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Lithium borohydride

physical science Maturity 11-13 Next article →

Lithium borohydride, known by the chemical formula LiBH4, is a specialized chemical salt. In the field of organic synthesis, it serves as a powerful reducing agent. A reducing agent is a substance that helps change other molecules by providing electrons or specific atoms. This particular salt is used to transform various chemical structures into new forms. It occupies a unique middle ground in chemistry. It is a stronger reducing agent than sodium borohydride. However, it is safer for scientists to handle than lithium aluminium hydride.

Li+
Li+

The primary mechanism of lithium borohydride involves its ability to act as a source of hydride ions. A hydride is a group consisting of a hydrogen atom and another atom, denoted as H–. When lithium borohydride reacts with carbonyl substrates, it facilitates the formation of a hydrogen–carbon bond. This process is essential for building complex organic molecules. The reactivity of this salt is enhanced through a process called complexation. This occurs when the lithium cation, or Li+, interacts with the substrate. This interaction polarizes the substrate, making it easier for the hydride to react.

Borhydrid-
Borhydrid-

Because of its specific reactivity, lithium borohydride can perform several distinct types of chemical reductions. It can reduce esters into alcohols. It can also transform nitriles and primary amides into amines. Additionally, it has the ability to open epoxides, which are cyclic chemical structures. While it is powerful, it is selective. It does not react with nitro groups, carbamic acids, or alkyl halides. It also avoids reacting with secondary or tertiary amides. This selectivity makes it a precise tool for chemists.

Borhydrid-
Borhydrid-

Scientists use different methods to prepare lithium borohydride in a laboratory. One common method is a metathesis reaction. This process involves ball-milling sodium borohydride with lithium bromide to produce lithium borohydride and sodium bromide. Another synthesis method involves treating boron trifluoride with lithium hydride. This second reaction takes place in a liquid called diethyl ether. Both methods require careful control to ensure the correct chemical products are formed. These processes allow researchers to create the salt for various industrial and scientific uses.

Beyond organic synthesis, lithium borohydride is a significant candidate for energy storage. It is recognized as one of the highest-energy-density chemical energy carriers. When the solid is treated with atmospheric oxygen, it liberates 65 MJ/kg of heat. This is a measurement of its specific energy density. If the lithium borohydride is in a liquid state, its oxidation provides 43 MJ/L.

Volvsgrav
Volvsgrav

To put this in perspective, regular gasoline provides 44 MJ/kg and 35 MJ/L. Liquid hydrogen provides much more specific energy at 120 MJ/kg, but its volumetric energy density is much lower at only 8.0 MJ/L. This makes lithium borohydride very dense in terms of stored energy.

Due to these high energy numbers, researchers have proposed using it as fuel for automobiles and rockets. However, there are major practical challenges. One difficulty is that lithium borohydride is very complex to recycle, also known as recharging. Because recycling is difficult, the energy conversion efficiency remains low. In contrast, lithium-ion batteries can reach a DC-to-DC conversion efficiency as high as 90%. Current technology makes it hard to achieve such high efficiency with metal hydrides.

4002726 schematics
4002726 schematics

One proposed recycling method involves turning lithium borate into a methyl borate intermediate.

Finally, the chemical properties of lithium borohydride allow it to generate hydrogen gas. This happens when the salt reacts with water or other Brønsted–Lowry-acidic substances. This reaction is often spontaneous and can be quite violent. Because of this rapid release, it is studied for hydrogen generation technologies. The substance can exist in four different structural forms known as polymorphs. The stable versions of these structures feature tetrahedral BH4- anions. These various properties make lithium borohydride a subject of intense study in both chemistry and energy science.

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File:Li+.svg
Li+.svg
File:Borhydrid-.svg
Borhydrid-.svg
File:Volvsgrav.png
Volvsgrav.png
File:4002726 schematics.png
4002726 schematics.png
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