Syn-hydroxylation
With OsO4
In syn-hydroxylation, an alkene is turned into a 1,2-diol, a glycol. This is another example of “Giving it the horns.” In this case, osmium tetroxide (OsO4) acts like the bull with two oxides as the horns. Osmium tetroxide reacts with the pi-bond of the alkene to make an osmate ester. The osmate ester can be oxidized with hydrogen peroxide and hydroxide to make a glycol with syn-stereochemistry. The bull has given it the horns. OsO4 is a catalyst in this reaction. The reaction starts with OsO4 and ends with OsO4. Since it is regenerated, only a small amount is needed in this reaction.
One drawback of this reaction is that OsO4 is expensive, toxic, and volatile. Volatile means it easily evaporates. This is especially problematic if that compound is toxic!
With KMnO4
A safer, cheaper, and easier method to perform a syn-hydroxylation is to a cold, dilute, sometimes neutral aqueous solution of potassium permanganate (KMnO4). The active species is the permanganate (MnO4-). The permanganate acts like the bull and uses its oxides as horns. The mechanism is similar to the reaction with OsO4. In this case, a manganate ester is formed. Following the addition of water and hydroxide, a glycol is formed with syn-stereochemistry.
Potassium permanganate is not a catalyst because it is not regenerated in this reaction. It is reduced. Notice, in the reaction with OsO4, the oxidizer H2O2 is used to add more oxygen atoms to regenerate the OsO4 catalyst. In the reaction with KMnO4, no oxidizer is used. That is why the KMnO4 is not a catalyst. This is OK since KMnO4 is so cheap.
Potassium permanganate is a quick and easy chemical test for alkenes. Aqueous potassium permanganate is a purple solution. We know that it reacts with alkenes. If an unknown organic compound is added to the purple potassium permanganate and nothing happens, we could reason that the unknown organic compound is NOT an alkene. On the other hand, if the purple potassium permanganate solution turns brown, we would know a reaction occurred because the brown MnO2 was formed and the unknown organic compound that was added must be an alkene.