sodium borohydride

The compound is soluble in alcohols, certain ethers, and water, although it slowly hydrolyzes.{{cite encyclopedia|author1=Banfi, L. |author2=Narisano, E. |author3=Riva, R. |author4=Stiasni, N. |author5=Hiersemann, M. |encyclopedia=Encyclopedia of Reagents for Organic Synthesis|year=2004|publisher=J. Wiley & Sons |location=New York|doi=10.1002/047084289X.rs052|isbn=978-0471936237|chapter=Sodium Borohydride}}

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.Brown, H. C. "Organic Syntheses via Boranes" John Wiley & Sons, Inc. New York: 1975. {{ISBN|0-471-11280-1}}. page 260-261 Sodium borohydride is soluble in protic solvents such as water and lower alcohols. It also reacts with these protic solvents to produce {{chem2|H2}}; however, these reactions are fairly slow. Complete decomposition of a methanol solution requires nearly 90 min at 20 °C.{{cite journal|last1=Lo|first1=Chih-ting F.|last2=Karan|first2=Kunal|last3=Davis|first3=Boyd R.|title=Kinetic Studies of Reaction between Sodium Borohydride and Methanol, Water, and Their Mixtures|journal=Industrial & Engineering Chemistry Research|volume=46|issue=17|pages=5478–5484|doi=10.1021/ie0608861|year=2007}} It decomposes in neutral or acidic aqueous solutions, but is stable at pH 14.

{{chem2|NaBH4}} is a salt, consisting of the tetrahedral Borohydride anion. The solid is known to exist as three polymorphs: α, β and γ. The stable phase at room temperature and pressure is α-{{chem2|NaBH4}}, which is cubic and adopts an NaCl-type structure, in the Fm{{overline|3}}m space group. At a pressure of 6.3 GPa, the structure changes to the tetragonal β-{{chem2|NaBH4}} (space group P42₁c) and at 8.9 GPa, the orthorhombic γ-{{chem2|NaBH4}} (space group Pnma) becomes the most stable.{{ cite journal | journal = Appl. Phys. Lett. | year = 2005 | volume = 87 | page = 261916 | doi = 10.1063/1.2158505 | title = Structural transitions in NaBH[sub 4] under pressure | issue = 26 }}{{ cite journal | journal = Phys. Rev. B | year = 2007 | volume = 76 | page = 092104 | doi = 10.1103/PhysRevB.76.092104 | title = High-pressure phase of NaBH₄: Crystal structure from synchrotron powder diffraction data | issue = 9 | bibcode = 2007PhRvB..76i2104F | last1 = Filinchuk | first1 = Y. | last2 = Talyzin | first2 = A. V. | last3 = Chernyshov | first3 = D. | last4 = Dmitriev | first4 = V. | s2cid = 122588719 }}{{cite journal | journal = J. Phys. Chem. B | year = 2007 | volume = 111 | issue = 50 | pages = 13873–13876 | doi = 10.1021/jp709840w | pmid = 18031032 | title = Pressure-driven phase transitions in NaBH₄: theory and experiments | last1 = Kim | first1 = E. | last2 = Kumar | first2 = R. | last3 = Weck | first3 = P. F. | last4 = Cornelius | first4 = A. L. | last5 = Nicol | first5 = M. | last6 = Vogel | first6 = S. C. | last7 = Zhang | first7 = J. | last8 = Hartl | first8 = M. | last9 = Stowe | first9 = A. C. | last10 = Daemen | first10 = L. | last11 = Zhao | first11 = Y.}}

Alpha-sodium-borohydride-xtal-2007-3D-balls.png|α-{{chem2|NaBH4}}

Beta-sodium-borohydride-xtal-2007-3D-balls.png|β-{{chem2|NaBH4}}

Gamma-sodium-borohydride-xtal-2007-3D-balls.png|γ-{{chem2|NaBH4}}

For commercial {{chem2|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 {{chem2|H2}}) and trimethyl borate at 250–270 °C:

:{{chem2|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.{{cite book | last1 = Wietelmann | first1 = Ulrich | last2 = Felderhoff | first2 = Michael | last3 = Rittmeyer | first3 = Peter | chapter = Hydrides | publisher = Wiley-VCH Verlag GmbH & Co. KGaA | doi = 10.1002/14356007.a13_199.pub2 | title = Ullmann's Encyclopedia of Industrial Chemistry | publication-place = Weinheim, Germany | date = 2002 | isbn = 978-3-527-30673-2 | oclc = 751968805 }} In the Bayer process, it is produced from inorganic borates, including borosilicate glassSchubert, 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" and borax ({{chem2|Na2B4O7}}):

