Criegee intermediate decomposition

Table 6.10 gives the ranges of observed yields of the stabilized Criegee intermediates at 1 atm pressure in air and at room temperature. Clearly, significant decomposition of the intermediates occurs under typical tropospheric conditions. 

The mechanism of decomposition of the Criegee intermediates is believed to occur via several reaction channels shown for the [(R,CH2)(R2)CHOO] Criegee intermediate in Fig. 6.4. The oxygen-atom elimination channel for simple alkenes is not believed to be important. However, the ester and hydroperoxide channels are important and explain the production of free radicals such as OH. Theoretical calculations have shed some light on this (e.g., Gutbrod et al., 1996, 1997a ... 

Horie, O., and G. K. Moortgat, Decomposition Pathways of the Excited Criegee Intermediates in the Ozonolysis of Simple Alkenes, Atmos. Environ., 25A, 1881-1896 (1991). 

POZ). The reaction enthalpy is retained as the internal energy of the products, resulting in formation of the vibrationally excited ozonide, which subsequently undergoes unimolecular decomposition to yield a chemically activated biradical, known as the carbonyl oxide or Criegee intermediate (Cl), and an aldehyde (e.g., MVK, MACR or formaldehyde). A total of nine carbonyl oxides (methyl vinyl carbonyl oxide, derived from 1,2-ozonide isopropyl carbonyloxide, derived from... 

As for the reactions of O3 with olefins, the reaction of O3 with ROCH=CH2 proceeds by initial addition of O3 to the carbon-carbon double bond, followed by decomposition of the ozonide to form formaldehyde and a formate as stable eompoxmds in addition to two Criegee intermediates ... 

Horie, O., and Moortgat, G. K., Decomposition pathways of the excited Criegee intermediates in the ozonolysis of simple alkenes. Atmos. Environ. 25A, 1881 (1992). 

According to currently accepted Criegee mechanism [20], the formation of the reaction products in the ozonolysis of ethene is explained in terms of the formation of the excited Criegee intermediate CH2OO and its subsequent decomposition and collisional stabilisation ... 

The reaction of ozone and alkenes is sufficiently fast that it can compete with other removal processes and provide sinks for both ozone and alkenes in the troposphere. While kinetic data for a series of alkene/ozone reactions have been reported, not much is known about details of the reaction mechanisms, the role that carbonyl O oxides play, and the role that free radicals play in these processes. Our laboratory experiments provide the spectroscopic data (both infrared and UV/visible) that are important for the spectroscopic identification of Criegee intermediates in the troposphere. In addition, we were able to characterize secondary partially oxidized products (aldehydes, peroxides etc.) that are produced during the gas-phase ozonolysis. These products might lead to a net increase of ozone, if oxygen atoms are formed during their decomposition. 

Decomposition pathways of the excited Criegee intermediates in the ozonolysis of simple alkanes,... 

The reaction of ozone with allyl alcohol is expected to involve, as for other unsaturated alcohols and olefins, electrophilic addition at the carbon-carbon double bond leading to the formation of a trioxide followed by its unimolecular decomposition into two carbonyls and two biradicals (Criegee intermediates) ... 

The reaction of O3 with MPAN is expected to proceed by O3 addition to the C=C bond followed by decomposition of the 1,2,3-trioxolane adduct into two carbonyls and two Criegee intermediates as for the reaction with unsaturated organics. Grosjean et al. (1993d) have reported CH2O as the major reaction product with a formation yield of 60 10%. 

Peroxyeslers may undergo non-radical decomposition via ibe Criegee rearrangement (Scheme 3.35). This process is analogous to the earboxy inversion process described for diacyl peroxides (see 3.3.2.1,3) and probably involves ionic intermediates. 

Decomposition of this intermediate, the Criegee split, forms an aldehyde, which is electronically excited formaldehyde in the case of a terminal alkene, and a diradical,... 

A mechanism has been proposed recently by O Neal and Blumstein for the gas-phase ozone-olefin reaction. This mechanism postulates that molozonide-biradical equilibrium is reached fast and postulates a competition between a-, 8-, and y-hydrogen abstraction reactions and the classical mechanism proposed by Criegee for the liquid-phase reaction. The main features of the Criegee mechanism (Figure 3-9) are the formation, from the initial molozonide, of the major carbonyl products and a second biradical intermediate, the zwitterion. The decomposition pathways of the zwitterion comprise unimolecular re-... 

Paul R. Story In the first place we cannot rule out some homolytic decomposition of the proposed intermediate. However, it is worth comparing our proposed seven-membered ring trioxide with the molozonide for which an ionic decomposition to zwitterion and carbonyl is readily envisioned our intermediate and the proposed ionic decomposition are quite analogous to Criegee s molozonide or primary ozonide and its decomposition. 

Current ideas about the mechanism of C=C double bond ozonolysis in solution are summarized in Schemes 1 and 2 [19, 21, 34]. As a result of the decomposition of the initial reaction product, PO, zwitterionic species is formed, termed as Criegee s intermediates or carbonyl oxides (hereafter referred to as Cl) (Scheme 1, reactions 2 and 2 ). Two intermediates are formed from as5munetric olefins monosubstituted Cl (MCI) and disub-stituted Cl (DCI), if their syn and anti stereoisomers are not taken into account. 

Macroscopically, ozonation leads to cracking of the rubber surface and ultimate product failure. A polar mechanism was proposed by Criegee for ozonation of unsaturated hydrocarbon rubbers, with addition of ozone to the carbon-carbon double bond followed by decomposition of the intermediate ozonide leading to cleavage of the polymer chain (80). Polymers without carbon bond imsaturation in the backbone do not crack in the presence of ozone. Zhang recently proposed a free-radical mechanism and attributed PPD antiozonant activity to its low ionization potential and ability to quench poljuner radicals (81). 

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