Lithium borohydride
A borohydride reducing agent and high-energy-density fuel candidate.
Lithium borohydride (LiBH4) is a borohydride used in organic synthesis to reduce esters. While it is less widely used than sodium borohydride, the lithium version is a stronger reducing agent and dissolves well in ethers. It is also safer to work with than lithium aluminium hydride.
It can be made by grinding sodium borohydride with lithium bromide, which swaps the metals to produce lithium borohydride and sodium bromide. Another method involves reacting boron trifluoride with lithium hydride in diethyl ether.
As a source of hydride ions, lithium borohydride adds hydrogen to carbonyl compounds and other polarized carbon structures. It also reacts with acidic substances to release hydrogen gas.
As a reducing agent, it is more powerful than sodium borohydride but less so than lithium aluminium hydride. Unlike sodium borohydride, it can reduce esters to alcohols, convert nitriles and primary amides into amines, and open epoxides. This extra reactivity often comes from the lithium cation polarizing the carbonyl group. Unlike lithium aluminium hydride, it does not attack nitro groups, carbamic acids, alkyl halides, or secondary and tertiary amides.
Lithium borohydride reacts with water to produce hydrogen, a reaction that could be used for hydrogen generation. This reaction is usually violent, but stable aqueous solutions can be made at low temperatures using degassed, distilled water and avoiding oxygen.
It is known for its high energy density. Although not practical, the solid releases 65 MJ/kg when burned in air. With a density of 0.67 g/cm³, the liquid form gives 43 MJ/L. For comparison, gasoline provides 44 MJ/kg (or 35 MJ/L), and liquid hydrogen gives 120 MJ/kg (or 8.0 MJ/L). This high specific energy has led to proposals for use in cars and rockets, but it has not seen wide use. Like other chemical hydrides, recycling (recharging) lithium borohydride is complex and inefficient. While lithium-ion batteries have energy densities up to 0.72 MJ/kg and 2.0 MJ/L with DC-to-DC efficiency as high as 90%, such efficiency is not achievable with current metal hydride recycling technology.
Four crystal forms (polymorphs) are known. The stable ones contain tetrahedral BH₄⁻ ions.
- Chemical formula
- LiBH4
- Molar mass
- 21.78 g/mol
- Density
- 0.67 g/cm3
- Energy density
- 65 MJ/kg (oxidation with oxygen)
- Known for
- Reducing agent for esters; high-energy-density chemical energy carrier
Lore & Background
Lithium borohydride may be prepared by the metathesis reaction of sodium borohydride and lithium bromide via ball-milling, or by treating boron trifluoride with lithium hydride in diethyl ether. It is useful as a source of hydride (H−) and can react with carbonyl substrates and other polarized carbon structures to form hydrogen–carbon bonds, as well as with Brønsted–Lowry-acidic substances to produce hydrogen gas.
As a hydride reducing agent, lithium borohydride is stronger than sodium borohydride but weaker than lithium aluminium hydride. Unlike the sodium analog, it can reduce esters to alcohols, nitriles and primary amides to amines, and open epoxides. Unlike the aluminium analog, it does not react with nitro groups, carbamic acids, alkyl halides, or secondary and tertiary amides.
Lithium borohydride reacts with water to produce hydrogen, a reaction that can be used for hydrogen generation. Although usually spontaneous and violent, somewhat-stable aqueous solutions can be prepared at low temperature with degassed, distilled water and avoidance of oxygen. It is renowned as one of the highest-energy-density chemical energy carriers, liberating 65 MJ/kg upon treatment with atmospheric oxygen.
Reader's Guide
Lithium borohydride holds significance primarily in organic synthesis as a selective reducing agent, bridging the reactivity gap between sodium borohydride and lithium aluminium hydride. Its ability to reduce esters, nitriles, and primary amides, while tolerating nitro groups and alkyl halides, makes it valuable for specific transformations. Additionally, its high energy density—65 MJ/kg upon oxidation—has attracted research interest for automobile and rocket fuel applications, though practical use remains limited. The complexity of recycling the hydride and low energy conversion efficiency compared to batteries have prevented widespread adoption. Its structure features tetrahedral BH4- anions in four known polymorphs. The compound's legacy lies in its dual role: a specialized reagent in organic chemistry and a theoretical high-density energy carrier that highlights the challenges of chemical hydride fuel systems.
Did You Know?
- Lithium borohydride can be prepared by ball-milling sodium borohydride and lithium bromide.
- It is a stronger reducing agent than sodium borohydride but weaker than lithium aluminium hydride.
- Lithium borohydride liberates 65 MJ/kg of heat upon treatment with atmospheric oxygen.
- It reacts with water to produce hydrogen, though the reaction is usually spontaneous and violent.
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