Oxidation of Alkynes

Before we discuss Oxidation of Alkynes we will talk about triple bond of Alkynes

What are Alkynes?

Alkynes are hydrocarbons that contain carbon–carbon triple bonds.

Alkynes are also called acetylenes because they are derivatives of acetylene, the simplest alkyne

Nomenclature of Alkynes

Nomenclature of Alkynes

– The chemistry of the carbon–carbon triple bond is similar to that of the double bond.

– Alkynes undergo most of the same reactions as alkenes, especially the additions and the oxidations.

– Reactions that are specific to alkynes: some that depend on the unique characteristics of the C ≡ C triple bond, and others that depend on the unusual acidity of the acetylenic  ≡ C – H bond.

– A triple bond gives an alkyne four fewer hydrogens than the corresponding alkane.

Oxidation of Alkynes

(1) Oxidation of Alkynes: Permanganate Oxidations

– Under mild conditions, potassium permanganate oxidizes alkenes to glycols, compounds with two -OH groups on adjacent carbon atoms.

– Recall that this oxidation involves adding a hydroxyl group to each end of the double bond (hydroxylation). A similar reaction occurs with alkynes.

– If an alkyne is treated with cold, aqueous potassium permanganate under nearly neutral conditions, an α-diketone results.

– This is conceptually the same as hydroxylating each of the two pi bonds of the alkyne, then losing two molecules of water to give the diketone.

Oxidation of Alkynes

– For example, when pent-2-yne is treated with a cold, dilute solution of neutral permanganate, the product is pentane-2,3-dione.

Oxidation of Alkynes

– Terminal alkynes probably give a keto-aldehyde at first, but the aldehyde quickly oxidizes to an acid under these conditions.

– If the reaction mixture becomes warm or too basic, the diketone undergoes oxidative cleavage.

– The products are the carboxylate salts of carboxylic acids, which can be converted to the free acids by adding dilute acid.

– For example, warm, basic permanganate cleaves the triple bond of pent-2-yne to give acetate and propionate ions.

– Acidification reprotonates these anions to acetic acid and propionic acid.

– Terminal alkynes are cleaved similarly to give a carboxylate ion and formate ion.

– Under these oxidizing conditions, formate oxidizes further to carbonate, which becomes after protonation

Oxidation of Alkynes

(2) Oxidation of Alkynes: Ozonolysis

– Ozonolysis of an alkyne, followed by hydrolysis, cleaves the triple bond and gives two carboxylic acids.

– Either permanganate cleavage or ozonolysis can be used to determine the position of the triple bond in an unknown alkyne.

Oxidation of Alkynes

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