8.11: Oxidation of Alkenes: Syn Dihydroxylation with Potassium Permanganate
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.