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Sodium Borohydride

CAS Number:16940-66-2
Molecular Formula:37.83
Molecular Weight:NaBH4

Sodium borohydride, also known as sodium tetrahydridoborate and sodium tetrahydroborate, is an inorganic compound with the formula NaBH4.
Sodium borohydride white solid, usually encountered as a powder, is a reducing agent that finds application in chemistry, both in the laboratory and on an industrial scale.
Sodium borohydride has been tested as pretreatment for pulping of wood, but is too costly to be commercialized.
The compound is soluble in alcohols, certain ethers, and water, although it slowly hydrolyzes.
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.

-Uses-
The primary use of NaBH4 is in the preparation of Na2S2O4 (sodium dithionite, which is a bleaching agent for wood pulp) from SO2.
Aldehydes & ketones can be converted into the corresponding alcohols with the help of this compound.
Several antibiotics such as thiophenicol, dihydrostreptomycin, and chloramphenicol are produced by employing NaBH4 as a reducing agent.
It can also be used to reduce foxing in aged documents and books.

-Preperation-
Sodium borohydride can be prepared on an industrial scale by treating trimethyl borate with sodium hydride at a temperature range of 250-27oC.

The balanced chemical equation for this reaction is given by:
4NaH + B(OCH3)3 → 3NaOCH3 + NaBH4

Alternately, this compound can also be prepared by reacting borax, metallic sodium, dihydrogen, and silicon dioxide at a temperature of 700oC. This reaction can be represented as follows:
Na2B4O7 + 8H2 + 16Na + 7SiO2 → 7Na2SiO3 + 4NaBH4

-Structure and propreties-
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.
Sodium borohydride is an odorless white to gray-white microcrystalline powder that often forms lumps. Sodium borohydride can be purified by recrystallization from warm (50 °C) diglyme.
Sodium borohydride is soluble in protic solvents such as water and lower alcohols.
Sodium borohydride also reacts with these protic solvents to produce H2; however, these reactions are fairly slow. Complete decomposition of a methanol solution requires nearly 90 min at 20 °C.
Sodium borohydride decomposes in neutral or acidic aqueous solutions, but is stable at pH 14.
The molar mass of NaBH4 is 37.83 grams per mole.
Sodium borohydride does not have any characteristic odour.
The density of sodium borohydride at STP corresponds to 1.07 grams per cubic centimetre.
Sodium borohydride has a melting point of 673K. However, it tends to undergo decomposition at this temperature.
Despite being soluble in most protic solvents (like water), sodium borohydride slowly reacts with the protic solvent, resulting in the formation of dihydrogen.
Sodium borohydride generally undergoes decomposition in acidic and aqueous media but not in basic media.
Sodium borohydride is a reducing agent and can reduce a wide spectrum of organic carbonyls.
NaBH4 also releases hydrogen when exposed to many metal catalysts.

Molecular Weight: 37.84
Hydrogen Bond Donor Count: 0
Hydrogen Bond Acceptor Count: 1
Rotatable Bond Count: 0
Exact Mass: 38.0303746
Monoisotopic Mass: 38.0303746
Topological Polar Surface Area: 0 Å²
Heavy Atom Count: 2
Formal Charge: 0
Complexity: 2
Isotope Atom Count: 0
Defined Atom Stereocenter Count: 0
Undefined Atom Stereocenter Count: 0
Defined Bond Stereocenter Count: 0
Undefined Bond Stereocenter Count: 0
Covalently-Bonded Unit Count: 2
Compound Is Canonicalized   : Yes
InChI: 1S/BH4.Na/h1H4;/q-1;+1
InChI key: YOQDYZUWIQVZSF-UHFFFAOYSA-N

