SPECTRA, IONIZATION-CONSTANTS, AND RATES OF OXIDATION OF 1,4-DIMERCAPTOBENZENE AND PROPERTIES OF THE P-MERCAPTOPHENYLTHIYL AND P-BENZODITHIYL ANION-RADICALS

In basic solution, p-dimercaptobenzene, which is found to have pK(a)'s of 6.0 and 7.7, is rapidly oxidized by azide radical (k = 7.4 x 10(9) M-1 s-1) top-benzodithiyl radical anion, the sulfur analog of p-benzosemiquinone radical anion. This radical has very strong absorption bands at 390 and approximately 700 nm. In acidic solutions, CO2.- abstracts a SH hydrogen atom (k = 3.1 x 10(9) M-1 s-1) to form the p-mercaptophenylthiyl radical, which has absorption maxima at 350 and 585 nm and a pK(a) of 2.7. From measurements of its electron-transfer equilibrium with the resorcinol dianion, the reduction potential of the dithiyl radical has been determined to be 0.33 V vs SHE. This value is 0.37 V below the reduction potential of phenylthiyl radical, indicating that resonance stabilization of approximately 36 kJ/mol results from delocalization of the unpaired spin between the two sulfur atoms. This stabilization energy is, however, approximately 39 kJ/mol less than that for the analogous difference of 0.78 V between the reduction potentials of p-benzosemiquinone anion and phenoxy radicals. At high pH, the dithiyl radical anion decays rapidly in second-order processes (2k = 2.8 x 10(9) M-1 s-1), in contrast to the relatively stable p-benzosemiquinone radical anion, which, because its reduction potential is below that of quinone, is stable toward disproportionation by electron transfer.