Ozonolysis

In ozonolysis ozone cleaves an alkene into carbonyl compounds.

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The ozonolysis process can be expressed this way:

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RCH=CR'2 + O3 → RC(H)=O + O=CR'2

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First the alkene reacts with ozone 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 present to reduce hydrogen peroxide that is liberated as a side-product and that would otherwise give secondary oxidation reactions.

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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.

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Ozonolysis was invented by Christian Friedrich Sch?nbein 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. An ozonolysis mechanism was first proposed by Rudolf Criegee in 1953.

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Ozone reacts with an alkene in a 1,3-dipolar cycloaddition to a molozonide which is a 5 membered ring structure with 3 neighboring oxygen atoms (a 1,2,3-trioxolane). This intermediate is unstable and one oxygen to oxygen bond is broken by heterolysis to form a zwitterion and then the carbon to carbon bond splits with formation of a carbonyl compound and a second zwitterion. In the third reaction step the carbonyl compound and the zwitterion recombine in a second cycloaddition to form the 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 (a 1,2,4-trioxolane). Evidence for this mechanism is found in isotopic labeling 1. Benzaldehyde labeled with 17oxygen reacts with the zwitterionic intermediate in an ozonolysis reaction and the isotope ends up exclusively in the ether linkage of the ozonide.

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There is still dispute over whether the molozonide collapses via a concerted or radical process; this may also exhibit a substrate dependence.

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In industry azelaic acid and pelargonic acids are produced from ozonolysis of oleic acid.

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Alkene: An alkene in organic chemistry is an unsaturated hydrocarbon containing at least one carbon to carbon double bond. The simplest alkenes, with only one double bond, form a homologous series, the alkenes with general formula CnH2n....

Carbonyl: In organic chemistry, a carbonyl group is a functional group composed of a carbon atom double-bonded to an oxygen atom....

Ozonide: Ozonide is an unstable, reactive polyatomic anion O3-, derived from ozone, or an organic compound similar to organic peroxide formed by a reaction of ozone with an unsaturated compound....