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

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Answer 1 · 2015-02-02T23:30:22

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

If the BH₃ attacks from the top of the alkene

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we get the borane

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

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If the BH₃ attacks from the bottom of the alkene

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we get the borane

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

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