Dihydroxylation of Alkenes (OsO₄)
Dihydroxylation adds two OH groups across the C=C double bond of an alkene. Step-by-step mechanism, reagents and practice.
What happens
Dihydroxylation adds two OH groups across the C=C double bond of an alkene. The double bond becomes a single bond, and each of the two carbons gets one OH group. The final product is a vicinal diol, meaning the two OH groups are on neighboring carbons.
Reaction summary
Cyclohexene + OsO₄, then workup → Cyclohexane-1,2-diol (syn addition)
Reagents and conditions
- Starting material: Cyclohexene (alkene)
- Reagents: OsO₄
- Conditions: OsO₄, followed by workup
- Products: Cyclohexane-1,2-diol (vicinal diol)
Mechanism
- Cyclic Osmate Ester Formation: OsO₄ reacts with the alkene and forms a cyclic osmate ester.
- Same-Side Delivery: Both oxygen atoms are delivered to the double bond from the same side.
- Workup: Workup breaks the osmate ester and gives the final vicinal diol.
- Product: The vicinal diol, cyclohexane-1,2-diol: each former alkene carbon now carries one OH.
Selectivity
- Stereoselectivity: Syn addition: both OH groups are added to the same side of the double bond, so the reaction is stereospecific.
Common mistakes
- Don't confuse OsO₄ dihydroxylation with reactions that break the double bond completely.
Memory trick
OsO₄ gives OH + OH. Same side = SYN. The carbons stay together, they just each gain an OH.