Neutral hydrolysis and imidazole-catalysed decomposition of bis(4-nitrophenyl) oxalate. 1,1′-Oxalyldiimidazole as an intermediate

Abstract

Neutral hydrolysis and imidazole-catalysed decomposition of a peroxyoxalate chemiluminescence reagent type compound, bis(4-nitrophenyl) oxalate (4-NPO), have been studied in acetonitrile and in acetonitrilewater mixtures. For comparison, the rate coefficients for the neutral hydrolysis of 4-nitrophenyl dichloroacetate have also been measured. The first step of the neutral hydrolysis of 4-NPO apparently proceeds by the B_AC3 mechanism as evidenced by the solvent isotope effect and by the effect of the solvent composition on the rate coefficient. The second step of the reaction is significantly slower than the first one, presumably owing to the retarding inductive effect of the dissociated carboxylate group. The imidazolecatalysed degradation of 4-NPO proceeds by the successive release of the two 4-nitrophenol groups and includes the formation and decomposition of 1,1′-oxalyldiimidazole. Although the hydrolytic reactivity of 4-NPO and 4-nitrophenyl dichloroacetate are close to each other, the reactivity of imidazole toward 4-NPO is considerably higher than toward 4-nitrophenyl dichloroacetate. The difference in reactivity is possibly due to the neighbouring carbonyl group stabilization of the transition state for the partitioning of an addition intermediate in the direction of the substituted phenoxide ion expulsion in the reaction of 4-NPO.