Sodium Borohydride - Reactivity

Reactivity

NaBH₄ will reduce many organic carbonyls, depending on the precise conditions. Most typically, it is used in the laboratory for converting ketones and aldehydes to alcohols. It will reduce acyl chlorides, thiol esters and imines. Under typical conditions, it will not reduce esters, amides, or carboxylic acids. At room temperature, the only acid derivatives it reduces are acyl chlorides, which are exceptionally electrophilic.

Many other hydride reagents are more strongly reducing. These usually involve replacing hydride with alkyl groups, such as lithium triethylborohydride and L-Selectride (lithium tri-sec-butylborohydride), or replacing B with Al. Variations in the counterion also affect the reactivity of the borohydride.

Oxidation of NaBH₄ with iodine in tetrahydrofuran gives the BH₃-THF complex, which can reduce carboxylic acids. Likewise, the NaBH₄-MeOH system, formed by the addition of methanol to sodium borohydride in refluxing THF, reduces esters to the corresponding alcohols, for instance, benzyl benzoate to benzyl alcohol. Mixing water or an alcohol with the borohydride converts some of it into unstable hydride ester, which is more efficient at reduction, but the reductant will eventually decompose spontaneously to give hydrogen gas and borates. The same reaction can run also intramolecularly: an α-ketoester converts into a diol, since the alcohol produced will attack the borohydride to produce an ester of the borohydride, which then reduces the neighboring ester.

Aqueous solutions of sodium borohydride are decomposed by catalytic amounts of cobalt(II) ions to yield sodium borate and hydrogen. Pellets of cobalt-doped sodium borohydride are commercially available for use in hydrogen generators, for applications where cylinders of hydrogen would be inconvenient.