8.13:
Oxidative Cleavage of Alkenes: Ozonolysis
Ozonolysis of alkenes is an oxidative cleavage reaction where the carbon–carbon double bond cleaves completely to form two carbon–oxygen bonds.
The reaction proceeds with the addition of ozone to alkenes followed by reduction in the presence of mild reducing agents, such as dimethyl sulfide or zinc dust in water, to form corresponding aldehydes and ketones.
Ozone is a colorless gas generated by passing oxygen through an electric discharge. It is a symmetrical bent molecule that exists as a resonance hybrid of two contributing Lewis structures.
The first step of the mechanism involves an electrophilic addition of ozone across the alkene double bond, forming a molozonide, which splits into a carbonyl oxide and a carbonyl compound. Next, the intermediates rearrange to form a relatively stable ozonide.
In the second step, a mild reducing agent such as dimethyl sulfide converts the ozonide into carbonyl compounds.
Ozonolysis of alkenes yields different products based on the structure of the starting alkene and the reaction conditions.
For example, ozonolysis of a monosubstituted alkene such as 1-butene followed by a reductive workup with dimethyl sulfide yields a mixture of aldehydes. A trisubstituted alkene such as 2-methyl-2-butene forms an aldehyde and a ketone.
However, an oxidative workup using hydrogen peroxide further oxidizes the aldehydes to carboxylic acids while the ketones remain intact.
8.13:
Oxidative Cleavage of Alkenes: Ozonolysis
In ozonolysis, ozone is used to cleave a carbon–carbon double bond to form aldehydes and ketones, or carboxylic acids, depending on the work-up.
Ozone is a symmetrical bent molecule stabilized by a resonance structure.
Ozonolysis proceeds through an oxidative cleavage reaction. The first step is the electrophilic addition of ozone across the alkene double bond, forming an unstable molozonide intermediate, which reacts further to form a carbonyl and a carbonyl oxide. These intermediates rearrange to form an ozonide.
The ozonide is treated with a mild reducing agent such as dimethyl sulfide or zinc to yield the carbonyl compounds as the final product.
Ozonolysis with Different Substituted Alkenes
The conversion of ozonide to aldehydes, ketones, or carboxylic acids depends on the structure of the alkene starting material and different reaction conditions.
When a reductive work-up is used, ozonolysis of monosubstituted alkenes such as 1-butene yields a mixture of aldehydes.
Trisubstituted alkenes, such as 2-methyl-2-butene, on the other hand, form an aldehyde and a ketone.
When an oxidative work-up is used, the reaction yields a ketone and an aldehyde that is further oxidized to the corresponding carboxylic acid.
