Sodium borohydride

Sodium borohydride, also known as sodium tetrahydridoborate and sodium tetrahydroborate,[3] is an inorganic compound with the formula NaBH4. This white solid, usually encountered as a powder, is a reducing agent that finds application in chemistry, both in the laboratory and on an industrial scale. It has been tested as pretreatment for pulping of wood, but is too costly to be commercialized.[4][5] The compound is soluble in alcohols, certain ethers, and water, although it slowly hydrolyzes.[6]

Sodium borohydride
Names
IUPAC name
Sodium tetrahydridoborate(1–)
Systematic IUPAC name
Sodium boranuide
Identifiers
3D model (JSmol)
ChEBI
ChemSpider
ECHA InfoCard 100.037.262
EC Number
  • 241-004-4
23167
MeSH Sodium+borohydride
RTECS number
  • ED3325000
UNII
UN number 1426
Properties
NaBH4
Molar mass 37.83 g/mol
Appearance white crystals
hygroscopic
Density 1.07 g/cm3[1]
Melting point 400 °C (752 °F; 673 K)(decomposes)[1]
550 g/L[1]
Solubility soluble in liquid ammonia, amines, pyridine
Structure[2]
Cubic (NaCl), cF8
Fm3m, No. 225
a = 0.6157 nm
Hazards
GHS hazard statements
 H260, H301, H311, H314
P223, P231, P232, P280, P301+310, P370+378, P422
NFPA 704 (fire diamond)
Flammability code 1: Must be pre-heated before ignition can occur. Flash point over 93 °C (200 °F). E.g. canola oilHealth code 3: Short exposure could cause serious temporary or residual injury. E.g. chlorine gasReactivity code 2: Undergoes violent chemical change at elevated temperatures and pressures, reacts violently with water, or may form explosive mixtures with water. E.g. white phosphorusSpecial hazard W: Reacts with water in an unusual or dangerous manner. E.g. sodium, sulfuric acid
1
3
2
Flash point 70 °C (158 °F; 343 K)
ca. 220 °C (428 °F; 493 K)
Explosive limits 3%
Lethal dose or concentration (LD, LC):
160 mg/kg (Oral – Rat)
230 mg/kg (Dermal – Rabbit)
Related compounds
Other anions
 Sodium cyanoborohydride
Sodium hydride
Sodium borate
Borax
Sodium aluminum hydride
Other cations
 Lithium borohydride
Related compounds
 Lithium aluminium hydride
Sodium triacetoxyborohydride
Except where otherwise noted, data are given for materials in their standard state (at 25 °C [77 °F], 100 kPa).
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Infobox references

The compound was discovered in the 1940s by H. I. Schlesinger, who led a team seeking volatile uranium compounds.[7][8] Results of this wartime research were declassified and published in 1953.

Properties
SolventSolubility (g/100 mL)[6]
MeOH13
EtOH3.16
Diglyme5.15
Et2Oinsoluble

Sodium borohydride is an odorless white to gray-white microcrystalline powder that often forms lumps. It can be purified by recrystallization from warm (50 °C) diglyme.[9] Sodium borohydride is soluble in protic solvents such as water and lower alcohols. It also reacts with these protic solvents to produce H2; however, these reactions are fairly slow. Complete decomposition of a methanol solution requires nearly 90 min at 20 °C.[10] It decomposes in neutral or acidic aqueous solutions, but is stable at pH 14.[6]

Structure

NaBH4 is a salt, consisting of the tetrahedral [BH4]− anion. The solid is known to exist as three polymorphs: α, β and γ. The stable phase at room temperature and pressure is α-NaBH4, which is cubic and adopts an NaCl-type structure, in the Fm3m space group. At a pressure of 6.3 GPa, the structure changes to the tetragonal β-NaBH4 (space group P421c) and at 8.9 GPa, the orthorhombic γ-NaBH4 (space group Pnma) becomes the most stable.[11][12][13]

α-NaBH4
β-NaBH4
γ-NaBH4

Synthesis and handling

For commercial NaBH4 production, the Brown-Schlesinger process and the Bayer process are the most popular methods. In the Brown-Schlesinger process Sodium borohydride is industrially prepared from sodium hydride (produced by reacting Na and H2) and trimethyl borate at 250–270 °C:

B(OCH3)3 + 4 NaH → NaBH4 + 3 NaOCH3

Millions of kilograms are produced annually, far exceeding the production levels of any other hydride reducing agent.[4] Sodium borohydride can also be produced by the action of NaH on powdered borosilicate glass.[14]

