Sodium borohydride, also known as
sodium tetrahydridoborate, is an
inorganic compoundInorganic compounds have traditionally been considered to be of inanimate, non-biological origin. In contrast, organic compounds have an explicit biological origin. However, over the past century, the classification of inorganic vs organic compounds has become less important to scientists,...
with the
formulaA chemical formula or molecular formula is a way of expressing information about the atoms that constitute a particular chemical compound....
NaSodium is a chemical element with the symbol Na and atomic number 11. It is a soft, silvery-white, highly reactive metal and is a member of the alkali metals; its only stable isotope is 23Na. It is an abundant element that exists in numerous minerals, most commonly as sodium chloride...
BBoron is the chemical element with atomic number 5 and the chemical symbol B. Boron is a metalloid. Because boron is not produced by stellar nucleosynthesis, it is a low-abundance element in both the solar system and the Earth's crust. However, boron is concentrated on Earth by the...
HHydrogen is the chemical element with atomic number 1. It is represented by the symbol H. With an average atomic weight of , hydrogen is the lightest and most abundant chemical element, constituting roughly 75% of the Universe's chemical elemental mass. Stars in the main sequence are mainly...
4. This white solid, usually encountered as a powder, is a versatile
reducing agentA reducing agent is the element or compound in a reduction-oxidation reaction that donates an electron to another species; however, since the reducer loses an electron we say it is "oxidized"...
that finds wide application in chemistry, both in the laboratory and on a technical scale. Large amounts are used for bleaching
wood pulpPulp is a lignocellulosic fibrous material prepared by chemically or mechanically separating cellulose fibres from wood, fibre crops or waste paper. Wood pulp is the most common raw material in papermaking.-History:...
. The compound is insoluble in
etherDiethyl ether, also known as ethyl ether, simply ether, or ethoxyethane, is an organic compound in the ether class with the formula . It is a colorless, highly volatile flammable liquid with a characteristic odor...
, and soluble in glyme solvents, methanol and water, but reacts with the latter two in the absence of base.
The compound was discovered in the 1940s by
H. I. SchlesingerHermann Irving Schlesinger was an American inorganic chemist, working in boron chemistry.He and Herbert C. Brown discovered sodium borohydride in 1940 and both were involved in further development of borohydride chemistry....
, who led a team that developed metal borohydrides for wartime applications. Their work was classified and published only in 1953.
Physical properties
Sodium borohydride is an odorless white to gray-white
microcrystallineA microcrystalline material is a crystallized substance or rock that contains small crystals visible only through microscopic examination.-See also:* Macrocrystalline* Microcrystalline silicon* Protocrystalline* Rock microstructure...
powder which often forms lumps. It is soluble in water, without decomposition, however it reacts vigorously with acid solutions.
Structure
NaBH
4 has three known
polymorphsPolymorphism in materials science is the ability of a solid material to exist in more than one form or crystal structure. Polymorphism can potentially be found in any crystalline material including polymers, minerals, and metals, and is related to allotropy, which refers to chemical elements...
:
α,
β and
γ. The stable phase at room temperature and pressure is
α-NaBH
4, which is cubic and adopts an NaCl-type structure, in the
Fm3m space groupIn mathematics and geometry, a space group is a symmetry group, usually for three dimensions, that divides space into discrete repeatable domains.In three dimensions, there are 219 unique types, or counted as 230 if chiral copies are considered distinct...
. At a pressure of 6.3 GPa, the structure changes to the tetragonal
β-NaBH
4 (space group
P421c) and at 8.9 GPa, the orthorhombic
γ-NaBH
4 (space group
Pnma) becomes the most stable.
Synthesis and handling
Sodium borohydride is prepared by two routes of industrial significance. In one method, based on the original work of Schlesinger,
sodium hydrideSodium hydride is the chemical compound with the empirical formula NaH. It is primarily used as a strong base in organic synthesis. NaH is representative of the saline hydrides, meaning it is a salt-like hydride, composed of Na+ and H− ions, in contrast to the more molecular hydrides such as...
is treated with
trimethyl borateTrimethyl borate, or boron trimethoxide, has formula B3. It is a clear liquid. It melts at −34 °C and boils at 68-69 °C. It decomposes in contact with water...
at 250-270 °C:
- B(OCH3)3 + 4 NaH → NaBH4 + 3 NaOCH3
Alternatively, sodium borohydride is also produced by the action of NaH on powdered
borosilicate glassBorosilicate glass is a type of glass with the main glass-forming constituents silica and boron oxide. Borosilicate glasses are known for having very low coefficients of thermal expansion , making them resistant to thermal shock, more so than any other common glass...
