Reassessment of positional substituent effects: Influence in the chemistry of electrogenerated anion radicals from nitrobenzenesulfonamides during self-protonation pathways

The electrochemical reduction processes of 2-nitrobenzenesulfonamide and 4-nitrobenzenesulfonamide were studied by cyclic voltammetry and ESR-spectroelectrochemical experiments in acetonitrile. From the voltammetric responses, two reductions signals (Ic and IIc) were detected; the first one was associated to the formation of an anion radical which is destabilized by a self-protonation process while the second electrochemical signal corresponds to the formation of a stable dianion radical, confirming the proposed pathway. The self-protonation rate constant (k1+) was determined by fitting experimental ξpvs log Λ responses; this second order rate constant was similar for both ortho and para compounds: k1+=3.9 M−1 s−1. Small deviations in the determination of k1+ occurred at potential scan rates higher than 10 V s−1; this effect was associated to the passage from an ECE into a DISP2 mechanism. The rate of these disproportionation steps are different for ortho and para substituted nitrobenzenesulfonamides: kdisportho=700 M−1 s−1 and kdisppara=7000 M−1 s−1, indicating that the differences in the overall reduction of the nitro group is related the disproportionation rate which in turn is dependent to the specific structure of the nitrobenzensulfonamide studied. ESR spectra suggested that there is a difference in the properties of the electrogenerated dianion radicals; the spectra of the ortho substituted compound indicates a probable solvent deprotonation reaction, which did not occur for the para-nitrobenzenesulfonamide dianion radical, as confirmed by employing deuterated acetonitrile.