One-Electron Redox Potential of Thiobenzoic Acid. Kinetic Characteristics of Benzoylthiyl Radical β-Fragmentation

By means of pulse radiolysis, one-electron oxidation of benzoylthiolate (PhCOS - ) was achieved by the azide radical (N 3 • ) 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 N 3 • /N 3 - as reference couple, the reduction potential E °(PhCOS • /PhCOS - ) was measured to be 1.21 V vs NHE. Using the p K 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 × 10 3 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( k 2 /s - 1 ) = (12.3 ± 0.1) − (11.4 ± 0.2)/θ, where θ = 2.3 RT kcal/mol. Strong evidence for this process was provided byproduct identification by GC-MS, where the main products of the γ-irradiation-induced N 3 • radical oxidation of PhCOS - in 0.1 M NaN 3 solution were found to be phenyl azide (PhN 3 ) and aniline (PhNH 2 ). These products were formed via addition of the Ph • radical to N 3 - to form the PhN 3 - • 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.