:{{chem2|Na2B4O7 + 16 Na + 8 H2 + 7 SiO2 → 4 NaBH4 + 7 Na2SiO3}}

Magnesium is a less expensive reductant, and could in principle be used instead:Wu, Ying et al. (2004) [https://www1.eere.energy.gov/hydrogenandfuelcells/pdfs/review_chemical_processes.pdf Review of Chemical Processes for the Synthesis of Sodium Borohydride]. Millennium Cell Inc. {{cite journal|doi=10.3390/inorganics6010010|title=A Recycling Hydrogen Supply System of NaBH4 Based on a Facile Regeneration Process: A Review|journal=Inorganics|volume=6|pages=10|year=2018|last1=Ouyang|first1=Liuzhang|last2=Zhong|first2=Hao|last3=Li|first3=Hai-Wen|last4=Zhu|first4=Min|doi-access=free}}

:{{chem2|8 MgH2 + Na2B4O7 + Na2CO3 → 4 NaBH4 + 8 MgO + CO2}}

and

:{{chem2|2 MgH2 + NaBO2 → NaBH4 + 2 MgO}}

{{chem2|NaBH4}} reduces many organic carbonyls, depending on the conditions. Most typically, it is used in the laboratory for converting ketones and aldehydes to alcohols. These reductions proceed in two stages, formation of the alkoxide followed by hydrolysis:

:{{chem2|NaBH4 + 4 R2C\dO -> NaO\sCHR2 + B(O\sCHR2)3}}

:{{chem2|NaO\sCHR2 + B(O\sCHR2)3 + 4 H2O -> 4 HO\sCHR2 + NaOH + B(OH)3}}

It also 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.{{Citation|last1=Banfi|first1=Luca|title=Sodium Borohydride|date=2014|encyclopedia=Encyclopedia of Reagents for Organic Synthesis|pages=1–13|publisher=John Wiley & Sons|doi=10.1002/047084289x.rs052.pub3|isbn=9780470842898|last2=Narisano|first2=Enrica|last3=Riva|first3=Renata|last4=Stiasni|first4=Nikola|last5=Hiersemann|first5=Martin|last6=Yamada|first6=Tohru|last7=Tsubo|first7=Tatsuyuki}}

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,{{cite book|title=Organic chemistry|author=Carey, Francis A.|others=Giuliano, Robert M., 1954–|isbn=9780073511214|edition= Tenth |location=New York, NY|oclc=915135847|date = 2016-01-07}} or a six-membered transition state involving a molecule of the alcohol solvent.{{cite book|title=Organic chemistry|last=Loudon|first=Marc|date=2009|publisher=Roberts and Co|isbn=9780981519432|edition=5th|location=Greenwood Village, Colo.|oclc=263409353|url-access=registration|url=https://archive.org/details/organicchemistry0000loud}} 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.{{cite journal|last1=Wigfield|first1=Donald C.|last2=Gowland|first2=Frederick W.|date=March 1977|title=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|journal=The Journal of Organic Chemistry|volume=42|issue=6|pages=1108–1109|doi=10.1021/jo00426a048}}{{cite journal|last=Wigfield|first=Donald C.|date=January 1979|title=Stereochemistry and mechanism of ketone reductions by hydride reagents|journal=Tetrahedron|volume=35|issue=4|pages=449–462|doi=10.1016/0040-4020(79)80140-4|issn=0040-4020}}

α,β-Unsaturated ketones tend to be reduced by {{chem2|NaBH4}} in a 1,4-sense, although mixtures are often formed. Addition of cerium chloride 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 {{chem2|NaBH4}}.

The {{chem2|NaBH4}}-MeOH system, formed by the addition of methanol to sodium borohydride in refluxing THF, reduces esters to the corresponding alcohols.{{cite journal | first1 = Jorge C.S. | last1 = da Costa | first2= Karla C. | last2= Pais | first3= Elisa L. | last3 = Fernandes | first4 = Pedro S. M. | last4 = de Oliveira | first5 = Jorge S. | last5 = Mendonça | first6 =Marcus V. N. | last6 = de Souza | first7 = Mônica A. | last7 = Peralta | first8 = Thatyana R.A. | last8 = Vasconcelos | title = Simple reduction of ethyl, isopropyl and benzyl aromatic esters to alcohols using sodium borohydride-methanol system | journal = Arkivoc | year = 2006 | pages = 128–133 | url = http://www.arkat-usa.org/ark/journal/2006/I01_General/1523/05-1523A%20as%20published%20mainmanuscript.pdf | access-date = 29 August 2006 }} 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.{{cite journal|title=Mechanistic rationale for the NaBH₄ reduction of α-keto esters|journal=Tetrahedron Letters|volume=40|issue=28|pages=5193–5196|doi=10.1016/S0040-4039(99)01006-0|year=1999|last1=Dalla|first1=V.|last2=Catteau|first2=J.P.|last3=Pale|first3=P.}}

