New aspects concerning the mechanism of the ketone-catalysed decomposition of Caro’s acid
| Autor: | Hans-Dieter Brauer, Markus Hild, Andreas Lange |
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| Rok vydání: | 1999 |
| Předmět: | |
| Zdroj: | Journal of the Chemical Society, Perkin Transactions 2. :1343-1350 |
| ISSN: | 1364-5471 0300-9580 |
| Popis: | 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 ≥ pKa1d (where Ka1d denotes the first dissociation constant of the diol form). In addition, the apparent activation energy Eaa′ = (60 ± 6) kJ mol–1 obtained for the N,N-dimethyl-4-oxopiperidinium nitrate catalysed decomposition of Caro’s acid at pH = 9.0 pKa1d(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 (SO52–) 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. |
| Databáze: | OpenAIRE |
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