Electrogenerated chemiluminescence in mechanistic investigations of electroorganic reactions: Part III. Reduction of some disulfides at the dropping mercury electrode

Abstract

In the simultaneous cathodic reduction of diphenyl disulfide (Ph−S−S−Ph) or dibenzoyl disulfide (Ph−CO−S−S−CO−Ph) and a series of fluorescent aromatic hydrocarbons A at the dropping mercury electrode in dimethylformamide the emission of A is observed. This electrogenerated chemiluminescence (ECL) originates from the electron transfer between A − and the radicals Ph−S or Ph−COS, which are formed in a one-electron reductive cleavage of the disulfide bond by A −. As an intermediate the anion radical Ph−S−S−Ph − or Ph−CO−S−S−CO−Ph − is assumed. In the case of Ph−S−S−Ph the ECL intensity is enhanced by proton donors (water or benzoic acid), which increase the cleavage rate of Ph− S−S−Ph − in an electrophilic attack by the proton. The relatively negative threshold reduction potential of A (−1.4 to −1.6 V) for the ECL in comparison with the half-wave potential (−0.85 V) supports a mercury-assisted heterogeneous reduction mechanism of Ph−S−S−Ph. The intensity-potential curves and the intensity—time curves at the mercury drop were measured for different concentrations of Ph−S−S−Ph and A and for different mercury pressures. No luminescence could be detected with o,o′-dinitrodiphenyl disulfide and diethyl disulfide.