9.10 : Alcynes en acides carboxyliques: clivage oxydatif

The triple bond of an alkyne can be cleaved completely by using oxidizing reagents, like potassium permanganate or ozone, to yield carboxylic acids.

Alkynes yield carboxylate salts in the presence of warm and basic aqueous potassium permanganate. The salts form via the oxidative cleavage of the alkyne through an unstable α-diketone intermediate.

Further, mild acid treatment protonates the anions to generate free carboxylic acids.

When an alkyne is subjected to ozonolysis, it generates the intermediate ozonide, which is then cleaved by hydrolysis to yield carboxylic acids.

Irrespective of the choice of the reagent, an internal alkyne is oxidatively cleaved to yield only carboxylic acids. In comparison, a terminal alkyne generates carbon dioxide along with the acid.

A terminal alkyne is oxidatively cleaved to give a carboxylate salt and a formate anion. The formate then oxidizes to a carbonate, which is then protonated to an acid, followed by the release of carbon dioxide.

Oxidative cleavage helps locate the triple bond in an unknown alkyne.

The carbonyl groups in the products are a key to determine the position of the cleaved triple bond in the reactant. Thus, if the identity of the acids is known, the structure of the unknown alkyne can be deduced.

The usefulness of oxidative cleavage becomes apparent when warm and basic aqueous potassium permanganate is used to test for the presence of unsaturation in an unknown sample.

To demonstrate, if the sample contains an alkyne, the purple color of the oxidizing reagent decolorizes as it oxidatively cleaves the alkyne, and subsequently, a brown precipitate of manganese dioxide is formed.