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Sodium borohydride



Sodium borohydride
IUPAC name Sodium tetrahydridoborate
Identifiers
CAS number 16940-66-2
Properties
Molecular formula NaBH4
Molar mass 37.83 g/mol
Density 1.0740 g cm-3
Melting point

36 °C

Boiling point

400 °C (dec.)

Hazards
NFPA 704
1
2
2
W
Related Compounds
Other anions Sodium cyanoborohydride
Sodium hydride
Sodium borate
Borax
Other cations Lithium borohydride
Related compounds Lithium aluminium hydride
Except where noted otherwise, data are given for
materials in their standard state
(at 25 °C, 100 kPa)

Infobox disclaimer and references

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

Contents

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

  1. ^ Banfi, L.; Narisano, E.; Riva, R. “Sodium Borohydride” in Encyclopedia of Reagents for Organic Synthesis (Ed: L. Paquette) 2004, J. Wiley & Sons, New York,doi:10.1002/047084289X.rs052.
  2. ^ Schlesinger, H. I.; Brown, H. C.; Abraham, B.; Bond, A. C.; Davidson, N.; Finholt, A. E.; Gilbreath, J. R.; Hoekstra, H.; Horvitz, L.; Hyde, E. K.; Katz, J. J.; Knight, J.; Lad, R. A.; Mayfield, D. L.; Rapp, L.; Ritter, D. M.; Schwartz, A. M.; Sheft, I.; Tuck, L. D.; Walker, A. O. “New developments in the chemistry of diborane and the borohydrides. General summary” Journal of the American Chemical Society 1953, volume 75, pages 186-90,doi:10.1021/ja01097a049.
  3. ^ Schubert, F.; Lang, K.; Burger, A. “Alkali metal borohydrides” (Bayer), 1960. German patent DE 1088930 19600915 (ChemAbs: 55:120851). Supplement to . to Ger. 1,067,005 (CA 55, 11778i). From the abstract: “Alkali metal borosilicates are treated with alkali metal hydrides in approx. 1:1 ratio at >100° with or without H pressure”.
  4. ^ Brown, H. C. “Organic Syntheses via Boranes” John Wiley & Sons, Inc. New York: 1975. ISBN 0-471-11280-1. page 260-1.
  5. ^ Banfi, L.; Narisano, E.; Riva, R. “Sodium Borohydride” in Encyclopedia of Reagents for Organic Synthesis (Ed: L. Paquette) 2004, J. Wiley & Sons, New York,doi:10.1002/047084289X.rs052.
  6. ^ SPECIAL FEATURE SECTION: HYDRIDE REDUCTIONS Christian T. Goralski and Bakthan Singaram Org. Process Res. Dev.; 2006; 10(5) pp 947 - 948; (Editorial) doi:10.1021/op0601363
  7. ^ Seyden-Penne, J. "Reductions by the Alumino- and Borohydrides in Organic Synthesis"; VCH–Lavoisier: Paris, 1991.
  8. ^ da Costa, Jorge C.S.; Karla C. Pais, Elisa L. Fernandes, Pedro S. M. de Oliveira, Jorge S. Mendonça, Marcus V. N. de Souza, Mônica A. Peralta, and Thatyana R.A. Vasconcelos (2006). "Simple reduction of ethyl, isopropyl and benzyl aromatic esters to alcohols using sodium borohydride-methanol system" (PDF). Arkivoc: 128-133. Retrieved on 2006-08-29.
 
This article is licensed under the GNU Free Documentation License. It uses material from the Wikipedia article "Sodium_borohydride". A list of authors is available in Wikipedia.
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