syn dihydroxylation: practice and mechanism
OsO₄ or cold KMnO₄ puts one OH group on each carbon (syn): a vicinal diol forms and the C–C bond stays.
Example
Osmium tetroxide forms a cyclic ester with the double bond, which is cleaved to the vicinal diol. Both OH groups come from the same side (syn).
Mechanism step by step
- 1
OsO₄ adds to the double bond in one step, forming a cyclic osmate ester.
- 2
Both oxygens come from the same side (syn).
- 3
Hydrolysis cleaves the ester to give the cis diol. NMO re-oxidises the osmium, so little OsO₄ is needed.
Practice tasks
Which product forms, which reagent is needed? After each answer you see the explanation.
Which main product forms?
Typical mistakes
Often confused with: Epoxidation · Ozonolysis (reductive work-up)
Frequently asked questions
Why only cold KMnO₄?
Warm KMnO₄ oxidises further and cleaves the C–C bond, like ozonolysis but up to the acid.
How do I get a trans diol?
Via the epoxide (mCPBA) followed by acid-catalysed ring opening with water.
Why does a syn diol form?
OsO₄ forms a cyclic osmate ester with the alkene; both oxygens come from the same face, so the OH groups end up syn.