Question

What is meant by hydroboration-oxidation reaction? Illustrate it with an example.

Illustrate the hydroboration-oxidation reaction with an example.

Answer 1

Diborane (BH₃)₂ reacts with alkenes to yield trialkylboranes as the addition products. This is oxidised to alcohol by hydrogen peroxide in the presence of aqueous sodium hydroxide.

\[\begin{array}{cc}
\ce{CH3 - CH = CH2 + (H - BH2)2 -> CH3 - CH - CH2 ->[CH3 - CH = CH2] (CH3 - CH2 - CH2)2BH ->[CH3 - CH = CH2] (CH3 - CH2 - CH2)3B ->[3H2O2, \overset{-}{O}H][H2O] \underset{Propan-1-ol}{3CH3 - CH2 - CH2 - OH} + B(OH)3\\
|\phantom{.........}|\phantom{................................................................................................................................}\\
\ce{H}\phantom{.....}\ce{BH2}\phantom{..............................................................................................................................}\\}
\end{array}\]

The incorporation of borane into the double bond occurs so that the boron atom binds to the sp² carbon with a higher hydrogen atom count. The resulting alcohol appears to have been produced by the addition of water to the alkene in a manner contrary to Markovnikov’s rule. This process produces alcohol in high yield.