Generalized ozonolysis mechanism

When appreciable concentrations of carbonyl compounds are present before complete formation of ozonide, crossed ozonides are formed. This occurs when the added carbonyl compound traps the zwitterion formed in the cleavage step. When c -stilbene is subjected to ozonolysis in the presence of 0-labeled benzal-dehyde, the label is incorporated into the ether rather than the peroxide portion of the ozonide  

This result is consistent with formation of the crossed ozonide via the cleavage-recombination mechanism  

Reactive solvent molecules can modify the course of ozonolysis reactions. We have already mentioned that when ozonolysis is carried out using certain carbonyl compounds as solvents, the reaction is diverted from its normal course. Under these 

When ozonolysis is performed in alcoholic solvents, the zwitterionic cleavage intermediates are trapped as a-hydroperoxy ethers/ Recombination is then prevented, and the carbonyl compound formed in the cleavage step can also be isolated under these conditions. 

Despite the complexities in the mechanism of ozonolysis, the reaction constitutes a high-yield method for cleavage of carbon-carbon double bonds. The oxidation states of the products that are actually isolated depend on the conditions used in processing the reaction mixture. If the carbonyl products are desired, it is advantageous to carry out ozonolysis in methanol, resulting in the formation of a-methoxyalkyl hydroperoxides. The reaction mixture is then treated with dimethyl sulfide, which reduces the hydroperoxide and permits isolation of the carbonyl compounds in good yield. This procedure prevents oxidation of the carbonyl products, especially aldehydes, by the peroxidic compounds present at the conclusion of ozonolysis. 

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We are still not able to discuss an over-all ozonolysis mechanism in any specific, detailed way. Instead we still must settle for broader generalizations and postulated intermediates which are given properties to rationalize the experimental results. Still as a result of continued experimentation there are clear differences appearing, such as the over-all differences between cis and trans olefins. Further work and new approaches may shed further light on this apparently complex process. 

Ozonolysis in the gas phase is not generally carried out in the laboratory. However, the reaction is important because it takes place in the atmosphere and contributes to air pollution. There is much evidence that the Criegee mechanism operates in the gas phase too, though the products are more complex beeause of other reactions that also take plaee. ... 

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 ... 

Targely based on our finding that the cis/trans ratios of cross ozonides 10) formed from unsymmetrical olefins depended on olefin geometry 11, 12), we have proposed a new mechanism of ozonolysis which takes account of this effect 14). The new mechanism, which considers only a limited type of olefin, namely trans-disubstituted and relatively unhindered cis olefins, differs significantly from the generally accepted Criegee mechanism 1,5). 

While the oxygen-18 labeling results described here confirm the molozonide-aldehyde mechanism for the types of olefins considered, the ozonolysis reaction in general is quite complex and seems to vary widely depending especially upon the stereochemistry of the olefin. To sum up, the molozonide-aldehyde mechanism 14) considered here appears to be applicable to any important degree only to trans-disubstituted olefins, relatively unhindered cis olefins, and perhaps to unhindered terminal olefins. As pointed out, more hindered olefins seem to react by one or more different pathways, which differ most notably from the present system in the apparent absence of a molozonide intermediate (2, 8, i2,14). 

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