. Millions of kilograms are produced annually, far exceeding the production levels of any other hydride reducing agent.
NaBH
4 can be recrystallized by dissolving in warm (50 °C)
diglymeDiglyme, or bis ether, is a solvent with a high boiling point. It is an organic compound which is the dimethyl ether of diethylene glycol. It is a clear, colorless liquid with a slight ether-like odor...
followed by cooling the solution.
Reactivity
NaBH
4 will
reduce many organic carbonylsCarbonyl reduction in organic chemistry is the organic reduction of any carbonyl group containing compound by a reducing agent. Typical carbonyl compounds are ketones , aldehydes, carboxylic acids and esters. A carbonyl group can be reduced to the alcohol or the oxygen atom can be removed...
, depending on the precise conditions. Most typically, it is used in the laboratory for converting ketones and aldehydes to alcohols. It will reduce
acyl chlorideIn organic chemistry, an acyl chloride is an organic compound with the functional group -CO-Cl. Their formula is usually written RCOCl, where R is a side chain. They are usually considered to be reactive derivatives of carboxylic acids. A specific example of an acyl chloride is acetyl chloride,...
s, thiol esters and
imineAn imine is a functional group or chemical compound containing a carbon–nitrogen double bond, with the nitrogen attached to a hydrogen atom or an organic group. If this group is not a hydrogen atom, then the compound is known as a Schiff base...
s. However, unlike the powerful reducing agent
lithium aluminium hydrideLithium aluminium hydride, commonly abbreviated to LAH or known as LithAl, is an inorganic compound with the chemical formula LiAlH4. It was discovered by Finholt, Bond and Schlesinger in 1947. This compound is used as a reducing agent in organic synthesis, especially for the reduction of esters,...
, NaBH
4 typically will not reduce
esterEsters are chemical compounds derived by reacting an oxoacid with a hydroxyl compound such as an alcohol or phenol. Esters are usually derived from an inorganic acid or organic acid in which at least one -OH group is replaced by an -O-alkyl group, and most commonly from carboxylic acids and...
s,
amideIn chemistry, an amide is an organic compound that contains the functional group consisting of a carbonyl group linked to a nitrogen atom . The term refers both to a class of compounds and a functional group within those compounds. The term amide also refers to deprotonated form of ammonia or an...
s, or
carboxylic acidCarboxylic acids are organic acids characterized by the presence of at least one carboxyl group. The general formula of a carboxylic acid is R-COOH, where R is some monovalent functional group...
s. At room temperature, the only acid derivatives it reduces are
acyl chlorideIn organic chemistry, an acyl chloride is an organic compound with the functional group -CO-Cl. Their formula is usually written RCOCl, where R is a side chain. They are usually considered to be reactive derivatives of carboxylic acids. A specific example of an acyl chloride is acetyl chloride,...
s, which have exceptionally high reactivity. With high temperature and concomitant high pressure, sodium borohydride can be forced to react with esters, for example.
Many other hydride reagents are more strongly reducing. These usually involve replacing hydride with alkyl groups, such as
lithium triethylborohydrideLithium triethylborohydride , commonly abbreviated to LiTEBH or Superhydride, is a powerful and selective reducing agent used in inorganic and organic chemistry. LiTEBH is far more powerful than lithium borohydride and more powerful than lithium aluminium hydride in many cases...
and
L-SelectrideL-selectride is an organoborane. It is used in organic chemistry as a reducing agent, for example in the reduction of a ketone, as part of Overman's synthesis of strychnine....
(lithium tri-
sec-butylborohydride), or replacing B with Al. Variations in the counterion also affect the reactivity of the borohydride.
Oxidation of NaBH
4 with
iodineIodine is a chemical element with the symbol I and atomic number 53. The name is pronounced , , or . The name is from the , meaning violet or purple, due to the color of elemental iodine vapor....
in
tetrahydrofuranTetrahydrofuran is a colorless, water-miscible organic liquid with low viscosity at standard temperature and pressure. This heterocyclic compound has the chemical formula 4O. As one of the most polar ethers with a wide liquid range, it is a useful solvent. Its main use, however, is as a precursor...
gives the
BH3Diborane is the chemical compound consisting of boron and hydrogen with the formula B2H6. It is a colorless gas at room temperature with a repulsively sweet odor. Diborane mixes well with air, easily forming explosive mixtures. Diborane will ignite spontaneously in moist air at room temperature...
