Sodium borohydride
Template:Short description Template:Chembox
Sodium borohydride, also known as sodium tetrahydridoborate and sodium tetrahydroborate,<ref name="google">Template:Cite book</ref> is an inorganic compound with the formula Template:Chem2 (sometimes written as Template:Chem2). It is a white crystalline solid, usually encountered as an aqueous basic solution. Sodium borohydride is a reducing agent that finds application in papermaking and dye industries. It is also used as a reagent in organic synthesis.<ref name=eros>Template:Cite book</ref>
The compound was discovered in the 1940s by H. I. Schlesinger, who led a team seeking volatile uranium compounds.<ref>Template:Cite journal</ref><ref name=sch1945>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.</ref> Results of this wartime research were declassified and published in 1953.
PropertiesEdit
The compound is soluble in alcohols, certain ethers, and water, although it slowly hydrolyzes.<ref name=eEROS>Template:Cite encyclopedia</ref>
| Solvent | Solubility (g/(100 mL))<ref name=eEROS/> |
|---|---|
| [[Methanol|Template:Chem2]] | 13 |
| [[Ethanol|Template:Chem2]] | 3.16 |
| Diglyme | 5.15 |
| [[Diethyl ether|Template:Chem2]] | insoluble |
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.<ref>Brown, H. C. "Organic Syntheses via Boranes" John Wiley & Sons, Inc. New York: 1975. Template:ISBN. page 260-261</ref> Sodium borohydride is soluble in protic solvents such as water and lower alcohols. It also reacts with these protic solvents to produce Template:Chem2; however, these reactions are fairly slow. Complete decomposition of a methanol solution requires nearly 90 min at 20 °C.<ref>Template:Cite journal</ref> It decomposes in neutral or acidic aqueous solutions, but is stable at pH 14.<ref name=eEROS/>
StructureEdit
Template:Chem2 is a salt, consisting of the tetrahedral [[Borohydride|Template:Chem2]] anion. The solid is known to exist as three polymorphs: α, β and γ. The stable phase at room temperature and pressure is α-Template:Chem2, which is cubic and adopts an NaCl-type structure, in the FmTemplate:Overlinem space group. At a pressure of 6.3 GPa, the structure changes to the tetragonal β-Template:Chem2 (space group P421c) and at 8.9 GPa, the orthorhombic γ-Template:Chem2 (space group Pnma) becomes the most stable.<ref>Template:Cite journal</ref><ref>Template:Cite journal</ref><ref>Template:Cite journal</ref>
- Alpha-sodium-borohydride-xtal-2007-3D-balls.png
- Beta-sodium-borohydride-xtal-2007-3D-balls.png
- Gamma-sodium-borohydride-xtal-2007-3D-balls.png
Synthesis and handlingEdit
For commercial Template:Chem2 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 Template:Chem2) and trimethyl borate at 250–270 °C:
Millions of kilograms are produced annually, far exceeding the production levels of any other hydride reducing agent.<ref name=Ullmann>Template:Cite book</ref> In the Bayer process, it is produced from inorganic borates, including borosilicate glass<ref>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"</ref> and borax (Template:Chem2):
Magnesium is a less expensive reductant, and could in principle be used instead:<ref name="Production 1">Wu, Ying et al. (2004) Review of Chemical Processes for the Synthesis of Sodium Borohydride. Millennium Cell Inc. </ref><ref name="Production 2">Template:Cite journal</ref>
and
ReactivityEdit
Organic synthesisEdit
Template:Chem2 reduces many organic carbonyls, depending on the conditions. Most typically, it is used in the laboratory for converting ketones and aldehydes to alcohols.<ref name=eros/> These reductions proceed in two stages, formation of the alkoxide followed by hydrolysis:
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.<ref>Template:Citation</ref>
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,<ref>Template:Cite book</ref> or a six-membered transition state involving a molecule of the alcohol solvent.<ref>Template:Cite book</ref> 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.<ref>Template:Cite journal</ref><ref>Template:Cite journal</ref>
α,β-Unsaturated ketones tend to be reduced by Template:Chem2 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 Template:Chem2.<ref name="eEROS" />
The Template:Chem2-MeOH system, formed by the addition of methanol to sodium borohydride in refluxing THF, reduces esters to the corresponding alcohols.<ref name='Costa'>Template:Cite journal</ref> 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.<ref>Template:Cite journal</ref>
The reactivity of Template:Chem2 can be enhanced or augmented by a variety of compounds.<ref>Template:Cite journal</ref><ref>Template:Cite journal</ref>
Many additives for modifying the reactivity of sodium borohydride have been developed as indicated by the following incomplete listing.
