121 Oxidation
Hydroboration/oxidation is a two-step reaction that converts alkenes into alcohols with
anti-Markovnikov regiochemistry.
First, borane, BH 3 is allowed to react with the alkene. The boron atom will generally react
with the less substituted carbon, and it will also donate a proton to the other carbon across
the double bond. If there is an excess of the alkene, a trialkylborane will be formed. That
is, one molecule of borane can react with up to three molecules of the alkane if necessary.
Tetrahydrofuran, a cyclic ether, is usually used as a solvent for this first step in the reaction
because it can form a complex with the borane that gives the boron atom a complete
octet (boron only has 6 valence electrons when it is in pure borane, which makes it highly
unstable.)
Now, we have a trialkylborane, which may be fine if that is what one is trying to synthesize,
but most chemists are more interested in converting this borane intermediate into 3 alcohol
molecules. The way this is done is to react the trialkylborane with hydrogen peroxide in
a basic, aqueous solution. This causes the borane intermediate to be converted into three
molecules of an alcohol. B(OH) 3 is also produced, but is probably ignored by most chemists.