-Reactivity-
NaBH4 reduces many organic carbonyls, depending on the precise conditions.
Most typically, it is used in the laboratory for converting ketones and aldehydes to alcohols.
Sodium borohydride efficiently reduces acyl chlorides, anhydrides, α-hydroxylactones, thioesters, and imines at room temperature or below.
Sodium borohydride reduces esters slowly and inefficiently with excess reagent and/or elevated temperatures, while carboxylic acids and amides are not reduced at all.
NaBH4 reacts with water and alcohols, with evolution of hydrogen gas and formation of the corresponding borate, the reaction being especially fast at low pH.
Nevertheless, an alcohol, often methanol or ethanol, is generally the solvent of choice for sodium borohydride reductions of ketones and aldehydes.
The mechanism of ketone and aldehyde reduction has been scrutinized by kinetic studies, and contrary to popular depictions in textbooks, the mechanism does not involve a 4-membered transition state like alkene hydroboration, or a six-membered transition state involving a molecule of the alcohol solvent.
Hydrogen-bonding activation is required, as no reduction occurs in an aprotic solvent like diglyme.
However, the rate order in alcohol is 1.5, while carbonyl compound and borohydride are both first order, suggesting a mechanism more complex than one involving a six-membered transition state that includes only a single alcohol molecule.
Sodium borohydride was suggested that the simultaneous activation of the carbonyl compound and borohydride occurs, via interaction with the alcohol and alkoxide ion, respectively, and that the reaction proceeds through an open transition state.
α,β-Unsaturated ketones tend to be reduced by NaBH4 in a 1,4-sense, although mixtures are often formed.
Addition of cerium chloride improves the selectivity for 1,2-reduction of unsaturated ketones (Luche reduction).
α,β-Unsaturated esters also undergo 1,4-reduction in the presence of NaBH4.
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 eventually decomposes spontaneously to produce hydrogen gas and borates.
The same reaction can also occur intramolecularly: an α-ketoester converts into a diol, since the alcohol produced attacks the borohydride to produce an ester of the borohydride, which then reduces the neighboring ester.
The reactivity of NaBH4 can be enhanced or augmented by a variety of compounds.

-Oxidation-
Oxidation with iodine in tetrahydrofuran gives borane–tetrahydrofuran, which can reduce carboxylic acids.

Partial oxidation of borohydride with iodine gives octahydrotriborate:
3 BH4− + I2 → B3H8− + 2 H2 + 2 I−

-History-
Sodium borohydride (NaBH4) is one of the handiest reducing agents, especially for organic compounds in nonaqueous solvents. It was introduced in the 1940s for wartime applications and was an important enough reagent by 1970 to warrant a lengthy review.
Sodium borohydride has been used to reduce aldehydes, ketones, Schiff bases, carboxylic acids and esters, acid chlorides, disulfides, nitriles, and inorganic anions.
For many of these substrates, only gentle reaction conditions are required.

-Processing of Sodium Borohydride-
Sodium borohydride (NaBH4) was synthesized by reacting sodium metaborate (NaBO2) with magnesium hydride (MgH2) or magnesium silicide (Mg2Si) by annealing (350–750°C) under high H2 pressure (0.1–7MPa) for 2–4h.
As the temperature and the pressure increased, the yield increased to have a maximum value (97–98%) at 550°C under 7MPa, but the value was independent of time.
A concept for converting NaBO2 back to NaBH4 using coke or methane is described.

-Future Role of Sodium Borohydride-
In a time of unprecedented change in environmental, geopolitical and socio-economic world affairs, the search for new energy materials has become a topic of great relevance.
Sodium borohydride, NaBH4, seems to be a promising fuel in the context of the future hydrogen economy.
NaBH4 belongs to a class of materials with the highest gravimetric hydrogen densities, which has been discovered in the 1940s by Schlesinger and Brown.
In the present paper, the most relevant issues concerning the use of NaBH4 are examined.
Its basic properties are summarised and its synthesis methods are described.
The general processes of NaBH4 oxidation, hydrolysis, and monitoring are reviewed.
A comprehensive bibliometric analysis of the NaBH4 publications in the energy field opens the discussion for current perspectives and future outlook of NaBH4 as an efficient energy/hydrogen carrier.
Despite the observed exponential increase in the research on NaBH4 it is clear that further efforts are still necessary for achieving significant overchanges.

-Safety-
Sodium borohydride is a source of hydrogen or diborane, which are both flammable.
Spontaneous ignition can result from solution of sodium borohydride in dimethylformamide.
Bulk solutions of sodium borohydride are often prepared with excess sodium hydroxide, which is corrosive.

-Application-
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 can be used to reduce foxing in old books and documents.
This treatment should only be done by a skilled professional conservator/restorer as damage to paper can ensue if the reducing agent is not applied correctly, such as over bleaching and bubbling of paper.

-Manufacturing-
Prepared from methyl borate and sodium hydride at elevated temperatures.
Manufactured from sodium hydride & trimethyl borate in a mineral oil medium at 275 °C

-Synonyms-
sodium tetrahydroborate