The reactivity of {{chem2|NaBH4}} can be enhanced or augmented by a variety of compounds.{{cite journal|last=Periasamy|first=Mariappan|author2=Thirumalaikumar, Muniappan |title=Methods of enhancement of reactivity and selectivity of sodium borohydride for applications in organic synthesis|journal=Journal of Organometallic Chemistry|year=2000|volume=609|issue=1–2|pages=137–151|doi=10.1016/S0022-328X(00)00210-2}}{{cite journal|last=Nora de Souza|first=Marcus Vinícius|author2=Alves Vasconcelos |author3=Thatyana Rocha |title=Recent methodologies mediated by sodium borohydride in the reduction of different classes of compounds|journal=Applied Organometallic Chemistry|date=1 November 2006|volume=20|issue=11|pages=798–810|doi=10.1002/aoc.1137}}

Many additives for modifying the reactivity of sodium borohydride have been developed as indicated by the following incomplete listing.

Oxidation with iodine in tetrahydrofuran gives borane–tetrahydrofuran, which can reduce carboxylic acids to alcohols.{{cite book | doi=10.1002/047084289X.rn01598| chapter=Sodium Borohydride and Iodine| title=Encyclopedia of Reagents for Organic Synthesis| date=2013| last1=Brown| first1=Jack D.| last2=Haddenham| first2=Dustin| isbn=978-0471936237}}

Partial oxidation of borohydride with iodine gives octahydrotriborate:{{cite book |doi=10.1002/9780470132463.ch25|year=1974|last1=Ryschlewitsch|first1=G. E.|last2=Nainan|first2=K. C.|last5=Dewkett|first5=W. J.|last6=Grace|first6=M.|last7=Beall|first7=H.|title=Inorganic Syntheses |chapter=Octahydrotriborate (1-) ([B ₃ H ₈ ]) salts |pages=111–118|volume=15|isbn=9780470132463}}

:{{chem2|3 [BH4]− + I2 → [B3H8]− + 2 H2 + 2 I−}}

{{chem2|[BH4]−}} is a ligand for metal ions. Such borohydride complexes are often prepared by the action of {{chem2|NaBH4}} (or the {{chem2|LiBH4}}) on the corresponding metal halide. One example is the titanocene derivative:{{cite book | last1 = Lucas | first1 = C. R. | title = Inorganic Syntheses | chapter = Bis(η ⁵ -Cyclopentadienyl) [Tetrahydroborato(1−)]Titanium | year = 1977 | volume = 17 | page = 93 | doi = 10.1002/9780470132487.ch27 | isbn = 9780470132487}}

:{{chem2|2 (C5H5)2TiCl2 + 4 NaBH4 → 2 (C5H5)2TiBH4 + 4 NaCl + B2H6 + H2}}

{{chem2|NaBH4}} reacts with water and alcohols, with evolution of hydrogen gas and formation of the corresponding borate, the reaction being especially fast at low pH. Exploiting this reactivity, sodium borohydride has been studied as a prototypes of the direct borohydride fuel cell.

:{{chem2|NaBH4 + 2 H2O → NaBO2 + 4 H2}} (ΔH < 0)

The dominant 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.

It has been tested as pretreatment for pulping of wood, but is too costly to be commercialized.{{cite journal|author=Istek, A. and Gonteki, E. |title=Utilization of sodium borohydride (NaBH₄) in kraft pulping process|url=http://www.jeb.co.in/journal_issues/200911_nov09/paper_05.pdf|journal=Journal of Environmental Biology|volume=30|issue=6|pages= 951–953 |year=2009|pmid=20329388}}

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 way to store hydrogen for hydrogen-fueled vehicles, as it is safer (being stable in dry air) and more efficient on a weight basis than most other alternatives.Eun Hee Park, Seong Uk Jeong, Un Ho Jung, Sung Hyun Kim, Jaeyoung Lee, Suk Woo Nam, Tae Hoon Lim, Young Jun Park, Yong Ho Yuc (2007): "Recycling of sodium metaborate to borax". International Journal of Hydrogen Energy, volume 32, issue 14, pages 2982-2987. {{doi|10.1016/j.ijhydene.2007.03.029}}Z. P. Li, B. H. Liu. K. Arai, N. Morigazaki, S. Suda (2003): "Protide compounds in hydrogen storage systems". Journal of Alloys and Compounds, volumes 356–357, pages 469-474. {{doi|10.1016/S0925-8388(02)01241-0}} The hydrogen can be released by simple hydrolysis of the borohydride. However, such a usage would need a cheap, relatively simple, and energy-efficient process to recycle the hydrolysis product, sodium metaborate, back to the borohydride. No such process was available as of 2007.Hasan K. Atiyeh and Boyd R. Davis (2007): "Separation of sodium metaborate from sodium borohydride using nanofiltration membranes for hydrogen storage application". International Journal of Hydrogen Energy, volume 32, issue 2, pages 229-236. {{doi|10.1016/j.ijhydene.2006.06.003}}

