The electrochemistry of nitrobenzene and p-nitrobenzaldehyde studied by transmission spectroelectrochemical methods in sulfolane

Transmission spectroelectrochemistry experiments have been used to study the electrochemical reductions of nitrobenzene and p-nitrobenzaldehyde in sulfolane. The reduction intermediates of both these aromatic compounds are sufficiently long-lived in sulfolane to be amenable to study by these techniques. Nitrobenzene undergoes a single one-electron, chemically reversible reduction at E_p,c = -1.875 V vs. AgRE. This was verified by analysis of the visible absorption spectrum for the reduction species; two maxima were observed at 348 nm (ε 5.7 × 10³ M^-1 cm^-1) and 465 nm (ε 4.1 × 10³ M^-1 cm^-1). p-Nitrobenzaldehyde undergoes two one-electron reductions (E_p,1 = -1.528 V, E_p,2 = -2.114 V vs. AgRE), which are accompanied by following chemical reactions. A mechanism is shown involving formation of a parent-radical anion complex and an unusually low reproportionation reaction rate (k_f = 5.4 ± 0.7 × 10² M^-1 s^-1 at 50°C). Verification of the proposed mechanism is assisted by digital simulation techniques and a newly developed differential treatment of the spectroelectrochemical data.