- Structure
- Nomenclature
- Physical properties
- Physical state
- Molecular weight
- Autoignition temperature
- Decomposition temperature
- Density
- Solubility
- Chemical properties
- Risks
- Obtaining
- Applications
- In reduction chemical reactions
- In the production of H
- In the pulp and paper industry
- In various uses
- References
The sodium borohydride is an inorganic solid with the chemical formula NaBH 4. 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, NaBH 4 can generate heat and hydrogen H 2 which is combustible, so it must be handled with caution. Because of the ease with which it can generate H - hydride ions, sodium borohydride is a moderately reducing compound and is therefore widely used in chemical reactions for this purpose.
Sodium borohydride, NaBH 4, solid. Ondřej Mangl. 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, such as in the pharmaceutical industry.
Structure
Sodium borohydride is an ionic compound. It is formed by the borohydride anion - attached to the sodium cation Na +.
The anion - is tetrahedral.
Structure of sodium borohydride. Kemikungen. Source: Wikimedia Commons.
Its Lewis structure is as follows:
Lewis electronic structure of NaBH 4. Author: Marilú Stea.
Nomenclature
-Sodium borohydride
-Sodium tetrahydroborate
-Sodium tetrahydruroborate
Physical properties
Physical state
Crystalline white solid.
Molecular weight
37.84 g / mol
Autoignition temperature
∼ 220 ºC
Decomposition temperature
> 250 ºC
Density
1,074 g / cm 3 at 20 ºC
Solubility
NaBH 4 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.
- + H 2 O → B (OH) 3 + H 2 ↑ + OH -
This is explained because the - interacts with the H + of the water, where the H + takes a hydride anion H - to form H 2. It can be said that BH 3 competes with H + for the hydride ion H -.
In an acid medium, hydrolysis is complete due to the abundance of H + ions.
NaBH 4 is stable in dry air. It is not volatile.
Due to its content of hydride ions, sodium borohydride is a reducing compound. It allows reducing the carbonyl group C = O to C-OH, that is, carbonyl compounds to alcohols.
NaBH 4 by itself does not reduce C = C double bonds, not even those conjugated with carbonyl groups –C = CC (= O) -.
However, on contact with protonic acids (such as HCl) or Lewis acids (such as BCl 3 or AlCl 3), diborane B 2 H 6 is formed. And if this reaction is carried out in the presence of organic compounds with C = C double bonds, the diborane B 2 H 6 performs the 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.
NaBH 4 is easily ignited and burns easily.
Obtaining
The typical preparation reaction of NaBH 4 is with sodium hydride NaH and B (OCH 3) 3 at a temperature of about 250 ºC:
4 NaH + B (OCH 3) 3 → NaBH 4 + 3 NaOCH 3
Applications
In reduction chemical reactions
As a source of H - ions, NaBH 4 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.
It 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, the reduction 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 = CC (= O) -, makes it possible to prepare unsaturated alcohols -C = C-CH 2 -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 the reduction of carboxylic acids, esters, amides, nitriles, imines, epoxides and even double and triple bonds.
In the production of H
NaBH 4 can be converted into a fuel cell.
NaBH 4 in the presence of an alkaline KOH or NaOH solution is hydrolyzed and produces hydrogen H 2 which can be used as fuel in a polymer electrolyte fuel cell.
It has also been investigated as a reversible storage material for H 2.
NaBH 4 nanoparticles are synthesized and stabilized with a surfactant. After treatment with NiCl 2, a protective layer or coating is formed that regulates the release of H 2 in a completely reversible way.
This new nanometric material would allow the use of H 2 as a clean fuel produced in a renewable way.
Hydrogen fuel cell powered vehicle. Dr. Artur Braun (Arturbraun). 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.
By treating the pulp or paper with NaBH 4, it reduces aldehydes and ketones to -OH groups without affecting the carboxylic acid groups, improving stability and gloss at higher than initial values.
NaBH 4 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
- Christian, ML and Aguey-Zinsou, K.-F. (2012). Core-Shell Strategy Leading to High Reversible Hydrogen Storage Capacity for NaBH 4. ACS Nano 2012, 6, 9, 7739-7751. Recovered from pubs.acs.org.
- Nora de Souza, MV and Alves V., TR (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.
- Imamoto, T. (1991). Reduction. Sodium Borohydride. In Comprehensive Organic Synthesis. Recovered from sciencedirect.com.
- Tang, LC (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.
- Cotton, F. Albert and Wilkinson, Geoffrey. (1980). Advanced Inorganic Chemistry. Fourth Edition. John Wiley & Sons.
- Morrison, Robert Thornton; and Boyd, Robert Neilson. 1992. Organic Chemistry. Prentice-Hall. ISBN 81-203-0765-8.
- US National Library of Medicine. (2019). Sodium borohydride. Recovered from: pubchem.ncbi.nlm.nih.gov.