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Ozonolysis of Alkenes

Ozonolysis of Alkenes Followed by Reduction: Mechanism

Ozonolysis proceeds in two steps. Initially, ozone is added in a 1,3-dipolar cycloaddition to the double bond. The resulting primary ozonide is labile and decomposes immediately in a 1,3-dipolar cycloreversion to give aldehyde or ketone, depending on the alkene used, and a zwitterionic intermediate (carbonyl oxide). It has been observed that the less alkylated end of the olefin forms the carbonyl compounds. The more alkylated end becomes part of the intermediate, because the higher +I-effect of the substituents better compensates the positive charge of the oxygen.

Fig.1

2D animation: Ozonolysis

3D animation: Ozonolysis

In the second step of the reaction, the two fragments recombine again in a 1,3-dipolar addition to form the secondary ozonide.

Fig.2

In order to obtain the carbonyl compounds, the ozonide has to be cleaved. A reductive cleavage is employed in most cases to prevent oxidation of the products, particularly aldehydes, by hydrogen peroxide formed during hydrolysis. However, oxidative cleavages have been used.

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