Oxidation of Primary Alcohols To Carboxylic Acids - Potassium Permanganate

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 very efficiently primary alcohols to carboxylic acids. 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 till 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 alkaline conditions using a ca. 1N NaOH or KOH solution, 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. As the extent of this decomposition is difficult to estimate during the oxidation of primary alcohols, the quantity of KMnO₄ must be adjusted during the oxidation by adding it sequentially until the oxidation is complete.