Sodium borohydride

Sodium borohydride, also known as sodium tetrahydroborate, has the chemical formula NaBH4. It is a selective specialty reducing agent used in the manufacture of pharmaceuticals and other organic compounds. It is a white solid, usually encountered as a powder. It is soluble in methanol and water, but reacts with both unless strong base is added.

The compound was discovered in the 1940's by H. I. Schlessinger, who led a team that developed metal borohydrides for wartime applications.

Synthesis and handling
Sodium borohydride is prepared by the reaction of sodium hydride on trimethylborate at 250-270°C:
 * B(OCH3)3 +  4 NaH  &rarr;  NaBH4  +  3 NaOCH3

It can also be generated by the action of NaH on powdered borosilicate glass.

NaBH4 can be recrystallized by dissolving in warm (50 °C) diglyme followed by cooling the solution.

Organic synthetic applications
Sodium borohydride reduce aldehydes and ketones into alcohols, but unlike the powerful reducing agent lithium aluminium hydride, NaBH4 will generally not reduce esters, amides, or carboxylic acids. . An example of the use of sodium borohydride is the industrial production of fexofenadine which includes a reduction step :



In general the reactivity of boron hydrides can be modified by seeming subtle changes in its structure. For example, sodium cyanoborohydride (NaCNBH3) converts certain alcohol groups to methylene groups. The reagent known as L-Selectride, lithium tri-sec-butylborohydride), is a still more powerful reducing agent. . Another modification is sodium triacetoxyborohydride (NaBH(OCOCH3)3).

Other reactions
Oxidation of NaBH4 with iodine in tetrahydrofuran creates the BH3-THF complex which can reduce esters. 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.

BH4- is an excellent ligand for metal ions. Such borohydride complexes are often prepared by the action of NaBH4 (or the LiBH4) on the corresponding metal halide, e.g. Zr(BH4)4.

Fuel cells
Sodium borohydride is also used in experimental fuel cell systems as a means of storing hydrogen. As a fuel it is less flammable and less volatile than gasoline but more corrosive. It is relatively environmentally friendly because it will quickly degrade into inert salts when released into the environment. The hydrogen is generated for a fuel cell by catalytic decomposition of the aqueous borohydride solution:


 * NaBH4 + 2H2O → NaBO2 + 4H2