Kinetic and spectral properties of the photogenerated p-nitrobenzyl carbanion in aqueous media

Abstract

The p-nitrobenzyl carbanion is a photoelimination product from various p-nitrobenzyl (p-NO2C6H4CH2R, R = CO2–, PO32–, CH2OH, etc.) compounds in aqueous media. Following millisecond flash photolysis of N2-saturated p-nitrophenylacetate (pH > 6) solutions, we have shown that the carbanion decays via mixed first-(solvent protonation) and second-(dimerisation) order processes. Rate constants k1= 1.3 ± 0.1 × 10–2 s–1 and 2k2= 1.7 ± 0.3 × 103 dm3 mol–1 s–1, respectively, are determined. Reactions of the anion with O2, H+, and an efficient one-electron oxidant, IrCl62–, are also examined. Studies in dimethyl sulphoxide–water mixtures indicate that the anion exists as a hydrated form (λmax 356 nm) in water and an ‘unspecifically’ solvated species in dimethyl sulphoxide (λmaxca. 440 nm). A reaction mechanism is developed which accounts for transformation of the carbanion to p,p′-dinitrobibenzyl and p-nitrotoluene, the major isolable nitro products from these systems.