|CAS number||, (2D4)|
|PubChem||, (2D4) , (3T4)|
|ChemSpider||, (2D4) , (3T4)|
|Jmol-3D images||Image 1|
|Molar mass||37.83 g/mol|
|Melting point||400 °C|
|Boiling point||500 °C (dec.)|
|Solubility in water||soluble, reacts with water|
|Solubility||soluble in liquid ammonia, amines, pyridine|
|Flash point||70 °C (158 °F; 343 K)|
|Autoignition temperature||ca. 220 °C|
|Other anions||Sodium cyanoborohydride
|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 (77 °F), 100 kPa)|
|(what is: / ?)|
Sodium borohydride, also known as sodium tetrahydridoborate, and sodium tetrahydroborate is an inorganic compound with the formula NaBH4. This white solid, usually encountered as a powder, is a versatile reducing agent that finds wide application in chemistry, both in the laboratory and on a technical scale. Large amounts are used for bleaching wood pulp. The compound is soluble in alcohols and certain ethers but reacts with water in the absence of a base.
Sodium borohydride is an odorless white to gray-white microcrystalline powder which often forms lumps. It is soluble in water, but will decompose rapidly in neutral or acidic solutions (only stable at pH 14). The salt can be recrystallized by dissolving in warm (50 °C) diglyme followed by cooling the solution.
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.
Synthesis and handling
- 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. Sodium borohydride can also be produced by the action of NaH on powdered borosilicate glass.
NaBH4 will reduce many organic carbonyls, depending on the precise conditions. Most typically, it is used in the laboratory for converting ketones and aldehydes to alcohols. It will reduce acyl chlorides, thiol esters and imines. Under typical conditions, it will not reduce esters, amides, or carboxylic acids. At room temperature, the only acid derivatives it reduces are acyl chlorides, which are exceptionally electrophilic.
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.
The reactivity of NaBH4 can be enhanced or augmented by a variety of compounds. Oxidation with iodine in tetrahydrofuran gives the BH3-THF complex, which can reduce carboxylic acids. Likewise, the NaBH4-MeOH system, formed by the addition of methanol to sodium borohydride in refluxing THF, reduces esters to the corresponding alcohols. 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 will eventually decompose spontaneously to give hydrogen gas and borates. The same reaction can run also intramolecularly: an α-ketoester converts into a diol, since the alcohol produced will attack the borohydride to produce an ester of the borohydride, which then reduces the neighboring ester. The combination of NaBH4 with carboxylic acids results in the formation of acyloxyborohydride species. These can perform a variety of reductions not normally associated with borohydride chemistry, such as alcohols to hydrocarbons and nitriles to primary amines.
- 2 (C5H5)2TiCl2 + 4 NaBH4 → 2 (C5H5)2TiBH4 + 4 NaCl + B2H6 + H2
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)
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.
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. Sodium borohydride is used as reducing agent in the synthesis of gold nano particles.
- MSDS data (carl roth)
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- 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 (American Chemical Society). doi:10.1021/nn3030018. Retrieved 20 August 2012.
- Low and Bansal, 2009
- National Pollutant Inventory – Boron and compounds
- MSDS for Sodium Borohydride
- Materials & Energy Research Institute Tokyo, Ltd.
- Chemo- and stereoselectivity using Borohydride reagents