Different from it, the Bayer process is based on the reaction among borax (Na2B4O7), Na, H2, and silicon oxide (SiO2) at 700 °C producing sodium borohydride and sodium silicate

Na2B4O7 + 16 Na + 8 H2 + 7 SiO2 → 4 NaBH4 + 7 Na2SiO3

There is currently an effort to modify the Bayer Process by employing the less expensive reducing metal magnesium (Mg) in place of sodium. Reactions such as:

8 MgH2 + Na2B4O7 + Na2CO3 → 4 NaBH4 + 8 MgO + CO2

and

2 MgH2 + NaBO2 → NaBH4 + 2 MgO

are promising modifications to the Bayer Process, but have not been developed far enough to exhibit both high yield and fast reaction rates.[15][16]

Reactivity

NaBH4 reduces many organic carbonyls, depending on the precise conditions. Most typically, it is used in the laboratory for converting ketones and aldehydes to alcohols. It efficiently reduces acyl chlorides, anhydrides, α-hydroxylactones, thioesters, and imines at room temperature or below. It reduces esters slowly and inefficiently with excess reagent and/or elevated temperatures, while carboxylic acids and amides are not reduced at all.[17] NaBH4 reacts with water and alcohols, with evolution of hydrogen gas and formation of the borate salt, the reaction being especially fast at low pH.

Nevertheless, an alcohol, often methanol or ethanol, is generally the solvent of choice for sodium borohydride reductions of ketones and aldehydes. The mechanism of ketone and aldehyde reduction has been scrutinized by kinetic studies, and contrary to popular depictions in textbooks, the mechanism does not involve a 4-membered transition state like alkene hydroboration,[18] or a six-membered transition state involving a molecule of the alcohol solvent.[19] Hydrogen-bonding activation is required, as no reduction occurs in an aprotic solvent like diglyme. However, the rate order in alcohol is 1.5, while carbonyl compound and borohydride are both first order, suggesting a mechanism more complex than one involving a six-membered transition state that includes only a single alcohol molecule. It was suggested that the simultaneous activation of the carbonyl compound and borohydride occurs, via interaction with the alcohol and alkoxide ion, respectively, and that the reaction proceeds through an open transition state.[20][21]

α,β-Unsaturated ketones tend to be reduced by NaBH4 in a 1,4-sense, although mixtures are often formed. Addition of cerium chloride as an additive greatly improves the selectivity for 1,2-reduction of unsaturated ketones (Luche reduction). α,β-Unsaturated esters also undergo 1,4-reduction in the presence of NaBH4.[6]

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.[22]

The reactivity of NaBH4 can be enhanced or augmented by a variety of compounds.[23][24] Oxidation with iodine in tetrahydrofuran gives the borane–tetrahydrofuran complex, which can reduce carboxylic acids.[25] Likewise, the NaBH4-MeOH system, formed by the addition of methanol to sodium borohydride in refluxing THF, reduces esters to the corresponding alcohols.[26] 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 eventually decomposes spontaneously to produce hydrogen gas and borates. The same reaction can also occur intramolecularly: an α-ketoester converts into a diol, since the alcohol produced attacks the borohydride to produce an ester of the borohydride, which then reduces the neighboring ester.[27] The combination of NaBH4 with carboxylic acids results in the formation of acyloxyborohydride species, such as STAB. These can perform a variety of reductions not normally associated with borohydride chemistry, such as alcohols to hydrocarbons and nitriles to primary amines.[28]

Coordination chemistry

BH4− is a ligand for metal ions. Such borohydride complexes are often prepared by the action of NaBH4 (or the LiBH4) on the corresponding metal halide. One example is the titanocene derivative:[29]

2 (C5H5)2TiCl2 + 4 NaBH4 → 2 (C5H5)2TiBH4 + 4 NaCl + B2H6 + H2

Hydrogen source

In the presence of metal catalysts, sodium borohydride releases hydrogen. Exploiting this reactivity, sodium borohydride is used in prototypes of the direct borohydride fuel cell. The hydrogen is generated for a fuel cell by catalytic decomposition of the aqueous borohydride solution:

NaBH4 + 2 H2O → NaBO2 + 4 H2 (ΔH < 0)

Applications

The principal application of sodium borohydride is the production of sodium dithionite from sulfur dioxide: Sodium dithionite is used as a bleaching agent for wood pulp and in the dyeing industry.

