8.11 : Oxidação de Alcenos: Diidroxilação Sin com Permanganato de Potássio

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.