Hydroboration–oxidation: practice and mechanism
Boron goes to the less substituted carbon and is then replaced by OH: the anti-Markovnikov alcohol.
Example
Boron adds to the less substituted, sterically more accessible carbon; oxidation replaces it with OH. Result: the anti-Markovnikov alcohol (syn addition of H and OH).
Mechanism step by step
- 1
BH₃ adds to the double bond in one step: B to the less substituted carbon, H to the more substituted one (syn).
- 2
This repeats until a trialkylborane has formed.
- 3
H₂O₂ and NaOH replace each B with OH, keeping the stereochemistry.
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: Acid-catalysed hydration · Radical addition of HBr (anti-Markovnikov)
Frequently asked questions
Why does boron go to the less substituted carbon?
For steric reasons and because a slight positive charge builds up on the more substituted carbon in the transition state.
Are there rearrangements as with carbocations?
No, no carbocation forms, which makes hydroboration very reliable.