Although practical temperatures and pressures for hydrogen storage have not been achieved, in 2012 a core–shell nanostructure of sodium borohydride was used to store, release and reabsorb hydrogen under moderate conditions.Stuart Gary, "[http://www.abc.net.au/science/articles/2012/08/16/3569478.htm Hydrogen storage no longer up in the air]" in ABC Science 16 August 2012, citing {{cite journal |last1=Christian |first1=Meganne |author-link=Meganne Christian |last2=Aguey-Zinsou |first2=Kondo François |year=2012 |title=Core–Shell Strategy Leading to High Reversible Hydrogen Storage Capacity for NaBH₄ |journal=ACS Nano |volume=6 |issue=9 |pages=7739–7751 |doi=10.1021/nn3030018 |pmid=22873406}}

Skilled professional conservator/restorers have used sodium borohydride to minimize or reverse foxing in old books and documents.{{cite web|url=https://blog.bookstellyouwhy.com/bid/230209/how-to-prevent-and-reverse-foxing-in-rare-books|title=How to Prevent and Reverse Foxing in Rare Books|first=Kristin|last=Masters|website=bookstellyouwhy.com|access-date=3 April 2018}}

A common laboratory demonstration "uncooks" eggs with sodium borohydride, as hydride reagents reduce disulfides to thiols.Hervé This. Can a cooked egg white be uncooked? The Chemical Intelligencer (Springer Verlag), 1996 (14), 51. To uncook an egg, breaking the hydrogen and hydrophobic bonds is not enough.{{Citation |last=This-Benckhard |first=Hervé |title=Can a Cooked Egg White Be “Uncooked”? |date=2015 |work=Culture of Chemistry: The Best Articles on the Human Side of 20th-Century Chemistry from the Archives of the Chemical Intelligencer |pages=71–71 |editor-last=Hargittai |editor-first=Balazs |url=https://link.springer.com/chapter/10.1007/978-1-4899-7565-2_18 |access-date=2025-01-17 |place=Boston, MA |publisher=Springer US |language=en |doi=10.1007/978-1-4899-7565-2_18 |isbn=978-1-4899-7565-2 |editor2-last=Hargittai |editor2-first=István|url-access=subscription }} As sodium borohydride is toxic, the egg white uncooked after three hours is not edible, but Vitamin C can be used instead.{{Cite web |title=The man who unboiled an egg {{!}} Compare and buy {{!}} The Observer |url=https://www.theguardian.com/observer/foodmonthly/futureoffood/story/0,,1969723,00.html |access-date=2025-01-17 |website=www.theguardian.com}}

Many derivatives and analogues of sodium borohydride exhibit modified reactivity of value in organic synthesis.Seyden-Penne, J. (1991) [http://rushim.ru/books/mechanizms/reductions.pdf Reductions by the Alumino- and Borohydrides in Organic Synthesis]. VCH–Lavoisier: Paris. p. 9. {{ISBN|978-0-471-19036-3}}

• Sodium triacetoxyborohydride, a milder reductant owing to the presence of more electron-withdrawing acetate in place of hydride.
• Sodium triethylborohydride, a stronger reductant owing to the presence of electron-donating ethyl groups in place of hydride.
• sodium cyanoborohydride, a milder reductant owing to the presence of more electron-withdrawing cyanide in place of hydride. Useful for reductive aminations.

• Lithium borohydride, a more strongly reducing reagent.
• L-selectride (lithium tri-sec-butylborohydride), a more strongly reducing derivative.

• Lithium aluminium hydride, a more strongly reducing reagent, capable of reducing esters and amides.

{{Reflist}}

• [https://web.archive.org/web/20060209040519/http://www.npi.gov.au/database/substance-info/profiles/15.html National Pollutant Inventory – Boron and compounds]
• [https://archive.today/20130202223031/http://www.sodiumborohydride.com/wcm/products/product_detail.page?display-mode=msds&product=1120465&application=1120785 MSDS for Sodium Borohydride]
• [https://web.archive.org/web/20050305180812/http://merit.hydrogen.co.jp/ Materials & Energy Research Institute Tokyo, Ltd.]
• [https://web.archive.org/web/20051201011849/http://www.sodiumborohydride.com/technical.html Chemo- and stereoselectivity using Borohydride reagents]
• [http://www.sciencelab.com/msds.php?msdsId=9924969 Material Safety Data Sheet] {{Webarchive|url=https://web.archive.org/web/20170710022359/http://www.sciencelab.com/msds.php?msdsId=9924969 |date=2017-07-10 }}

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Category:Sodium compounds

Category:Borohydrides

Category:Reducing agents

Category:Substances discovered in the 1940s