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Concept
Oxidation of primary alcohols to carboxylic acids
Summary
The oxidation of primary alcohols to carboxylic acids is an important oxidation reaction in organic chemistry.
When a primary alcohol is converted to a carboxylic acid, the terminal carbon atom increases its oxidation state by four. Oxidants able to perform this operation in complex organic molecules, featuring other oxidation-sensitive functional groups, must possess substantial selectivity. The most common oxidants are alkaline potassium permanganate (KMnO4) or acidified potassium dichromate. Jones reagent, PCC in DMF, Heyns oxidation, ruthenium tetroxide (RuO4) and TEMPO are also used.
Potassium permanganate
Potassium permanganate (KMnO4) is a very strong oxidant able to react with many functional groups, such as secondary alcohols, 1,2-diols, aldehydes, alkenes, oximes, sulfides and thiols. Under controlled conditions, KMnO4 oxidizes primary alcohols to carboxylic acids very efficiently. This reaction, which was first described in detail by Fournier, is typically carried out by adding KMnO4 to a solution or suspension of the alcohol in an alkaline aqueous solution. The resulting mixture is stirred until the oxidation is complete. For the reaction to proceed efficiently, the alcohol must be at least partially dissolved in the aqueous solution. This can be facilitated by the addition of an organic co-solvent such as dioxane, pyridine, acetone or t-BuOH. KMnO4 will readily react with a carbon-carbon double bond before oxidizing a primary alcohol.
Normally, these oxidations are performed under strong basic conditions, because this promotes a greater oxidation speed and selectivity. In substrates sensitive to strong base, the reaction can be carried out at a lower pH—or even under acidic conditions—at the cost of a greatly decreased reaction velocity.
KMnO4 is decomposed in water, resulting in formation of manganese dioxide (MnO2) and gaseous oxygen. This decomposition is catalyzed by acid, base and MnO2.
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