A criegee biradical, also known as a carbonyl oxide, is an organic compound with the formula R-C(=O)-O-R. They are reactive intermediates in the ozonolysis of alkenes. Criegee biradicals are named after the German chemist Siegfried Criegee, who first proposed their existence in 1953.
Criegee biradicals are resonance stabilized species with a short lifetime at ambient temperature and pressure. They decompose rapidly to formaldehydes or ketones by loss of CO2. The formation of criegee biradicals can be observed in the ozonolysis of alkenes:
O3 + R-CH=CH2 → R-CHO + HO2In this reaction, an alkene is converted into a carbonyl compound by way of a criegee biradical intermediate. The carbonyl group can be further oxidized to yield a carboxylic acid:
O3 + 2R-CHO → R-CO2H + 2HO2This sequence of reactions is called the “ozonolytic oxidation” of an alkene. It should be noted that not all alkenes undergo this type of oxidation; only those with at least one tertiary carbon atom adjacent to the double bond are susceptible. Additionally, other functional groups present on the molecule can influence its reactivity towards ozone.
The mechanism for the formation of criegee biradicals begins with the addition of ozone across an alkene π bond:
O3 + R-CH=CH2 → Ozonide IntermediateSubsequent homolytic cleavage of one O–O bond produces two radicals:
Ozonide Intermediate → R•+ •OOORadiative stabilization then takes place via delocalization of electrons within the π* molecular orbital:
•OOOR•→ Resonance Stabilized Criegee BiradicalThe final step in the mechanism involves thermal decomposition to formaldehyde or ketone products and release CO2 gas:
Resonance Stabilized Criegee Biradical → Products + CO2