Hydroboration/oxidation is a two-step reaction that converts alkenes into alcohols with anti-Markovnikov regiochemistry. First, borane, BH3 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.
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