Ozonolysis
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Ozonolysis is a chemical reaction in which ozone oxidizes an alkene forming carbonyl compounds.<ref>Claus, R. E.; Schreiber, S. L. Org. Syn., Coll. Vol. 7, p.168 (1990); Vol. 64, p.150 (1986). (Article)</ref><ref>Bailey, P. S.; Erickson, R. E. Org. Syn., Coll. Vol. 5, p.489 (1973); Vol. 41, p.41 (1961). (Article)</ref><ref>Tietze, L. F.; Bratz, M. Org. Syn., Coll. Vol. 9, p.314 (1998); Vol. 71, p.214 (1993). (Article)</ref> This reaction is an example of 1,3-dipolar cycloaddition.
Ozonolysis was invented by Christian Friedrich Schönbein in 1840 and in the early days of chemistry it was an important tool in organic structure elucidation. Ozonolysis of an unknown alkene yields smaller carbonyl fragments easier to identify and from which the structure of the original alkene could be derived.
First the alkene reacts with ozone<ref>Smith, L. I.; Greenwood, F. L.; Hudrlik, O. Org. Syn., Coll. Vol. 3, p.673 (1955); Vol. 26, p.63 (1946). (Article)</ref> to form an intermediate ozonide which is hydrolysed to form two carbonyl compounds. These will be aldehydes or ketones depending on the structure of the initial alkene. A reducing agent like zinc or dimethyl sulfide is usually present as well.
The ozonide can be reduced with lithium aluminium hydride to the corresponding alcohols. Ozonolytic reaction can also be conducted in an oxidizing environment, which generates carboxylic acids and ketones instead of aldehydes and ketones. Due to the inherent toxic properties of ozone, it is sometimes desirable to cleave a double bond into carbonyl containing compounds under different conditions. Such can be achieved using hot and concentrated potassium permanganate under basic conditions followed by an acid quench. Unfortunately, this method of cleavage won't produce aldehydes, only ketones and carboxylic acids.
In the chemical industry, azelaic acid and pelargonic acids are produced from ozonolysis of oleic acid.
[edit] Reaction mechanism
An ozonolysis mechanism was first proposed by Rudolf Criegee in 1953 <ref>Criegee, R. Angew. Chem. Int. Ed. Engl. 1975, 87, 745-752. (DOI:10.1002/anie.197507451)</ref>.
Note that the molozonide breakdown step above has incorrect or incompletely drawn mechanism arrows. Below represents the correct mechanism.
Ozone reacts with an alkene in a 1,3-dipolar cycloaddition to a molozonide (also called a primary ozonide) which is a 5 membered ring structure with 3 neighboring oxygen atoms (1,2,3-trioxolane). This intermediate is unstable and opens up in a cycloreversion <ref>The arrows on the molozonide are wrong, look at the charges involved and the movement of electrons. For the correct mechanism see Clayden, Greeves, Warren and Wothers page 938.</ref> to form a zwitterion and a carbonyl compound, often called the Criegee intermediate. In the third reaction step the carbonyl compound and the zwitterion recombine in another cycloaddition to form the secondary ozonide which is also a five membered ring but with 2 neighboring oxygen atoms (peroxide bond) and the third inserted in the original carbon carbon double bond as an ether linkage (1,2,4-trioxolane). Evidence for this mechanism is found in isotopic labeling. Benzaldehyde labeled with 17O (oxygen-17) reacts with the zwitterionic intermediate in an ozonolysis reaction and the isotope ends up exclusively in the ether linkage of the ozonide.<ref>The Mechanism of Ozonolysis Revisited by 17O-NMR Spectroscopy Geletneky, C.; Berger, S. Eur. J. Org. Chem. 1998, 1625-1627. (Abstract)</ref>
Note that if the carbon carbon double bond is at the end of the chain, that is R1-CH=CH2, carbon dioxide (CO2) will be formed.
There is still dispute over whether the molozonide collapses via a concerted or radical process; this may also exhibit a substrate dependence.
[edit] References
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