Criegee zwitterion mechanism

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. 

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

T he Criegee (1) mechanism of ozonolysis postulates that unsymmetrical olefins should give two zwitterions and two carbonyl compounds and hence postulates the possible formation of three different ozonides. This prediction has now been realized in a number of cases (2-9). It has also been shown that in many cases the ozonide stereoisomer ratio depends upon olefin stereochemistry in both normal (3, 6-12) and cross (6-9) ozonides. Since the original Criegee mechanism did not provide for these stereochemical results, a number of additional suggestions for the mechanism have been made (6,9,13, 14), all of which retain the fundamentals of the Criegee mechanism. 

In accordance with Criegee s mechanism as applied to abnormal ozonizations (4), an unsaturated substance which contains electron-donating groups attached to a carbon atom adjacent to a double bond will produce zwitterions which will rearrange in such a manner as to produce abnormal products. This can be illustrated by the following sequence of reactions, in which R contains an electron-donating atom attached to a carbon adjacent to the double bond ... 

Criegee (2) mechanism of ozonolysis allows for the possibility of two zwitterions and two carbonyl moieties upon ozonolysis of an unsymmetrical olefin. This possibility leads to the prediction that such ozonolyses could lead to the production of three ozonide cis-trans pairs. 

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. 

Ozonation ofAlkenes. The most common ozone reaction involves the cleavage of olefinic carbon—carbon double bonds. Electrophilic attack by ozone on carbon—carbon double bonds is concerted and stereospecific (54). The modified three-step Criegee mechanism involves a 1,3-dipolar cycloaddition of ozone to an olefinic double bond via a transitory TT-complex (3) to form an initial unstable ozonide, a 1,2,3-trioxolane or molozonide (4), where R is hydrogen or alkyl. The molozonide rearranges via a 1,3-cycloreversion to a carbonyl fragment (5) and a peroxidic dipolar ion or zwitterion (6). 

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-... 

Cyclo-addition (Criegee mechanism) — As a result of its dipolar structure, an ozone molecule may lead to three dipolar cyclo-additions on unsaturated bonds, with the formation of primary ozonide (I) corresponding to the reaction shown in Figure 4.8. In a protonic solvent such as water, this primary ozonide decomposes into a carbonyl compound (aldehyde or ketone) and a zwitterion (II) that quickly leads to a hydroxy-hyperoxide (III) stage that, in turn, decomposes into a carbonyl compound and hydrogen peroxide (see Figure 4.9). 

This mechanism proceed via a peroxidic Zwitterion what is now largely accepted by all the scientific community. Product 1, an ozone-olefin adduct is a very unstable compound giving rapidly product 3, probably through intermediate 2. The Criegee intermediate 3 can lead to different structures like ... 

The primary step of the propylene ozonolysis was taken to be the generally accepted Criegee mechanism 46, 47), which leads to the formation of zwitterions and aldehydes ... 

According to the Criegee mechanism, formyl and acetyl zwitterions, HC HOO and CHaC HOO, are reactive intermediates in the ozonolysis of propylene. Among several thermochemically feasible reactions of the zwitterions, the following scheme provides a chain oxidation mechanism which is consistent with the observed data. 

The Criegee mechanism 13-15, 17) for ozonolysis has been of tremendous importance in giving a better understanding of the course of the ozonolysis reaction—in particular, the nature and fate of the active oxygen-containing ozonolysis products. It leaves in doubt, however, the nature of the initial attack of ozone on an unsaturated system and of the intermediates leading to the formation of the primary cleavage products, the zwitterion (IV) and the aldehyde or ketone (V). 

Ethene, like other alkenes, reacts also with ozone in the atmosphere. The older work on ozone reactions has been reviewed by Leighton (1961) and by Bufalini and Altshuller (1965). More recent work has done much to clarify the principal reaction mechanisms involved. Criegee (1957, 1962, 1975), who had studied the ozonolysis of alkenes in solution, suggested that ozone adds to the C=C double bond, forming an unstable intermediate, which then decomposes toward a carbonyl compound and a zwitterion fragment, for example ... 

Essentially, we have proposed that ozonide (5) is formed not only by the Criegee mechanism but also by reaction of the molozonide with aldehyde. Thus, a competition exists between molozonide fragmentation and molozonide reaction with aldehyde. In addition, ozonide may also be formed by the reaction of molozonide with zwitterion followed by regeneration of a new zwitterion (Reaction 2). As yet we have no evi-... 

The detailed mechanism of the reaction of ozone with olefins proposed by Criegee et involves initial attack upon the olefin to form a primary ozonide which suffers oxygen-oxygen and then carbon-carbon bond fission to form a reactive zwitterionic intermediate (III) which provides the normal ozonide , together with the products of rearrangement, polymerisation, or... 

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. 

Unsymmetrical olefins, according to the Criegee mechanism, should be able to yield two different zwitterions and two different carbonyl compounds. Inspection of the structures shows that these are theoretically capable of combining to form three different type (IV) ozonides. Several instances have now been discovered where under the appropriate reaction conditions all three ozonides may be produced. 

The mechanism proposed by Criegee best describes the degradation initiated by ozone called ozonolysis. Ozone, a very reactive material, reacts at the surface, across the double bond, in an unsaturated polymer to form a trioxolane stmcture. This stmcture undergoes decomposition to give a carbonyl compound and a zwitterion, resulting in a severed molecular chain. The zwitterion can recombine to form either an ozonide, diperoxide, or higher peroxide. 

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