# Sodium borohydride

Structural formula
General
Surname Sodium borohydride
other names
• Sodium boranate
• Sodium tetrahydroborate
• Sodium tetrahydridoborate
Molecular formula NaBH 4
Brief description

White dust

External identifiers / databases
 CAS number 16940-66-2 EC number 241-004-4 ECHA InfoCard 100.037.262 PubChem 4311764 ChemSpider 26189 Wikidata Q407895
properties
Molar mass 37.83 g mol −1
Physical state

firmly

density

1.07 g cm −3

Melting point

approx. 400 ° C

solubility

good in water with slow decomposition (550 g l −1 at 25 ° C)

safety instructions
GHS labeling of hazardous substances

danger

H and P phrases H: 260-301-314-360F
EUH: 014
P: 201-231 + 232-280-308 + 313-370 + 378-402 + 404
Toxicological data
Thermodynamic properties
ΔH f 0

−188.6 kJ / mol

As far as possible and customary, SI units are used. Unless otherwise noted, the data given apply to standard conditions .

Sodium borohydride is a complex salt consisting of a sodium cation (Na + ) and a complex tetrahydridoborate anion (BH 4 - ). The compound is a reducing agent and is widely used in organic chemistry .

## history

Sodium borohydride was discovered in 1942 by Hermann Irving Schlesinger's group at the University of Chicago as part of their work on the isolation of volatile uranium compounds. The later Nobel Prize winner Herbert Charles Brown played a key role in this work. During the Second World War , at the suggestion of the US Army Signal Corps , the same working group investigated the hydrolysis of sodium borohydride to generate hydrogen for military applications. For reasons of confidentiality, however, the research results on sodium borohydride were not published until 1953. In the 1960s, the first attempts to use sodium borohydride solutions in fuel cells were made.

## Presentation and extraction

Sodium borohydride is prepared from the alkali metal hydride sodium hydride (NaH) and boric acid trimethyl ester [B (OCH 3 ) 3 ]. The reaction equation is:

${\ displaystyle {\ ce {4NaH + B (OCH3) 3 -> NaBH4 + 3NaOCH3}}}$

Furthermore, the compound can be obtained industrially from borosilicate glass , sodium and hydrogen .

## properties

### Physical Properties

Sodium borohydride forms colorless, corrosive and flammable crystals. A melting point of 505 ° C can only be observed under a hydrogen pressure of 10 atm. The three polymorphic forms α-, β- and γ-sodium borohydride are known of the compound . At room temperature and normal pressure, the α-form that occurs in a cubic crystal lattice is the stable form. From a pressure of 6.3 GPa, the structure is converted into the tetragonal β-form and from 8.9 GPa into the orthorhombic γ-form. From aqueous solution, below a crystallized from 36.4 ° C dihydrate . The α- anhydrate form is stable above this temperature or in the absence of water . The compound dissolves very well in water. Below 36.4 ° C, the solubility curve is determined by the dihydrate form, above this it corresponds to the anhydrous anhydrate.

The solutions of sodium borohydride in water are basic. The pH value depends on the concentration and increases with increasing content.

 pH values ​​of sodium borohydride solutions at 24 ° C concentration in mol·l −1 0.01 0.1 1 PH value 9.56 ± 0.02 10.05 ± 0.02 10.48 ± 0.02

### Chemical properties

When heated, the connection is stable in dry air up to 600 ° C. In humid air, decomposition starts at 300 ° C. With water it is decomposed with hydrolysis and formation of elemental hydrogen, whereby one gram of substance gives 2.4 liters of hydrogen .

${\ displaystyle {\ ce {NaBH4 + 4H2O -> NaB (OH) 4 + 4H2}}}$

The rate of hydrolysis is strongly pH-dependent. A quick reaction only takes place at low pH values , so that the compound can also be used in aqueous and alcoholic solvents . The hydrolysis kinetics in the acidic as well as in the basic pH range follows a pseudo-first order time law. The pH and temperature dependency of the rate of hydrolysis can be calculated using an empirical formula

${\ displaystyle \ log _ {10} (t_ {1/2}) = pH- (0 {,} 034 \ cdot T-1 {,} 92)}$ (with t 1/2 in min and T in K)

be estimated. In acidic conditions, hydrolysis takes place almost spontaneously. In the strongly alkaline, the solution is stable in normal use.

 Half-life of hydrolysis at 25 ° C pH 4.0 5.0 5.5 6.0 7.0 8.0 9.0 10.0 11.0 12.0 13.0 14.0 t 1/2 3.7 ms 37 ms 0.12 s 0.37 s 3.7 s 36.8 s 6.1 min 61.4 min 10.2 h 4.3 d 42.6 d 426 d

Sodium borohydride is a powerful reducing agent . The reaction with metal ions can lead to either the reduction of the metal, the formation of metal borides or the formation of volatile metal hydrides .

Due to its reducing effect, sodium borohydride attacks organic tissue, so any contact, including with the skin , should be avoided.

## Usage and reactions

### Reducing agents in organic chemistry

Sodium borohydride is mainly used as a reducing agent in organic chemistry . This application proceeds with low atom economy , since considerable stoichiometric amounts of inorganic salts are always obtained when working up the reaction mixture. The polarization of the boron-hydrogen bond enables the compound to function as a hydride ion donor; the hydrogen is transferred with its binding electrons . This particle is a strong nucleophile and reacts easily with, for example, carbonyl groups and reduces them to the corresponding alcohol , which is obtained after acid-aqueous work-up. It is particularly suitable for reactions in aqueous or methanolic solutions, since it does not react as violently with water as, for example, lithium aluminum hydride . Under suitable conditions, aldehyde and keto groups can be selectively reduced with sodium borohydride . Methanol increases the reactivity, so that ester groups are also reduced in this solvent .

### Analytical chemistry

In analytical chemistry , sodium borohydride is used to convert semimetals such as arsenic and selenium into volatile compounds, which can be more easily detected with atomic absorption spectrometry (hydride AAS).

### Fuel cells

Another possible use envisaged for sodium borohydride is its use in fuel cell vehicles ; the sodium prototype built by DaimlerChrysler exists here . The functional principle is a catalytic hydrolysis of the boron hydride, which creates elemental hydrogen, which is used in the fuel cell. The product is sodium metaborate (NaBO 2 ), which can be made usable again as fuel by reacting with hydrogen . The advantage of using sodium borohydride as a proton source in fuel cells is the significantly higher cell voltage compared to H2 / O2 cells, which allows a higher power density. Another advantage is the less complex handling compared to H 2 , since neither deep freezing nor high pressure is necessary.

## Individual evidence

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