| additive | synthetic applications | page in Smith and March<ref>Template:March6th</ref> | comment |
|---|---|---|---|
| AlCl3 | reduction of ketones to methylene | 1837 | |
| BiCl3 | converts epoxides to allylic alcohols | 1316 | |
| (C6H5Te)2 | reduction of nitroarenes | 1862 | |
| CeCl3 | reduction of ketones in the presence of aldehydes | 1794 | Luche reduction |
| CoCl2 | reduction of azides to amines | 1822 | |
| InCl3 | hydrogenolysis of alkyl bromides, double reduction of unsaturated ketones | 1825, 1793 | |
| LiCl | amine oxides to amines | 1846 | lithium borohydride |
| NiCl2 | deoxygenation of sulfoxides, hydrogenolysis of aryl tosylates, desulfurization, reduction of nitriles | 1851,1831, 991, 1814 | nickel boride |
| TiCl4 | denitrosatation of nitrosamines | 1823 | |
| ZnCl2 | reduction of aldehydes | 1793 | |
| ZrCl4 | reduction of disulfides, reduction of azides to amines, cleavage of allyl aryl ethers | 1853, 1822, 582 |
OxidationEdit
Oxidation with iodine in tetrahydrofuran gives borane–tetrahydrofuran, which can reduce carboxylic acids to alcohols.<ref>Template:Cite book</ref>
Partial oxidation of borohydride with iodine gives octahydrotriborate:<ref>Template:Cite book</ref>
Coordination chemistryEdit
Template:Chem2 is a ligand for metal ions. Such borohydride complexes are often prepared by the action of Template:Chem2 (or the Template:Chem2) on the corresponding metal halide. One example is the titanocene derivative:<ref>Template:Cite book</ref>
Protonolysis and hydrolysisEdit
Template:Chem2 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.
- Template:Chem2 (ΔH < 0)
ApplicationsEdit
Paper manufactureEdit
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.<ref name=Ullmann/><ref name=kraftpulping>Template:Cite journal</ref>
Chemical synthesisEdit
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.<ref name=Ullmann/>
Niche or abandoned applicationsEdit
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.<ref name=park2007>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. {{#invoke:doi|main}}</ref><ref name=zpli2003>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. {{#invoke:doi|main}}</ref> 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.<ref name=atiy2007>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. {{#invoke:doi|main}}</ref>
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.<ref name="gary2012">Stuart Gary, "Hydrogen storage no longer up in the air" in ABC Science 16 August 2012, citing Template:Cite journal</ref>
Skilled professional conservator/restorers have used sodium borohydride to minimize or reverse foxing in old books and documents.<ref>{{#invoke:citation/CS1|citation |CitationClass=web }}</ref>
EducationEdit
A common laboratory demonstration "uncooks" eggs with sodium borohydride, as hydride reagents reduce disulfides to thiols.<ref>Hervé This. Can a cooked egg white be uncooked? The Chemical Intelligencer (Springer Verlag), 1996 (14), 51. </ref> To uncook an egg, breaking the hydrogen and hydrophobic bonds is not enough.<ref name=":0">Template:Citation</ref> As sodium borohydride is toxic, the egg white uncooked after three hours is not edible,<ref name=":0" /> but Vitamin C can be used instead.<ref>{{#invoke:citation/CS1|citation |CitationClass=web }}</ref><ref name=":0" />
See alsoEdit
Many derivatives and analogues of sodium borohydride exhibit modified reactivity of value in organic synthesis.<ref>Seyden-Penne, J. (1991) Reductions by the Alumino- and Borohydrides in Organic Synthesis. VCH–Lavoisier: Paris. p. 9. Template:ISBN</ref>
- 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.