dihydroxylation
/ dy-hy-drox-il-AY-shun /
Dihydroxylation adds two hydroxyl groups across a double bond, one on each carbon, turning an alkene into a 1,2-diol (also called a glycol, two -OH groups on neighbouring carbons). Unlike ozonolysis, which cuts the molecule apart, dihydroxylation keeps the carbon skeleton whole and merely decorates the former double bond with two OH groups.
The classic reagents are osmium tetroxide (OsO4) or cold dilute potassium permanganate (KMnO4). Both work the same way: the metal-oxygen reagent adds to the double bond in one concerted step, both oxygens reaching the same face of the alkene at once, forming a five-membered cyclic intermediate (an osmate or manganate ester). When this ring is hydrolyzed, it leaves both new OH groups on that same face. So dihydroxylation is a syn addition — the two hydroxyls end up cis to each other.
Dihydroxylation matters as the standard way to make a 1,2-diol with controlled, syn stereochemistry, which complements anti-selective routes (like opening an epoxide with water). It is the conceptual partner of ozonolysis — same oxidant family attacking the same bond, but one leaves the molecule intact with two OH groups while the other cleaves it into carbonyls. Practically, OsO4 is highly toxic and expensive, so it is usually used in tiny catalytic amounts with a cheap co-oxidant to regenerate it.
Cyclopentene treated with OsO4 (then aqueous workup) gives cis-cyclopentane-1,2-diol: both OH groups land on the same face of the ring, the hallmark syn outcome.
Both OH groups add to the same face, so the diol is syn (cis).
Hot or concentrated KMnO4 does not stop at the diol — it keeps oxidizing and cleaves the double bond, much like ozonolysis, so the cold and dilute conditions are essential to isolate the diol.