Lithium aluminium hydride
[CAS# 16853-85-3]

Identification

• Classification: Inorganic chemical industry >> Inorganic salt >> Hydride, nitride, azide >> Hydride
• Name: Lithium aluminium hydride
• Synonyms: LAH; Lithium tetrahydridoaluminate
• Molecular Formula: LiAlH₄
• Molecular Weight: 37.95
• CAS Registry Number: 16853-85-3
• EC Number: 240-877-9
• SMILES: [Li+].[AlH4-]

Properties

• Density: 0.917 g/mL
• Melting point: 125 ºC (Decomposes) (Expl.)
• Flash point: -17 ºC
• Solubility: Decomposes in water, Soluble in ether (Expl.)

Safety Data

• Hazard Symbols: GHS02;GHS05 DangerGHS02
• Hazard Statements: H220-H260-H314-H318
• Precautionary Statements: P203-P210-P222-P223-P231+P232-P260-P264-P264+P265-P280-P301+P330+P331-P302+P335+P334-P302+P361+P354-P304+P340-P305+P354+P338-P316-P317-P321-P363-P370+P378-P377-P381-P402+P404-P403-P405-P501

Hazard Classification

Hazard	Class	Category Code	Hazard Statement
Skin corrosion	Skin Corr.	1A	H314
Substances or mixtures which in contact with water emit flammable gases	Water-react.	1	H260
Serious eye damage	Eye Dam.	1	H318
Flammable gases	Flam. Gas	1	H220
Acute hazardous to the aquatic environment	Aquatic Acute	1	H400
Organic peroxides	Org. Perox.	D	H242
Acute toxicity	Acute Tox.	4	H312
Acute toxicity	Acute Tox.	4	H302
Flammable liquids	Flam. Liq.	3	H226
Acute toxicity	Acute Tox.	4	H332

• Transport Information: UN 1410;UN 1411

Discovory and Applicatios

Lithium aluminium hydride, with the chemical formula LiAlH₄, is a widely used inorganic compound known for its strong reducing capabilities in organic synthesis. It appears as a white to gray crystalline powder and is highly reactive, particularly with water. It consists of lithium cations (Li⁺) and tetrahydridoaluminate anions (AlH₄⁻), and is classified as a complex metal hydride.

The compound was first synthesized and studied in the 1940s as part of efforts to find new reducing agents that could replace or complement existing methods. Edward Wiberg and colleagues were among the earliest researchers to investigate lithium aluminium hydride, recognizing its potential due to its high hydrogen content and strong reactivity with a range of organic compounds. It quickly became an essential reagent in both academic and industrial chemical laboratories due to its unique ability to reduce various functional groups under mild conditions.

Lithium aluminium hydride is typically prepared by reacting lithium hydride (LiH) with aluminium chloride (AlCl₃) in an ether solvent such as diethyl ether or tetrahydrofuran. This reaction produces LiAlH₄ as a solid, which is isolated by filtration and purified by recrystallization. The product must be handled under anhydrous conditions, as it reacts violently with water and alcohols, releasing hydrogen gas and decomposing to lithium hydroxide and aluminium hydroxide.

The most significant application of lithium aluminium hydride is its role as a reducing agent in organic chemistry. It is capable of reducing a wide variety of carbonyl-containing compounds, including aldehydes, ketones, esters, carboxylic acids, acid chlorides, amides, nitriles, and even epoxides. In many cases, LiAlH₄ performs reductions that are difficult or impossible with milder agents such as sodium borohydride. It reduces esters and carboxylic acids to primary alcohols and nitriles to primary amines, making it particularly valuable in synthetic pathways for pharmaceuticals, agrochemicals, and fine chemicals.

LiAlH₄ also finds use in the reduction of inorganic compounds. For example, it can reduce metal halides to their corresponding metals and convert metal oxides or sulfates to hydrides or other reactive species. This versatility has led to its use in the preparation of high-purity materials and in specialized metallurgical processes.

In addition to its synthetic uses, lithium aluminium hydride has been investigated as a hydrogen storage material due to its high hydrogen content by weight. When heated, it decomposes to release hydrogen gas in multiple steps, producing intermediates such as Li₃AlH₆, LiH, and Al metal. Although this decomposition is not easily reversible under standard conditions, the study of its thermal behavior has contributed to the broader understanding of complex hydrides in energy storage applications.

Handling and storage of LiAlH₄ require strict safety precautions due to its high reactivity. It must be stored in airtight containers under an inert atmosphere, such as argon or nitrogen. Contact with moisture, air, or protic solvents must be avoided to prevent rapid and potentially hazardous decomposition. The compound’s reactivity also makes it incompatible with oxidizers, acids, and halogenated solvents, all of which can cause violent reactions.

Decomposition of lithium aluminium hydride can be triggered by heat or moisture and is exothermic, releasing hydrogen gas and forming lithium hydroxide and aluminium hydroxide. For this reason, reactions involving LiAlH₄ are typically conducted in dry, aprotic solvents under controlled conditions. Quenching procedures must be carefully designed to neutralize unreacted hydride safely, usually involving slow and controlled addition of alcohols followed by water under cooling.

Despite its reactivity and handling challenges, LiAlH₄ remains a cornerstone reagent in organic chemistry. Its ability to efficiently reduce a wide variety of substrates with relatively simple procedures makes it invaluable for both routine and complex syntheses. The compound continues to be a subject of research in areas such as catalysis, hydrogen storage, and advanced materials development, reflecting its importance across multiple domains of chemistry.

References

1954. A simplified synthesis of optically pure L- and D-alpha-polyglutamic acids. Experientia, 10(4).
DOI: 10.1007/bf02157190

2001. Reaction of Lithium Aluminum Hydride with Elemental Selenium: Its Application as a Selenating Reagent into Organic Molecules. Journal of the American Chemical Society, 123(31).
DOI: 10.1021/ja005800o

2024. Development of a multigram synthetic process to clinical candidate TMP195, a class IIa histone deacetylase selective inhibitor. Chemical Papers, 79(3).
DOI: 10.1007/s11696-024-03769-9