Ozonolysis of Alkenes
Alkenes can be oxidized with ozone to form alcohols, aldehydes or ketones, or carboxylic acids. In a typical procedure, ozone is bubbled through a solution of the alkene in methanol at −78 °C until the solution takes on a characteristic blue color. This indicates consumption of the alkene. A reagent is then added to convert the intermediate ozonide to a carbonyl derivative. Reductive work-up conditions are far more commonly used than oxidative conditions. The use of triphenylphosphine, thiourea, zinc dust, or dimethyl sulfide produces aldehydes or ketones while the use of sodium borohydride produces alcohols. The use of hydrogen peroxide produces carboxylic acids. Recently, the use of amine N-oxides has been reported to produce aldehydes directly. Other functional groups, such as benzyl ethers, can also be oxidized by ozone. It has been proposed that small amounts of acid may be generated during the reaction from oxidation of the solvent, so pyridine is sometimes used to buffer the reaction. Dichloromethane is often used as a 1:1 cosolvent to facilitate timely cleavage of the ozonide. Azelaic acid and pelargonic acids are produced from ozonolysis of oleic acid on an industrial scale.
A widely used alternative method for the ozonolysis of symmetrical alkenes allows differentially terminated hydrocarbons to be generated:
- with O3/MeOH/CH2Cl2; TsOH; sodium hydrogencarbonate (NaHCO3); dimethyl sulfide (DMS), aldehyde and dimethyl acetal termini
- with O3/MeOH/CH2Cl2; acetic anhydride (Ac2O), triethylamine (Et3N), methyl esters and aldehyde termini
- with O3/MeOH/CH2Cl2; TsOH; Ac2O, Et3N, methyl ester and dimethyl acetal termini.
An example is the ozonolysis of eugenol converting the terminal alkene to an aldehyde:
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