Criegee intermediate

In a first step the reactivity of the carbonyl group is increased by protonation at the carbonyl oxygen. The peracid then adds to the cationic species 3 leading to the so-called Criegee intermediate 4 ... 

A Criegee intermediate has been detected for the ozonolysis of 2-butene see Fajgar, R. Vitek, J. Haas, Y Pola, J. Tetrahedron Lett., 1996, 37, 3391. 

Solvent effects on relative stability and electronic and molecular structure of carbonyl oxide (Criegee) intermediates in ozonolysis have been analysed by ab initio... 

The authors propose a working model relying on the commonly accepted mechanism for BV reactions (Fig. 13, a). Thus the sense of asymmetric induction is determined by the conformation of the Criegee intermediate, which is dictated by the chiral environment created by the catalyst. However, an alternative noncova-lent, bifunctional mechanism may be considered (Fig. 13, b) [80], This work... 

This primary ozonide is not stable. One of the two peroxy 0-0 bonds (marked a or b in Eq. 43) and the C-C bond (marked c) cleave simultaneously to give an aldehyde or ketone and an intermediate called the Criegee intermediate, named after the German chemist who originally proposed this mechanism (e.g., see Criegee, 1975) ... 

In the liquid phase, the Criegee intermediates have been assumed to be zwitterions and hence the term Criegee zwitterion is commonly used. In the gas phase, the structure is usually written as a biradical (although it may really be more zwitterionic in character e.g., see Cremer et al., 1993). Hence Criegee biradical is frequently used for this gaseous intermediate. Sander (1990), Bunnelle (1991), and Cremer et al. (1993) give a more detailed discussion of the structure and properties of the Criegee intermediate. 

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. 

TABLE 6.10 Yields of Stabilized Criegee Intermediates at Room Temperature and 1 atm Air ... 

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 ... 

FIGURE 6.5 Calculated energetics of two possible reaction pathways for the Criegee intermediate (CH- )2COO- (adapted from Gutbrod et at., 1996). 

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). 

Other reactions proposed for the Criegee intermediate include... 

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-... 

TABLE 6.12 Range of Reported Rate Constants for the Reactions of the Criegee Intermediate with Some Gases" and Associated Lifetimes of the Criegee Intermediate under Polluted Tropospheric Conditions... 

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). 

While OH is the major gas-phase oxidant for S02, Criegee biradicals may also contribute. This is so particularly at night when OH concentrations are small but significant concentrations of 03 and alkenes may exist, generating the Criegee intermediate (see Chapter 6.E.2). 

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... 

Since then, there have been a number of studies of this reaction, which have been summarized by Hatakeyama and Akimoto (1994). The mechanism appears to involve formation of an adduct that can either decompose to S02 and an isomerization product of the Criegee intermediate or, alternatively, react with a second S02 molecule to generate other products. For the Criegee intermediate formed in the ethene-ozone reaction, for example, the proposed reaction sequence is the following ... 

One can estimate the relative contribution of the Criegee intermediate (Cl) to SOz oxidation in the gas phase in the troposphere. The absolute value of the rate constant for the reaction of the Cl with SOz is not known, with estimates ranging from 1.7 X 10 " to 3 X 10 15 cm2 molecule-1 s-1 (Hatakeyama and Akimoto, 1994). Using the highest value and a concentration of the Cl of 1 X 105 molecules cm"3, one obtains 10"6 s"1 for the first-order rate of removal of S02 by this reaction. This can be compared to the rate of removal of SOz by reaction with 1 X 106 OH radicals cm3, which is also 10 6 s-1 using the effective bimolecular rate constant cited earlier. Using the lower estimates for the CI-S02 rate constant, which is more reasonable, would lower its contribution proportionately. 

In short, the Criegee intermediate from alkene-ozone reactions can contribute, in principle, to the gas-phase oxidation of S02. In practice, it is likely less important than reaction with OH. In addition, as we shall see, even the OH-SOz gas-phase reaction is, under many conditions, swamped out by reactions occurring in the liquid phase found in clouds and fogs. As a result, the CI-S02 reaction may contribute in some circumstances but is unlikely to be a major contributor to S02 oxidation as a whole. 

The S02 oxidation rates in the units of % h 1 expected from the gas-phase reactions of S02 with OH and the Criegee intermediate can be computed as shown in Box 8.1 for comparison to rates of oxidation observed in field studies (see Section C.l), if the rate constants and the concentrations of OH and Criegee intermediates are known. 

The H atoms formed in reaction 15a can react with 02 (reaction 11) to form H02. The stabilized Criegee intermediate (CH200) can participate in further reactions, some of which will result in the formation of peroxy radicals. Larger alkenes react with ozone to produce organic peroxy radicals. 

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