Sodium borohydride (NaBH4) structure, properties, uses

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Philip Kelley
Sodium borohydride (NaBH4) structure, properties, uses

The sodium borohydride is an inorganic solid whose chemical formula is NaBH4. It can be considered representative of the alkali borohydrides and is the most common of these. It is a white crystalline solid.

When in contact with water, NaBH4 can generate heat and hydrogen Htwo which is combustible, so it must be handled with caution. Because of the ease with which it can generate hydride ions H-, sodium borohydride is a moderately reducing compound, so it is widely used in chemical reactions for this purpose.

Sodium borohydride, NaBH4, solid. Ondřej Mangl [Public domain]. Source: Wikimedia Commons.

Its reducing property is also used in the paper industry, as it improves the brightness and stability characteristics of pulp and paper by acting against the oxidation of cellulose, the main component of paper..

Because it can easily form hydrogen in the presence of water, the possibility of using it as a reversible source of hydrogen in fuel cells has also been investigated..

It has other uses all based on its reducing property, for example in the pharmaceutical industry.

Article index

  • 1 Structure
  • 2 Nomenclature
  • 3 Physical properties
    • 3.1 Physical state
    • 3.2 Molecular weight
    • 3.3 Autoignition temperature
    • 3.4 Decomposition temperature
    • 3.5 Density
    • 3.6 Solubility
  • 4 Chemical properties
  • 5 Risks
  • 6 Obtaining
  • 7 Uses
    • 7.1 In chemical reduction reactions
    • 7.2 In the production of H2 in fuel cells
    • 7.3 In the pulp and paper industry
    • 7.4 In various uses
  • 8 References

Structure

Sodium borohydride is an ionic compound. It is formed by the borohydride anion [BH4]- attached to the sodium cation Na+.

The anion [BH4]- is tetrahedral.

Structure of sodium borohydride. Kemikungen [Public domain]. Source: Wikimedia Commons.

Its Lewis structure is as follows:

Lewis electronic structure of NaBH4. Author: Marilú Stea.

Nomenclature

-Sodium borohydride

-Sodium tetrahydroborate

-Sodium tetrahydruroborate

Physical properties

Physical state

Solid white crystalline.

Molecular weight

37.84 g / mol

Autoignition temperature

∼ 220 ºC

Decomposition temperature

> 250 ºC

Density

1.074 g / cm3 at 20 ºC

Solubility

The NaBH4 it is quite soluble in water (55 g / 100mL at 25 ºC), where it is partially hydrolyzed. It is soluble in tetrahydrofuran (THF) and is insoluble in diethyl ether.

Chemical properties

Sodium borohydride dissolves in water, where it undergoes initial decomposition or hydrolysis that makes the solution basic, preventing further hydrolysis..

[BH4]- + HtwoO → B (OH)3 + Htwo↑ + OH-

This is explained because the [BH4]- interacts with the H+ of the water, where the H+ takes a hydride anion H- to form Htwo. It can be said that the BH3 compete with the H+ by the hydride ion H-.

In an acid medium, hydrolysis is complete due to the abundance of H ions+.

The NaBH4 it is stable in dry air. It is not volatile.

Due to its hydride ion content, sodium borohydride is a reducing compound. It allows reducing the carbonyl group C = O to C-OH, that is, carbonyl compounds to alcohols.

The NaBH4 By itself it does not reduce C = C double bonds, not even those conjugated with carbonyl groups -C = C-C (= O)-.

However, when in contact with protonic acids (such as HCl) or Lewis acids (such as BCl3 or AlCl3) diborane B is formedtwoH6. And if this reaction is carried out in the presence of organic compounds with C = C double bonds, diborane BtwoH6 performs hydroboration of these.

In the presence of appropriate catalysts and specific conditions sodium borohydride can reduce various types of functional groups.

Risks

The heat of the hydrolysis reaction in water is sufficient to ignite the hydrogen that is formed in it. So it is important to exercise caution when handling it.

The NaBH4 it is easily ignited and burns easily.