Sodium borohydride reduces aldehydes and ketones to give the related alcohols. This reaction is used in the production of various antibiotics including chloramphenicol, dihydrostreptomycin, and thiophenicol. Various steroids and vitamin A are prepared using sodium borohydride in at least one step.

Sodium borohydride has been considered as a solid state hydrogen storage candidate. Although practical temperatures and pressures for hydrogen storage have not been achieved, in 2012 a core–shell nanostructure of sodium borohydride was used successfully to store, release and reabsorb hydrogen under moderate conditions.[30]

Sodium borohydride can be used to reduce foxing in old books and documents.[31]

Safety

Sodium borohydride is a source of basic borate salts which can be corrosive, and hydrogen or diborane, which are both flammable. Spontaneous ignition can result from solution of sodium borohydride in dimethylformamide.

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See also

References
  1. Haynes, William M., ed. (2011). CRC Handbook of Chemistry and Physics (92nd ed.). CRC Press. p. 4.89. ISBN 978-1439855119.
  2. Ford, P. T. and Powell, H. M. (1954). "The unit cell of potassium borohydride, KBH4, at 90° K". Acta Crystallogr. 7 (8): 604–605. doi:10.1107/S0365110X54002034.CS1 maint: multiple names: authors list (link)
  3. Busch, D.H. (2009). Inorganic Syntheses. 20. Wiley. p. 137. ISBN 9780470132869. Retrieved 20 May 2015.
  4. Rittmeyer, Peter and Wietelmann, Ulrich (2002) "Hydrides" in Ullmann's Encyclopedia of Industrial Chemistry, Wiley-VCH, Weinheim. doi:10.1002/14356007.a13_199
  5. Istek, A. and Gonteki, E. (2009). "Utilization of sodium borohydride (NaBH4) in kraft pulping process" (PDF). Journal of Environmental Biology. 30 (6): 951–953.CS1 maint: multiple names: authors list (link)
  6. Banfi, L.; Narisano, E.; Riva, R.; Stiasni, N.; Hiersemann, M. (2004). "Sodium Borohydride". Encyclopedia of Reagents for Organic Synthesis, 8 Volume Set. Encyclopedia of Reagents for Organic Synthesis. New York: J. Wiley & Sons. doi:10.1002/047084289X.rs052. ISBN 978-0471936237.
  7. 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. (1953). "New developments in the chemistry of diborane and the borohydrides. General summary". J. Am. Chem. Soc. 75: 186–90. doi:10.1021/ja01097a049.
  8. Hermann I Schlesinger and Herbert C Brown (1945) "Preparation of alkali metal compounds". US Patent 2461661. Granted on 1949-02-15; expired on 1966-02-15.
  9. Brown, H. C. “Organic Syntheses via Boranes” John Wiley & Sons, Inc. New York: 1975. ISBN 0-471-11280-1. page 260-1.
  10. Lo, Chih-ting F.; Karan, Kunal; Davis, Boyd R. (2007). "Kinetic Studies of Reaction between Sodium Borohydride and Methanol, Water, and Their Mixtures". Industrial & Engineering Chemistry Research. 46 (17): 5478–5484. doi:10.1021/ie0608861.
  11. "Structural transitions in NaBH[sub 4] under pressure". Appl. Phys. Lett. 87 (26): 261916. 2005. doi:10.1063/1.2158505.
  12. Filinchuk, Y.; Talyzin, A. V.; Chernyshov, D.; Dmitriev, V. (2007). "High-pressure phase of NaBH4: Crystal structure from synchrotron powder diffraction data". Phys. Rev. B. 76 (9): 092104. Bibcode:2007PhRvB..76i2104F. doi:10.1103/PhysRevB.76.092104.
  13. Kim, E.; Kumar, R.; Weck, P. F.; Cornelius, A. L.; Nicol, M.; Vogel, S. C.; Zhang, J.; Hartl, M.; Stowe, A. C.; Daemen, L.; Zhao, Y. (2007). "Pressure-driven phase transitions in NaBH4: theory and experiments". J. Phys. Chem. B. 111 (50): 13873–13876. doi:10.1021/jp709840w. PMID 18031032.
  14. Schubert, F.; Lang, K.; Burger, A. (1960) "Alkali metal borohydrides" (Bayer). 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 °C with or without H pressure".
