Zwitterion, Criegee

FIGURE 8-2 Mechanism of ozonolysis. A, Criegee zwitterion B, aldehyde C, ozonide. 

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. 

The chemistry of the ozonization of olefins has been reviewed (I, 2, 5). Although the exact details of the mechanism(s) have not been elucidated, the Criegee zwitterion mechanism plays an important role in olefin ozonization. According to this mechanism, ozonization occurs in the manner shown in Figure 2. 

According to the Criegee mechanism, ozonide is formed by combination of a zwitterion (3) and an aldehyde (4). Our mechanism does not discard the concept of the Criegee zwitterion. 

Using modern analytical methods, a number of transient intermediates and byproducts could be verified [19, 20]. The first step in the mechanism of ozonolysis is the 1,3-dipolar cycloaddition of the dipole ozone to the double bond of OA. A 1,2,3-trioxolane is formed, the unstable primary ozonide or molozonide. The primary ozonide collapses in a 1,3 dipolar cycloreversion to a carbonyl compound and a carbonyl oxide, the so-called Criegee zwitterion. Since OA is substituted with two diverse groups at the double bond, two different opportunities exist for the formation of carbonyl compound and carbonyl oxide. Again, a 1,3-dipolar cycloaddition of these intermediates leads to three different pairs of 1,2,4-trioxolane derivatives (cisltram), the secondary ozonides, which are more stable than the primary ones. Their oxidative cleavage results in AA and PA. 

Ozonolysis is now more extensively employed for both analytical and preparative purposes. Reaction occurs via the molozonide which breaks down to aldehyde and the Criegee zwitterion. The latter then reacts with aldehyde to give ozonide or with solvent to give alkoxyhydroperoxide (alcohol) or acyloxyhydroperoxide (carboxylic acid). All of these subsequently break down to the same ozonolysis products. 

Crambe abyssinica, 52, 65, 489 Crambe hispanicay 65 Crambe oil, 65 Crappie, fatty acids, 134 Crjanchiidae, wax esters, 147 Cream preparation, 224 Crepenynic acid, 13, 290 Crepis oils, 4,13,52,53 Criegee zwitterion, 463 Crude oils, 184,189,190,192,194 Crystallization, 171,179,214,368,470 Crystallization inhibitors, 214 Crystal structure, 321,325,327,343, 344-49, 370... 

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