Investigation of the direct and indirect reduction processes of some disulfides by electrochemical means

The free energy relationship for the reaction between aromatic radical anions A(.-) having about the same intrinsic barrier and three disulfides RSSR (diphenyl disulfide, dimethyl disulfide and di-tert-butyl disulfide) in N,N-dimethylformamide has been obtained. For each RSSR second-order rate constants were measured electrochemically in the interval 10(-3)-10(-5) M(-1) s(-1) by changing the nature of A(.-) and thereby the driving force. In the case of diphenyl disulfide the standard potential E(RSSR/RSSR.-)degrees and the self-exchange reorganization energy lambda(0) of the RSSR/RSSR(.-) couple could be extracted from the free energy relationship to values of -1.4 (+/-0.1) V vs. SCE and 65 kcal mol(-1), respectively. This knowledge may be combined with the kinetic features of the heterogeneous reduction process of diphenyl disulfide to obtain, among other parameters, the standard heterogeneous rate constant. The cleavage rate constant k(c) of the corresponding radical anion was estimated from kinetic measurements to be about 5 x 10(8) s(-1). No such information was accessible for the aliphatic disulfides due to a high self-exchange energy of the RSSR/RSSR(.-) couple but still a maximum value of k(c) could be determined at 2 x 10(8) s(-1). The minimum values of E(RSSR/RSSR.-)degrees and lambda(0) were estimated to be -1.9 V vs. SCE and 75 kcal mol(-1) and -2.2 V vs. SCE and 75 kcal mol(-1) for dimethyl disulfide and di-tert-butyl disulfide, respectively.