New aspects concerning the mechanism of the ketone-catalysed decompositionof Caro's acid

The kinetics of the ketone-catalysed decomposition of peroxymonosulfuric ac id (Caro's acid) have been investigated by measuring the phosphorescence of the released singlet molecular oxygen (O-1(2)) at 1270 nm. As catalysts fl uoroacetone, 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 resul ts are explained on the assumption that for ketones with strong electron-wi thdrawing groups the keto-gem-diol equilibrium and additionally the first d issociation equilibrium of the diol form must be taken into account, if pH greater than or equal to pK(a1)(d) (where K-a1(d) denotes the first dissoci ation constant of the diol form). In addition, the apparent activation ener gy E-aa' = (60 +/- 6) kJ mol(-1) obtained for the N,N-dimethyl-4-oxopiperid inium nitrate catalysed decomposition of Care's acid at pH = 9.0 much less than pK(a1)(d)(N,N-dimethyl-4-oxopiperidinium-diol) is in agreement with th e assumption that a preliminary keto-gem-diol equilibrium is involved in th e dioxirane formation. Furthermore, the second-order rate constant k(6) for the reaction between dimethyldioxirane and the dianion of Care's acid (SO5 2-) has been determined by measuring the bleaching of the anionic dye Brill iant Blue G250 by in situ generated and by isolated dimethyldioxirane, resp ectively. This rate constant was found to be k(6) = (1.6 +/- 0.3) x 10(3) d m(3) mol(-1) s(-1). Moreover, the bleaching of Brilliant Blue G250 by other ketone-Care's acid systems was studied using cyclohexanone, fluoroacetone and N,N-dimethyl-4-oxopiperidinium nitrate as catalysts. These bleaching ex periments yielded the ratio of rate constants k(13)/k(6), where k(13) is th e second-order rate constant far the reaction between Brilliant Blue G250 a nd a chosen dioxirane. The values of k(13)/k(6) vary in a small range betwe en 0.23 (fluoroacetone) and 0.42 (cyclohexanone). k(13) obtained for the bl eaching of Brilliant Blue G250 by dimethyldioxirane amounts to (5 +/- I) x 10(2) dm(3) mol(-1) s(-1).