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…
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.
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Q1: What happens when potassium permanganate oxidizes an alkyne?
Warm and basic aqueous potassium permanganate cleaves the triple bond of an alkyne through an unstable α-diketone intermediate, forming carboxylate salts. Mild acid treatment then protonates these anions to generate free carboxylic acids. This oxidative process completely breaks apart the alkyne's triple bond structure.
Q2: How does ozonolysis convert alkynes to carboxylic acids?
Ozonolysis generates an ozonide intermediate when an alkyne is subjected to ozone. This intermediate is then cleaved by hydrolysis to yield carboxylic acids. The process provides an alternative oxidative pathway to potassium permanganate for breaking apart the triple bond and converting it to carboxylic acid products.
Q3: Why do terminal and internal alkynes produce different products?
Internal alkynes yield only carboxylic acids upon oxidative cleavage. Terminal alkynes generate carbon dioxide along with a carboxylic acid because the terminal carbon is oxidized further. The formate anion from the terminal carbon oxidizes to carbonate, which releases CO₂ upon protonation.
Q4: How can oxidative cleavage help identify an unknown alkyne?
The carbonyl groups in the carboxylic acid products reveal the position of the cleaved triple bond in the original alkyne. If the identity of the acids produced is known, the structure of the unknown alkyne can be deduced by working backward from the product positions.
Q5: What is the color change test for detecting alkynes?
Warm and basic aqueous potassium permanganate is purple in color. When an alkyne is present, the oxidizing reagent decolorizes as it oxidatively cleaves the triple bond, and a brown precipitate of manganese dioxide forms. This color change confirms the presence of unsaturation in the sample.
Q6: What is the role of the α-diketone intermediate in permanganate oxidation?
The α-diketone is an unstable intermediate formed during the oxidative cleavage of alkynes by potassium permanganate. This intermediate is essential to the reaction mechanism, as it facilitates the complete cleavage of the triple bond and subsequent conversion to carboxylate salts before acid treatment yields the final carboxylic acids.
Q7: How does oxidative cleavage differ from other alkyne transformations?
Unlike reduction of alkynes to cis alkenes catalytic hydrogenation or other addition reactions, oxidative cleavage completely breaks the carbon-carbon triple bond and oxidizes the resulting carbons to carboxylic acids. This destructive transformation is unique in providing structural information about the original alkyne's position.