8.13 : Clivage oxydatif des alcènes : ozonolyse

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.