Question

What stereoisomers are obtained from hydroboration–oxidation of cis -2-butene?

Answer 1

The product is a racemic mixture of (`R`)-butan-2-ol and (`S`)-butan-2-ol.

If the BH₃ attacks from the top of the alkene

we get the borane

The product on oxidation with H₂O₂ is (`S`)-butan-2-ol.

If the BH₃ attacks from the bottom of the alkene

we get the borane

The product on oxidation with H₂O₂ is (`R`)-butan-2-ol.

There is an equal chance of attack from top or bottom, so we get a 50:50 mixture of the two stereoisomers.