Lithium borohydride (LiBH₄) is a

borohydride Borohydride refers to the anion , which is also called tetrahydroborate, and its salts. Borohydride or hydroborate is also the term used for compounds containing , where ''n'' is an integer from 0 to 3, for example cyanoborohydride or cyanotrihyd ...

and known in

organic synthesis Organic synthesis is a special branch of chemical synthesis and is concerned with the intentional construction of organic compounds. Organic molecules are often more complex than inorganic compounds, and their synthesis has developed into one o ...

as a reducing agent for esters. Although less common than the related sodium borohydride, the lithium salt offers some advantages, being a stronger reducing agent and highly soluble in ethers, whilst remaining safer to handle than lithium aluminium hydride.Luca Banfi, Enrica Narisano, Renata Riva, Ellen W. Baxter, "Lithium Borohydride" e-EROS Encyclopedia of Reagents for Organic Synthesis, 2001, John Wiley & Sons. .

Preparation

Lithium borohydride may be prepared by the metathesis reaction, which occurs upon ball-milling the more commonly available sodium borohydride and

lithium bromide Lithium bromide (LiBr) is a chemical compound of lithium and bromine. Its extreme hygroscopic character makes LiBr useful as a desiccant in certain air conditioning systems.Wietelmann, Ulrich and Bauer, Richard J. (2005) "Lithium and Lithium Compo ...

: : NaBH₄ + LiBr → NaBr + LiBH₄ Alternatively, it may be synthesized by treating

boron trifluoride Boron trifluoride is the inorganic compound with the formula BF3. This pungent, colourless, and toxic gas forms white fumes in moist air. It is a useful Lewis acid and a versatile building block for other boron compounds. Structure and bondin ...

with

lithium hydride Lithium hydride is an inorganic compound with the formula Li H. This alkali metal hydride is a colorless solid, although commercial samples are grey. Characteristic of a salt-like (ionic) hydride, it has a high melting point, and it is not solub ...

in diethyl ether: : BF₃ + 4 LiH → LiBH₄ + 3 LiF

Reactions

Lithium borohydride is a stronger reducing agent than sodium borohydride. In mixtures of methanol and diethyl ether, lithium borohydride is able to reduce esters to alcohols and primary

amide In organic chemistry, an amide, also known as an organic amide or a carboxamide, is a compound with the general formula , where R, R', and R″ represent organic groups or hydrogen atoms. The amide group is called a peptide bond when it is ...

s to

amine In chemistry, amines (, ) are compounds and functional groups that contain a basic nitrogen atom with a lone pair. Amines are formally derivatives of ammonia (), wherein one or more hydrogen atoms have been replaced by a substituent suc ...

s. In contrast, these substrates are unaffected by sodium borohydride. The enhanced reactivity is attributed to the polarization of the carbonyl substrate by complexation to the lithium cation.

Chemoselectivity

The use of lithium borohydride is particularly advantageous in some preparations due to its higher chemoselectivity relative to other popular reducing agents such as lithium aluminium hydride. For instance, unlike lithium aluminium hydride, lithium borohydride reduces esters,

nitrile In organic chemistry, a nitrile is any organic compound that has a functional group. The prefix '' cyano-'' is used interchangeably with the term ''nitrile'' in industrial literature. Nitriles are found in many useful compounds, including met ...

s, lactones, primary amides, and epoxides, while sparing

nitro group In organic chemistry, nitro compounds are organic compounds that contain one or more nitro functional groups (). The nitro group is one of the most common explosophores (functional group that makes a compound explosive) used globally. The nit ...

carbamic acid Carbamic acid, which might also be called aminoformic acid or aminocarboxylic acid, is the chemical compound with the formula . It can be obtained by the reaction of ammonia and carbon dioxide at very low temperatures, which also yields an equ ...

alkyl halides The haloalkanes (also known as halogenoalkanes or alkyl halides) are alkanes containing one or more halogen substituents. They are a subset of the general class of halocarbons, although the distinction is not often made. Haloalkanes are widely us ...

, and

secondary Secondary may refer to: Science and nature * Secondary emission, of particles ** Secondary electrons, electrons generated as ionization products * The secondary winding, or the electrical or electronic circuit connected to the secondary winding i ...

tertiary Tertiary ( ) is a widely used but obsolete term for the geologic period from 66 million to 2.6 million years ago. The period began with the demise of the non-avian dinosaurs in the Cretaceous–Paleogene extinction event, at the start ...

amides.

Hydrogen generation

Lithium borohydride reacts with water to produce hydrogen. This reaction can be used for hydrogen generation.

Energy storage

Lithium borohydride is renowned as one of the highest- energy-density chemical

energy carrier An energy carrier is a substance (fuel) or sometimes a phenomenon (energy system) that contains energy that can be later converted to other forms such as mechanical work or heat or to operate chemical or physical processes. Such carriers includ ...

s. Although presently of no practical importance, the solid liberates 65 MJ/ kg heat upon treatment with atmospheric oxygen. Since it has a density of 0.67 g/cm³, oxidation of liquid lithium borohydride gives 43 MJ/L. In comparison, gasoline gives 44 MJ/kg (or 35 MJ/L), while liquid hydrogen gives 120 MJ/kg (or 8.0 MJ/L).The greater ratio of energy density to specific energy for hydrogen is because of the very low mass density (0.071 g/cm³). The high specific energy density of lithium borohydride has made it an attractive candidate to propose for automobile and rocket fuel, but despite the research and advocacy, it has not been used widely. As with all chemical-hydride-based energy carriers, lithium borohydride is very complex to recycle (i.e. recharge) and therefore suffers from a low energy conversion efficiency. While batteries such as lithium-ion carry an energy density of up to 0.72 MJ/kg and 2.0 MJ/L, their DC-to-DC conversion efficiency can be as high as 90%. In view of the complexity of recycling mechanisms for metal hydrides, (1977) lithium borohydride recycling from lithium borate via a methyl borate intermediate. such high energy-conversion efficiencies are not practical with present technology.

Notes

References

{{Lithium compounds Borohydrides Lithium salts Reducing agents el:Υδρίδιο του λιθίου