9.10: Alkynes to Carboxylic Acids: Oxidative Cleavage

Alkynes undergo oxidative cleavage in the presence of oxidizing reagents like potassium permanganate and ozone. The triple bond — one σ bond and two π bonds — is completely cleaved, and the alkyne is oxidized to carboxylic acids. When warm and basic aqueous potassium permanganate is used as an oxidizing agent, alkynes are first converted to carboxylate salts via an unstable α-diketone intermediate. Further, a mild acid treatment protonates the carboxylate anions generating free carboxylic acid molecules. When an alkyne is subjected to ozonolysis, an ozonide intermediate is formed, which is then oxidatively cleaved through hydrolysis to yield carboxylic acids.

Oxidative cleavage of internal alkynes yields only carboxylic acids, while terminal alkynes generate carbon dioxide, in addition to an acid, irrespective of the oxidizing reagent used. Thus, oxidative cleavage can be used to locate the triple bond in unknown alkynes. The carbonyl groups in the products are a key to determine the position of the oxidatively cleaved triple bond in the reactant: if the identity of the acids is known, the structure of the unknown alkyne can be deduced.

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.