-
THFTetrahydrofuran is a colorless, water-miscible organic liquid with low viscosity at standard temperature and pressure. This heterocyclic compound has the chemical formula 4O. As one of the most polar ethers with a wide liquid range, it is a useful solvent. Its main use, however, is as a precursor...
complex, which can reduce carboxylic acids. Likewise, the NaBH
4-MeOH system, formed by the addition of
methanolMethanol, also known as methyl alcohol, wood alcohol, wood naphtha or wood spirits, is a chemical with the formula CH3OH . It is the simplest alcohol, and is a light, volatile, colorless, flammable liquid with a distinctive odor very similar to, but slightly sweeter than, ethanol...
to sodium borohydride in refluxing THF, reduces esters to the corresponding alcohols, for instance,
benzyl benzoateBenzyl benzoate is the ester of benzyl alcohol and benzoic acid, with the formula C6H5CH2O2CC6H5. This easily prepared compound has a variety of uses.-Synthesis:This colorless liquid is formally the condensation product of benzoic acid and benzyl alcohol...
to
benzyl alcoholBenzyl alcohol is an organic compound with the formula C6H5CH2OH. The benzyl group is often abbreviated "Bn", thus benzyl alcohol is denoted as BnOH. Benzyl alcohol is a colorless liquid with a mild pleasant aromatic odor. It is a useful solvent due to its polarity, low toxicity, and low vapor...
. 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.
Aqueous solutions of sodium borohydride are decomposed by catalytic amounts of
cobaltCobalt is a chemical element with symbol Co and atomic number 27. It is found naturally only in chemically combined form. The free element, produced by reductive smelting, is a hard, lustrous, silver-gray metal....
(II) ions to yield sodium borate and hydrogen gas. Pellets of cobalt-doped sodium borohydride are commercially available for use in hydrogen generators, for applications where cylinders of hydrogen would be inconvenient.
BH
4− is an excellent
ligandIn coordination chemistry, a ligand is an ion or molecule that binds to a central metal atom to form a coordination complex. The bonding between metal and ligand generally involves formal donation of one or more of the ligand's electron pairs. The nature of metal-ligand bonding can range from...
for metal ions. Such borohydride complexes are often prepared by the action of NaBH
4 (or the LiBH
4) on the corresponding metal halide. One example is the titanocene derivative:
- 2 (C5H5)2TiCl2 + 4 NaBH4 → 2 (C5H5)2TiBH4 + 4 NaCl + B2H6 + H2
Combustion
Sodium borohydride 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 + 2 H2O → NaBO2 + 4 H2 (ΔH < 0)
Applications
The principle application of sodium borohydride is the production of
sodium dithioniteSodium dithionite is a white crystalline powder with a weak sulfurous odor. It is a sodium salt of dithionous acid. Although it is stable under most conditions, it will decompose in hot water and in acid solutions...
, which is used as a bleaching agent for wood pulp. Sulfur dioxide reacts with the borohydride. In a related process, sodium dithionite is used in the dyeing industry. Sodium borohydride can also be used in
oxymercuration reactionsThe oxymercuration reaction is an electrophilic addition organic reaction that transforms an alkene into a neutral alcohol. In oxymercuration, the alkene reacts with mercuric acetate in aqueous solution to yield the addition of an acetoxymercuri group and a hydroxy group across the double bond...
.
Production of pharmaceuticals
Sodium borohydride reduces
aldehydeAn aldehyde is an organic compound containing a formyl group. This functional group, with the structure R-CHO, consists of a carbonyl center bonded to hydrogen and an R group....
s and
ketoneIn organic chemistry, a ketone is an organic compound with the structure RCR', where R and R' can be a variety of atoms and groups of atoms. It features a carbonyl group bonded to two other carbon atoms. Many ketones are known and many are of great importance in industry and in biology...
s into
alcoholIn chemistry, an alcohol is an organic compound in which the hydroxy functional group is bound to a carbon atom. In particular, this carbon center should be saturated, having single bonds to three other atoms....
s. This reaction is used in the production of various antibiotics including
chloramphenicolChloramphenicol is a bacteriostatic antimicrobial that became available in 1949. It is considered a prototypical broad-spectrum antibiotic, alongside the tetracyclines, and as it is both cheap and easy to manufacture it is frequently found as a drug of choice in the third world.Chloramphenicol is...
,
dihydrostreptomycinDihydrostreptomycin is an antibiotic....
,
and thiophenicol. Various steroids and
vitamin AVitamin A is a vitamin that is needed by the retina of the eye in the form of a specific metabolite, the light-absorbing molecule retinal, that is necessary for both low-light and color vision...
are prepared using sodium borohydride in at least one step.
Safety
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
dimethylformamideDimethylformamide is an organic compound with the formula 2NCH. Commonly abbreviated as DMF , this colourless liquid is miscible with water and the majority of organic liquids. DMF is a common solvent for chemical reactions...
.
External links