Criegee intermediate water, reaction with

While it is often assumed that only the stabilized form of the Criegee intermediate undergoes such reactions, Moortgat and co-workers (Horie et al., 1994) have suggested that in the ethene reaction, it is the excited form that reacts with water vapor to form the acid, while the reaction of the stabilized Criegee intermediate leads to a hydroxyhydroperoxide, corresponding to reaction (48b). 

However, the relative importance of these reactions is highly uncertain. Table 6.12 gives one estimate of the range of possible rate constants for these reactions of the Criegee intermediate and the calculated lifetimes of the intermediate under some typical atmospheric conditions. Reaction with water vapor is expected to be the major fate of the Criegee intermediate, with per-... 

The first indication of a reaction between the Criegee intermediate and S02 came from studies by Cox and Penkett (1971, 1972), who showed that although the oxidation of S02 by 03 alone was negligible, it was relatively fast in the presence of both ozone and alkenes. In addition, water vapor inhibited the S02 oxidation in this system. These observations can be understood in terms of the competition between the reactions of the Criegee intermediate with S02 and... 

Baeyer-ViUiger oxidation involves NADPH and flavin (FAD) as cofactors and was originally proposed by Walsh et al. based on data obtained from cyclohexanone monooxygenase (CHMO) from Acinetobacter calcoaceticus (Fig. 24) [156]. In a first step, enzyme-bound flavin is reduced, followed by the addition of oxygen yielding a hydroperoxide anion. Reaction with the ketone substrate gives a Criegee intermediate, which is then converted into the product under dissociation of water. The cofactor FAD is recovered via oxidation with NADP+. 

Special emphasis has been laid on the formation of H2O2, hydroperoxides and hydroxy-hydroperoxides. Systematic studies of their yields for a number of alkenes have been made with and without the addition of water vapour (Becker, Gab, Moortgat). In the case of ethene, the formation of 2-hydroxyethyl hydroperoxide clearly can be related to the reaction OH + ethene. It was also possible to compare the product yields of hydroperoxides from the alkene + O3 reactions in the gas phase with that in the aqueous phase (Gab), (Table 6b). The product distribution is different and, in general, the yields of a-hydroxy-hydroperoxides in the aqueous phase are 10 times higher than in the gas phase. In the presence of formaldehyde, the simplest Criegee intermediate, CH2OO, forms hydroxylmethyl formate (Becker, Moortgat), which eventually decomposes into formic acid. 

While there are still some unresolved differences between the various studies, it appears that the reaction initially forms (CH3)2C(0H)CH0 + [CH2OO] and CH2O + [(CH3)2C(0H)CH00] in approximately equal amounts, with the [(CH3)2C(0H)CH00] Criegee intermediate reacting to form at least part of the OH radicals observed (Alvarado et al 1999) and also reacting with water vapor (when present) to form additional (CH3)2C(OH)CHO. 

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