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Sodium borohydride
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Sodium borohydride, also known as sodium tetrahydroborate, 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.
The compound was discovered in the 1940s by H. I. Schlessinger, who led a team that developed metal borohydrides for wartime applications.
um borohydride is an odorless white to gray-white microcrystalline powder which often forms lumps.
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Encyclopedia
Sodium borohydride, also known as sodium tetrahydroborate, 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.
The compound was discovered in the 1940s by H. I. Schlessinger, who led a team that developed metal borohydrides for wartime applications.
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.
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.
NaBH4 can be recrystallized by dissolving in warm (50 °C) diglyme followed by cooling the solution.
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. An example of the use of sodium borohydride is the industrial production of fexofenadine which includes a reduction step:
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Related reagents
The activity of borohydride reducing agents can be attenuated by stabilizing the boron-hydride bond. This is done through attaching sterically bulky groups or electron withdrawing groups. Acetyl (sodium triacetoxyborohydride, (NaBH(OCOCH3)3) or cyano (sodium cyanoborohydride (NaCNBH3)) groups have been used for such a purpose. As a result, they find use in the reductive amination reaction. Here, the imine intermediate must be selectively reduced in the presence of the aldehyde or ketone starting material.
Stronger reducing agents can be generated by destabilising the boron-hydride bond. This is found in compounds such as superhydride (Lithium triethylborohydride) and L-Selectride (lithium tri-sec-butylborohydride).
Other reactions
Oxidation of NaBH4 with iodine in tetrahydrofuran creates the BH3-THF complex, which can reduce esters. 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.
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
Health and safety
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
External links
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