Overview

Alkenes can be dihydroxylated using potassium permanganate.  The method encompasses the reaction of an alkene with a cold, dilute solution of potassium permanganate under basic conditions to form a cis-diol along with a brown precipitate of manganese dioxide.

The mechanism begins with the syn addition of a permanganate ion (MnO₄⁻) across the same side of the alkene π bond, forming a cyclic manganate ester intermediate. Next, the hydrolysis of the cyclic ester with water gives a cis-diol with the retention of stereochemistry at the newly formed C–O bonds.

Potassium permanganate is inexpensive and safer compared to osmium tetroxide. However, its strong oxidizing nature leads to over-oxidation of the diol, thereby giving poor yields.

Syn Dihydroxylation with Hot Basic Potassium Permanganate

When hot potassium permanganate is used, it oxidatively cleaves the carbon–carbon double bond, forming ketones or acids depending on the nature of the substituents on the alkene. Thus, terminal alkenes are oxidized to form carbon dioxide, while monosubstituted and disubstituted alkenes give carboxylic acids and ketones, respectively.

Qualitative Analysis Using Potassium Permanganate

The basic potassium permanganate solution is also known as Baeyer's reagent, which is used in qualitative analysis to determine the presence of olefinic double bonds. During the reaction, the deep purple color of potassium permanganate solution decolorizes with the formation of a brown precipitate of manganese dioxide.

Procedure

Analogous to the syn dihydroxylation of alkenes with osmium tetroxide, oxidation of alkenes with potassium permanganate follows a similar mechanism to form glycols.

When treated with a dilute solution of cold, alkaline potassium permanganate, alkenes form a cyclic ester, which, on hydrolysis, yields a cis-diol along with manganese dioxide.

The first step of the mechanism involves a syn addition of the permanganate ion across the alkene double bond to form a cyclic manganate ester.

In the second step, the cyclic manganate ester is hydrolyzed by water in the presence of a base. The reaction proceeds with the retention of stereochemistry at the newly formed carbon–oxygen bond to give a cis-diol as the final product.

Since the permanganate ion is a strong oxidizing agent, it readily accepts electrons from the alkene, which reduces the oxidation state of manganese from +7 to +4.

Potassium permanganate is inexpensive and safer to use compared to osmium tetroxide. However, being a strong oxidizing agent, it can further oxidize the diol to carbonyl compounds.

Thus, syn dihydroxylation is most efficient under mild conditions using cold, basic potassium permanganate. In contrast, hot, basic conditions oxidatively cleave the carbon–carbon double bond to form ketones or acids depending on the different substituents on the alkene.

For example, under strongly oxidizing conditions, terminal alkenes form carbon dioxide, alkenes with monosubstituted carbons give carboxylic acids, whereas alkenes with disubstituted carbons yield ketones.

The oxidation of alkenes with a cold, basic solution of potassium permanganate, also known as Baeyer's reagent, provides a qualitative test to detect the presence of olefinic double bonds.

When added to alkenes, the deep purple color of potassium permanganate disappears, transforming into a brown precipitate of manganese dioxide.