New aspects concerning the mechanism of the ketone-catalysed decomposition of Caro’s acid

Abstract

The kinetics of the ketone-catalysed decomposition of peroxymonosulfuric acid (Caro’s acid) have been investigated by measuring the phosphorescence of the released singlet molecular oxygen (¹O₂) at 1270 nm. As catalysts fluoroacetone, 1,1,1-trifluoroacetone, hexafluoroacetone and N,N-dimethyl-4-oxopiperidinium nitrate were used. With the exception of fluoroacetone, for the other ketones their third-order rate constant for the formation of the corresponding dioxirane is determined by the pH of the solution. The results are explained on the assumption that for ketones with strong electron-withdrawing groups the keto–gem-diol equilibrium and additionally the first dissociation equilibrium of the diol form must be taken into account, if pH ≥ pK_a1^d (where K_a1^d denotes the first dissociation constant of the diol form). In addition, the apparent activation energy E_aa′ = (60 ± 6) kJ mol^–1 obtained for the N,N-dimethyl-4-oxopiperidinium nitrate catalysed decomposition of Caro’s acid at pH = 9.0  pK_a1^d(N,N-dimethyl-4-oxopiperidinium-diol) is in agreement with the assumption that a preliminary keto–gem-diol equilibrium is involved in the dioxirane formation. Furthermore, the second-order rate constant k₆ for the reaction between dimethyldioxirane and the dianion of Caro’s acid (SO₅^2–) has been determined by measuring the bleaching of the anionic dye Brilliant Blue G250 by in situ generated and by isolated dimethyldioxirane, respectively. This rate constant was found to be k₆ = (1.6 ± 0.3) × 10³ dm³ mol^–1 s^–1. Moreover, the bleaching of Brilliant Blue G250 by other ketone–Caro’s acid systems was studied using cyclohexanone, fluoroacetone and N,N-dimethyl-4-oxopiperidinium nitrate as catalysts. These bleaching experiments yielded the ratio of rate constants k₁₃/k₆, where k₁₃ is the second-order rate constant for the reaction between Brilliant Blue G250 and a chosen dioxirane. The values of k₁₃/k₆ vary in a small range between 0.23 (fluoroacetone) and 0.42 (cyclohexanone). k₁₃ obtained for the bleaching of Brilliant Blue G250 by dimethyldioxirane amounts to (5 ± 1) × 10² dm³ mol^–1 s^–1.