Obtaining

The typical preparation reaction of NaBH4 is through sodium hydride NaH and B (OCH3)3 at a temperature of about 250 ºC:

4 NaH + B (OCH3)3 → NaBH4 + 3 NaOCH3

Applications

In reduction chemical reactions

Because it is a source of H ions-, the NaBH4 It is a reducing compound and is used to prepare other chemical compounds. For this it is used in aprotic polar solvents, that is, without H protons+, such as dimethyl sulfoxide, hexamethylphosphoramide, and dimethylformamide.

It is used as a reducing agent in both organic and inorganic chemical reactions..

Allows to reduce aldehydes to primary alcohols and ketones to secondary alcohols.

It also reduces alkyl halides to hydrocarbons such as iododecane to decane, bromoundecanoic acid to undecanoic acid, and chlorododecane to dodecane..

If it acts alone, when carrying out the reduction it does not affect other functional groups such as ester, carboxylic acid, nitrile and sulfone.

Its property of not reducing C = C double bonds, not even those conjugated with carbonyl groups -C = C-C (= O) -, makes it possible to prepare unsaturated alcohols -C = C-CHtwo-Oh.

To reduce nitrated aromatic compounds to their corresponding anilines requires the presence of catalysts such as cobalt or tin chloride. Reduces disulfides to thiols.

In the presence of suitable conditions and catalysts it allows to reduce carboxylic acids, esters, amides, nitriles, imines, epoxides and even double and triple bonds.

In the production of Htwo in fuel cells

The NaBH4 can be converted into a fuel cell.

The NaBH4 in the presence of an alkaline solution of KOH or NaOH it hydrolyzes and produces hydrogen Htwo which can be used as fuel in a polymer electrolyte fuel cell.

It has also been investigated as a storage material for Htwo reversibly.

NaBH nanoparticles are synthesized4 and stabilized with a surfactant. After treatment with NiCltwo a protective layer or coating is formed that regulates the release of Htwo completely reversible.

This new nanometric material would allow the use of Htwo as a clean and renewable fuel.

Vehicle powered by hydrogen fuel cell. Dr. Artur Braun (Arturbraun) [CC BY-SA 4.0 (https://creativecommons.org/licenses/by-sa/4.0)]. Source: Wikimedia Commons.

In the pulp and paper industry

Sodium borohydride is used to improve the gloss and other physical properties of pulp and paper..

The degradation of the cellulosic matrix of the paper occurs through complex processes that involve oxidation. Hydroxyl groups are oxidized to carbonyl and carboxyl, this leads to discoloration of the paper and a decrease in its physical properties..

When treating pulp or paper with NaBH4, This reduces aldehydes and ketones to -OH groups without affecting the acidic carboxylic groups, improving stability and gloss at higher than initial values..

The NaBH4 improves the gloss of the paper. Author: Rawpixel. Source: Pixabay.

In various uses

Sodium borohydride is used to treat wastewater, as a flavor-enhancing agent in the food and beverage industry, as a coating agent and for surface treatment, in tobacco production, in the pharmaceutical, textile and industrial industries. Leather.

References

  1. Christian, M.L. and Aguey-Zinsou, K.-F. (2012). Core-Shell Strategy Leading to High Reversible Hydrogen Storage Capacity for NaBH4. ACS Nano 2012, 6, 9, 7739-7751. Recovered from pubs.acs.org.
  2. Nora de Souza, M.V. and Alves V., T.R. (2006). Recent methodologies mediated by sodium borohydride in the reduction of different classes of compounds. App. Organometal. Chem. 2006; 20: 798-810. Recovered from onlinelibrary.wiley.com.
  3. Imamoto, T. (1991). Reduction. Sodium Borohydride. In Comprehensive Organic Synthesis. Recovered from sciencedirect.com.
  4. Tang, L.C. (1986) Stabilization of Paper through Sodium Borohydride Treatment. In Historic Textile and Paper Materials. Chapter 24. Pages 427-441. Advances in Chemistry, Volume 212. Recovered from pubs.acs.org.
  5. Cotton, F. Albert and Wilkinson, Geoffrey. (1980). Advanced Inorganic Chemistry. Fourth Edition. John Wiley & Sons.
  6. Morrison, Robert Thornton; and Boyd, Robert Neilson. 1992. Organic Chemistry. Prentice-Hall. ISBN 81-203-0765-8.
  7. U.S. National Library of Medicine. (2019). Sodium borohydride. Recovered from: pubchem.ncbi.nlm.nih.gov.

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