Sodium borohydride

Sodium borohydride, also known as sodium tetrahydroborate, has the chemical formula NaBH₄. This white solid, usually encountered as a powder, is a specialty reducing agent used in the manufacture of pharmaceuticals and other organic and inorganic compounds. It is soluble in methanol and water, but reacts with both in the absence of base.^[1]

The compound was discovered in the 1940's by H. I. Schlessinger, who led a team that developed metal borohydrides for wartime applications.^[2]

Additional recommended knowledge

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Daily Sensitivity Test

Synthesis and handling

Sodium borohydride is prepared by the reaction of sodium hydride on trimethylborate at 250-270°C:

B(OCH₃)₃ + 4 NaH → NaBH₄ + 3 NaOCH₃

It can also be generated by the action of NaH on powdered borosilicate glass.^[3]

NaBH₄ can be recrystallized by dissolving in warm (50 °C) diglyme followed by cooling the solution.^[4]

Reactivity

Organic synthetic applications

Sodium borohydride reduces aldehydes and ketones into alcohols, but unlike the powerful reducing agent lithium aluminium hydride, NaBH₄ will generally not reduce esters, amides, or carboxylic acids.^[5] An example of the use of sodium borohydride is the industrial production of fexofenadine which includes a reduction step^[6]:

Related reagents

The activity of borohydride reducing agents can be attenuated by stabilising the boron-hydride bond. This is done through attaching sterically bulky groups or electron withdrawing groups. Acetyl (sodium triacetoxyborohydride, (NaBH(OCOCH₃)₃) or cyano (sodium cyanoborohydride (NaCNBH₃)) groups have been used for such a purpose. As a result, they find use in the reductive amination reaction. Here, the imine intermediate must be selectively reduced in the presence of the aldehyde or ketone starting material.

Stronger reducing agents can be generated by destabilising the boron-hydride bond. This is found in compounds such as superhydride (Lithium triethylborohydride) and L-Selectride (lithium tri-sec-butylborohydride).^[7]

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.^[8]

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

For more details on this topic, see Direct borohydride fuel cell.

Sodium borohydride is also used in experimental fuel cell systems. As a fuel it is less flammable and less volatile than gasoline but more corrosive. It is relatively environmentally friendly becauseof the low toxicity of borates. The hydrogen is generated for a fuel cell by catalytic decomposition of the aqueous borohydride solution:

NaBH₄ + 2H₂O → NaBO₂ + 4H₂

References