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Concept
Tétrahydruroborate de sodium
Résumé
Sodium borohydride, also known as sodium tetrahydridoborate and sodium tetrahydroborate, is an inorganic compound with the formula (sometimes written as ). 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.
The compound was discovered in the 1940s by H. I. Schlesinger, who led a team seeking volatile uranium compounds. Results of this wartime research were declassified and published in 1953.
The compound is soluble in alcohols, certain ethers, and water, although it slowly hydrolyzes.
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. Sodium borohydride is soluble in protic solvents such as water and lower alcohols. It also reacts with these protic solvents to produce ; however, these reactions are fairly slow. Complete decomposition of a methanol solution requires nearly 90 min at 20 °C. It decomposes in neutral or acidic aqueous solutions, but is stable at pH 14.
is a salt, consisting of the tetrahedral anion. The solid is known to exist as three polymorphs: α, β and γ. The stable phase at room temperature and pressure is α-, which is cubic and adopts an NaCl-type structure, in the Fmm space group. At a pressure of 6.3 GPa, the structure changes to the tetragonal β- (space group P421c) and at 8.9 GPa, the orthorhombic γ- (space group Pnma) becomes the most stable.
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 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 ) and trimethyl borate at 250–270 °C:
Millions of kilograms are produced annually, far exceeding the production levels of any other hydride reducing agent.
Source officielle
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