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  16. Ouyang, Liuzhang; Zhong, Hao; Li, Hai-Wen; Zhu, Min (2018). "A Recycling Hydrogen Supply System of NaBH4 Based on a Facile Regeneration Process: A Review". Inorganics. 6: 10. doi:10.3390/inorganics6010010.
  17. Banfi, Luca; Narisano, Enrica; Riva, Renata; Stiasni, Nikola; Hiersemann, Martin; Yamada, Tohru; Tsubo, Tatsuyuki (2014), "Sodium Borohydride", Encyclopedia of Reagents for Organic Synthesis, John Wiley & Sons, pp. 1–13, doi:10.1002/047084289x.rs052.pub3, ISBN 9780470842898
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  19. Loudon, Marc (2009). Organic chemistry (5th ed.). Greenwood Village, Colo.: Roberts and Co. ISBN 9780981519432. OCLC 263409353.
  20. Wigfield, Donald C.; Gowland, Frederick W. (March 1977). "The kinetic role of hydroxylic solvent in the reduction of ketones by sodium borohydride. New proposals for mechanism, transition state geometry, and a comment on the origin of stereoselectivity". The Journal of Organic Chemistry. 42 (6): 1108–1109. doi:10.1021/jo00426a048.
  21. Wigfield, Donald C. (January 1979). "Stereochemistry and mechanism of ketone reductions by hydride reagents". Tetrahedron. 35 (4): 449–462. doi:10.1016/0040-4020(79)80140-4. ISSN 0040-4020.
  22. Seyden-Penne, J. (1991) Reductions by the Alumino- and Borohydrides in Organic Synthesis. VCH–Lavoisier: Paris. p. 9. ISBN 978-0-471-19036-3
  23. Periasamy, Mariappan; Thirumalaikumar, Muniappan (2000). "Methods of enhancement of reactivity and selectivity of sodium borohydride for applications in organic synthesis". Journal of Organometallic Chemistry. 609 (1–2): 137–151. doi:10.1016/S0022-328X(00)00210-2.
  24. Nora de Souza, Marcus Vinícius; Alves Vasconcelos; Thatyana Rocha (1 November 2006). "Recent methodologies mediated by sodium borohydride in the reduction of different classes of compounds". Applied Organometallic Chemistry. 20 (11): 798–810. doi:10.1002/aoc.1137.
  25. Kanth, J. V. Bhaskar; Periasamy, Mariappan (1 September 1991). "Selective reduction of carboxylic acids into alcohols using sodium borohydride and iodine". The Journal of Organic Chemistry. 56 (20): 5964–5965. doi:10.1021/jo00020a052.
  26. da Costa, Jorge C.S.; Pais, Karla C.; Fernandes, Elisa L.; de Oliveira, Pedro S. M.; Mendonça, Jorge S.; de Souza, Marcus V. N.; Peralta, Mônica A.; Vasconcelos, Thatyana R.A. (2006). "Simple reduction of ethyl, isopropyl and benzyl aromatic esters to alcohols using sodium borohydride-methanol system" (PDF). Arkivoc: 128–133. Retrieved 29 August 2006.
  27. Dalla, V.; Catteau, J.P.; Pale, P. (1999). "Mechanistic rationale for the NaBH4 reduction of α-keto esters". Tetrahedron Letters. 40 (28): 5193–5196. doi:10.1016/S0040-4039(99)01006-0.
  28. Gribble, Gordon, W. (1998). "Sodium borohydride in carboxylic acid media: a phenomenal reduction system". Chemical Society Reviews. 27 (6): 395. doi:10.1039/A827395Z.
  29. Lucas, C. R. (1977). Bis(5-Cyclopentadienyl) [Tetrahydroborato(1-)]Titanium. Inorganic Syntheses. 17. p. 93. doi:10.1002/9780470132487.ch27. ISBN 9780470132487.
  30. Stuart Gary, "Hydrogen storage no longer up in the air" in ABC Science 16 August 2012, citing Christian, Meganne; Aguey-Zinsou, Kondo François (2012). "Core–Shell Strategy Leading to High Reversible Hydrogen Storage Capacity for NaBH4". ACS Nano. 6 (9): 7739–7751. doi:10.1021/nn3030018. PMID 22873406.
  31. Masters, Kristin. "How to Prevent and Reverse Foxing in Rare Books". bookstellyouwhy.com. Retrieved 3 April 2018.
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