Sodium borohydride

Sodium borohydride
IUPAC name	Sodium tetrahydridoborate(1–)
Identifiers
CAS number	16940-66-2 Y
PubChem	22959485
UN number	1426
RTECS number	ED3325000
Properties
Molecular formula	NaBH4
Molar mass	37.83 g/mol
Appearance	white crystals hygroscopic
Density	1.0740 g/cm3
Melting point	400 °C[1]
Boiling point	500 °C (dec.)[1]
Solubility in water	not soluble, reacts with water
Solubility	soluble in liquid ammonia, amines, pyridine
Hazards
MSDS	ICSC 1670
NFPA 704	1 2 2 W
Flash point	70 °C
Autoignition temperature	ca. 220 °C
LD50	160 mg/kg
Related compounds
Other anions	Sodium cyanoborohydride Sodium hydride Sodium borate Borax
Other cations	Lithium borohydride
Related compounds	Lithium aluminium hydride
Y (what is this?)  (verify) Except where noted otherwise, data are given for materials in their standard state (at 25 °C, 100 kPa)
Infobox references

Sodium borohydride, also known as sodium tetrahydridoborate, has the chemical formula NaBH₄. This white solid, usually encountered as a powder, is a specialty reducing agent used in the manufacture of pharmaceuticals and other organic and inorganic compounds. It is soluble in methanol and water, but reacts with both in the absence of base.^[2]

The compound was discovered in the 1940s by H. I. Schlesinger, who led a team that developed metal borohydrides for wartime applications.^[3]

Physical properties

Sodium borohydride is an odorless white to gray-white microcrystalline powder which often forms lumps. It is soluble in water, with which it reacts vigorously.

Structure

NaBH₄ has three known 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.^[4][5]

α-NaBH4	β-NaBH4	γ-NaBH4

Synthesis and handling

Sodium borohydride is prepared by the reaction of sodium hydride on trimethylborate at 250-270°C:

B(OCH₃)₃ + 4 NaH → NaBH₄ + 3 NaOCH₃

It can also be generated by the action of NaH on powdered borosilicate glass.^[6]

NaBH₄ can be recrystallized by dissolving in warm (50 °C) diglyme followed by cooling the solution.^[7]

Reactivity

Organic synthetic applications

Sodium borohydride reduces aldehydes and ketones into alcohols, as well as the more reactive carboxylic acid derivatives acyl chlorides and thiol esters. However, unlike the powerful reducing agent lithium aluminium hydride, sole use of NaBH₄ with gentle reaction conditions will not reduce esters, amides, or carboxylic acids.^[8]

Stronger reducing agents can be generated by destabilizing the boron-hydride bond. This is found in compounds such as superhydride (Lithium triethylborohydride) and L-Selectride (lithium tri-sec-butylborohydride).^[9]

Other reactions

Ball-and-stick model of Zr(BH₄)₄

Oxidation of NaBH₄ with iodine in tetrahydrofuran creates 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, for instance, benzyl benzoate to benzyl alcohol.^[10]

BH₄⁻ is an excellent ligand for metal ions. Such borohydride complexes are often prepared by the action of NaBH₄ (or the LiBH₄) on the corresponding metal halide, e.g. Zr(BH₄)₄.

Fuel cells

Sodium borohydride is also used in experimental fuel cell systems. As a fuel it is less flammable and less volatile than gasoline but more corrosive. It is relatively environmentally friendly because of the low toxicity of borates. The hydrogen is generated for a fuel cell by catalytic decomposition of the aqueous borohydride solution:

NaBH₄ + 2H₂O → NaBO₂ + 4H₂ + heat

Health and safety

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Sodium borohydride is a particularly dangerous laboratory reagent. It is highly corrosive, and will cause burns upon contact with any area of the body. It is harmful if swallowed, inhaled or absorbed through the skin. It is highly flammable and will react with water.

Toxicity

Inhalation of sodium borohydride damages the mucous membranes and upper respiratory tract. Symptoms of inhalation include irritation of the mouth, nose and throat, and difficulty breathing. It may also cause lung edema, a potentially fatal medical emergency.

Ingestion of sodium borohydride can cause severe burns in the mouth, throat, and stomach. It can also cause sore throat, vomiting, and diarrhea. Severe skin irritation or burns can result from contact with wet sodium borohydride, or from contact with moist skin. Eye contact with sodium borohydride can cause blurred vision, redness, pain and severe tissue burns.

Reactivity

Sodium borohydride is highly reactive, and supports combustion. It is a flammable solid. It can ignite in air in the presence of an open flame, and will continue to burn as hydrogen is evolved. It can react with water and steam to produce hydrogen, which is flammable. An explosion can occur by spontaneous ignition of the gases released from a saturated solution of sodium borohydride in dimethylformamide at 17 °C.

See also

• Lithium borohydride

References

• Material Safety Data Sheet

External links

• National Pollutant Inventory - Boron and compounds
• Hydrogen Storage via Sodium Borohydride, Millennium Cell inc. (PDF)(no more available)
• Materials & Energy Research Institute Tokyo, Ltd.
• Chemo- and stereoselectivity using Borohydride reagents