow the Arrhenius equation, log(k(2)/s(-1)) = (12.3 +/- 0.1) -(11.4 +/- 0.2)/theta, where theta = 2.3RT kcal/mol. Strong evid ence for this process was provided byproduct identification by GC-MS, where the main products of the gamma-irradiation-induced N-3(.) radica l oxidation of PhCOS- in 0.1 M NaN3 solution were found to be phenyl a zide (PhN3) and aniline (PhH2). These products were formed via additio n of the Ph-. radical to N--(3) to form the PhN3-. radical anion, whic h subsequently disproportionated. In the Discussion, we summarize the reduction potentials and the bond dissociation energies of related thi ols and their oxygen couterparts. The beta-fragmentation of PhCOS. rad icals resembles that of the oxygen counterpart, benzoyloxyl radical (P hCOO.), and their thermochemical properties are also compared.