Sodium borohydride

Sodium borohydride
Wireframe model of sodium borohydride
Names
IUPAC name
Sodium tetrahydridoborate(1–)
Systematic IUPAC name
Sodium boranuide
Identifiers
3D model (JSmol)
ChEBI
ChemSpider
ECHA InfoCard 100.037.262 Edit this at Wikidata
EC Number
  • 241-004-4
23167
MeSH Sodium+borohydride
RTECS number
  • ED3325000
UNII
UN number 1426
  • InChI=1S/BH4.Na/h1H4;/q-1;+1 checkY
    Key: YOQDYZUWIQVZSF-UHFFFAOYSA-N checkY
  • InChI=1S/BH4.Na/h1H4;/q-1;+1
  • Key: YOQDYZUWIQVZSF-UHFFF
  • [Na+].[BH4-]
Properties
Na[BH4]
Molar mass 37.83 g·mol−1
Appearance white crystals
hygroscopic
Density 1.07 g/cm3
Melting point 400 °C (752 °F; 673 K)(decomposes)
550 g/L
Solubility soluble in liquid ammonia, amines, pyridine
Structure
Cubic (NaCl), cF8
Fm3m, No. 225
a = 0.6157 nm
Thermochemistry
86.8 J·mol−1·K−1
101.3 J·mol−1·K−1
−188.6 kJ·mol−1
−123.9 kJ·mol−1
Hazards
GHS labelling:
GHS02: FlammableGHS06: ToxicGHS08: Health hazardGHS05: Corrosive
Danger
H260, H301, H314, H360F
P201, P231+P232, P280, P308+P313, P370+P378, P402+P404
NFPA 704 (fire diamond)
NFPA 704 four-colored diamondHealth 3: Short exposure could cause serious temporary or residual injury. E.g. chlorine gasFlammability 1: Must be pre-heated before ignition can occur. Flash point over 93 °C (200 °F). E.g. canola oilInstability 2: Undergoes violent chemical change at elevated temperatures and pressures, reacts violently with water, or may form explosive mixtures with water. E.g. white phosphorusSpecial hazard W: Reacts with water in an unusual or dangerous manner. E.g. sodium, sulfuric acid
3
1
2
Flash point 70 °C (158 °F; 343 K)
ca. 220 °C (428 °F; 493 K)
Explosive limits 3%
Lethal dose or concentration (LD, LC):
160 mg/kg (Oral – Rat)
230 mg/kg (Dermal – Rabbit)
Related compounds
Other anions
Sodium cyanoborohydride
Sodium hydride
Sodium borate
Borax
Sodium aluminum hydride
Other cations
Lithium borohydride
Related compounds
Lithium aluminium hydride
Sodium triacetoxyborohydride
Except where otherwise noted, data are given for materials in their standard state (at 25 °C [77 °F], 100 kPa).
checkY verify (what is checkY☒N ?)
Infobox references

Sodium borohydride, also known as sodium tetrahydridoborate and sodium tetrahydroborate, is an inorganic compound with the formula NaBH4 (sometimes written as Na[BH4]). 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.

Properties

The compound is soluble in alcohols, certain ethers, and water, although it slowly hydrolyzes.

Solvent Solubility (g/(100 mL))
CH3OH 13
CH3CH2OH 3.16
Diglyme 5.15
(CH3CH2)2O insoluble

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 H2; 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.

Structure

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.

Synthesis and handling

For commercial NaBH4 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 H2) and trimethyl borate at 250–270 °C:

B(OCH3)3 + 4 NaH → NaBH4 + 3 NaOCH3

Millions of kilograms are produced annually, far exceeding the production levels of any other hydride reducing agent. In the Bayer process, it is produced from inorganic borates, including borosilicate glass and borax (Na2B4O7):

Na2B4O7 + 16 Na + 8 H2 + 7 SiO2 → 4 NaBH4 + 7 Na2SiO3

Magnesium is a less expensive reductant, and could in principle be used instead:

8 MgH2 + Na2B4O7 + Na2CO3 → 4 NaBH4 + 8 MgO + CO2

and

2 MgH2 + NaBO2 → NaBH4 + 2 MgO

Reactivity

Organic synthesis

NaBH4 reduces many organic carbonyls, depending on the conditions. Most typically, it is used in the laboratory for converting ketones and aldehydes to alcohols. These reductions proceed in two stages, formation of the alkoxide followed by hydrolysis:

NaBH4 + 4 R2C=O → NaO−CHR2 + B(O−CHR2)3
NaO−CHR2 + B(O−CHR2)3 + 4 H2O → 4 HO−CHR2 + NaOH + B(OH)3

It also efficiently reduces acyl chlorides, anhydrides, α-hydroxylactones, thioesters, and imines at room temperature or below. It reduces esters slowly and inefficiently with excess reagent and/or elevated temperatures, while carboxylic acids and amides are not reduced at all.

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. It 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.

Many additives for modifying the reactivity of sodium borohydride have been developed as indicated by the following incomplete listing.

Additives for sodium borohydride
additive synthetic applications page in Smith and March comment
AlCl3 reduction of ketones to methylene 1837
BiCl3 converts epoxides to allylic alcohols 1316
(C6H5Te)2 reduction of nitroarenes 1862
CeCl3 reduction of ketones in the presence of aldehydes 1794 Luche reduction
CoCl2 reduction of azides to amines 1822
InCl3 hydrogenolysis of alkyl bromides, double reduction of unsaturated ketones 1825, 1793
LiCl amine oxides to amines 1846 lithium borohydride
NiCl2 deoxygenation of sulfoxides, hydrogenolysis of aryl tosylates, desulfurization, reduction of nitriles 1851,1831, 991, 1814 nickel boride
TiCl4 denitrosatation of nitrosamines 1823
ZnCl2 reduction of aldehydes 1793
ZrCl4 reduction of disulfides, reduction of azides to amines, cleavage of allyl aryl ethers 1853, 1822, 582

Oxidation

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

Partial oxidation of borohydride with iodine gives octahydrotriborate:

3 [BH4] + I2 → [B3H8] + 2 H2 + 2 I

Coordination chemistry

[BH4] is a ligand for metal ions. Such borohydride complexes are often prepared by the action of NaBH4 (or the LiBH4) on the corresponding metal halide. One example is the titanocene derivative:

2 (C5H5)2TiCl2 + 4 NaBH4 → 2 (C5H5)2TiBH4 + 4 NaCl + B2H6 + H2

Protonolysis and hydrolysis

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. Exploiting this reactivity, sodium borohydride has been studied as a prototypes of the direct borohydride fuel cell.

NaBH4 + 2 H2O → NaBO2 + 4 H2 (ΔH < 0)

Applications

Paper manufacture

The dominant 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.

It has been tested as pretreatment for pulping of wood, but is too costly to be commercialized.

Chemical synthesis

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.

Niche or abandoned applications

Sodium borohydride has been considered as a way to store hydrogen for hydrogen-fueled vehicles, as it is safer (being stable in dry air) and more efficient on a weight basis than most other alternatives. The hydrogen can be released by simple hydrolysis of the borohydride. However, such a usage would need a cheap, relatively simple, and energy-efficient process to recycle the hydrolysis product, sodium metaborate, back to the borohydride. No such process was available as of 2007.

Although practical temperatures and pressures for hydrogen storage have not been achieved, in 2012 a core–shell nanostructure of sodium borohydride was used to store, release and reabsorb hydrogen under moderate conditions.

Skilled professional conservator/restorers have used sodium borohydride to minimize or reverse foxing in old books and documents.

See also

Many derivatives and analogues of sodium borohydride exhibit modified reactivity of value in organic synthesis.

  • Sodium triacetoxyborohydride, a milder reductant owing to the presence of more electron-withdrawing acetate in place of hydride.
  • Sodium triethylborohydride, a stronger reductant owing to the presence of electron-donating ethyl groups in place of hydride.
  • sodium cyanoborohydride, a milder reductant owing to the presence of more electron-withdrawing cyanide in place of hydride. Useful for reductive aminations.

This page was last updated at 2024-04-18 05:12 UTC. Update now. View original page.

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