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^–, PO₃^2–, 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, IrCl₆^2–, are also examined. Studies in dimethyl sulphoxide–water mixtures indicate that the anion exists as a hydrated form (λ_max 356 nm) in water and an ‘unspecifically’ solvated species in dimethyl sulphoxide (λ_maxca. 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.