Ozonolysis


Ozonolysis is a process which uses ozone to cleave unsaturated organic bonds. Ozone’s property of not to react at saturated carbon centers but to attack in 1,3 - dipolar addition on unsaturated systems in a 2+3 concerted manner gives ozone its usefulness in organic chemistry.


The Ozonolysis mechanism was first proposed by Rudolf Criegee in 1953.
The Ozonolysis Mechanism is shown below.

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 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.[1] 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.The Practical Side of Ozonolysis Azelaic acid and pelargonic acids are produced from ozonolysis of oleic acid.


How to achieve the best results with Ozonolysis

Efficient ozonolysis reactions are best attained with high ozone concentrations. Ozone concentrations above 10% give excellent results; however, the higher the ozone concentration, the better the results. Quick conversion is important since usually treated samples are very small (between 100-250 ml). Smaller samples will dry out faster than larger samples since the gas going into the sample (ozone & oxygen) is extremely dry.