Lithium aluminium hydride
A powerful reducing agent used in organic synthesis.
Lithium aluminium hydride, or LAH, is an inorganic compound with the formula Li[AlH4] (also written LiAlH4). It appears as a white solid and was first made in 1947 by Finholt, Bond, and Schlesinger. In organic synthesis, LAH serves as a reducing agent, particularly for converting esters, carboxylic acids, and amides into alcohols or amines. The solid reacts dangerously with water, giving off hydrogen gas. Some related compounds were once considered for hydrogen storage.
Although pure LAH is colorless, commercial samples are often gray due to contamination. It can be purified by recrystallization from diethyl ether, and large-scale purification uses a Soxhlet extractor. Usually, the impure gray material is used directly in reactions because the impurities are harmless and easy to separate from organic products. The pure powder is pyrophoric, but large crystals are not. Some commercial products are mixed with mineral oil to protect against moisture, but more often they are packed in moisture-proof plastic bags.
LAH reacts violently with water, releasing hydrogen: Li[AlH4] + 4 H2O → LiOH + Al(OH)3 + 4 H2. This reaction can be used to generate hydrogen in the lab. Aged samples exposed to air often turn white as they absorb moisture, forming a mixture of lithium hydroxide and aluminum hydroxide.
In its crystal structure, LAH belongs to the monoclinic space group P21/c, with unit cell dimensions a = 4.82, b = 7.81, c = 7.92 Å, and β = 112°. Lithium cations are surrounded by five [AlH4]− anions, each with tetrahedral geometry. Each Li+ bonds to one hydrogen from each surrounding tetrahedron, forming a bipyramid arrangement. At pressures above 2.2 GPa, a phase transition may produce β-LAH.
LAH was first prepared from lithium hydride and aluminum chloride: 4 LiH + AlCl3 → Li[AlH4] + 3 LiCl. Industrially, sodium aluminium hydride is made first from the elements under high pressure and heat: Na + Al + 2 H2 → Na[AlH4]. Then a salt metathesis reaction yields LAH: Na[AlH4] + LiCl → Li[AlH4] + NaCl. Lithium chloride is removed by filtration from an ethereal solution, and LAH is precipitated, leaving about 1 wt% LiCl. Another method uses LiH and metallic aluminum with a small amount of TiCl3 (0.2%) as a catalyst in dimethylether, avoiding salt byproducts.
LAH dissolves in many ether solvents, but can decompose spontaneously due to catalytic impurities.
- Chemical formula
- Li[AlH4] or LiAlH4
- Discovered by
- Finholt, Bond and Schlesinger
- Year discovered
- 1947
- Appearance
- White solid (commercial samples often gray due to contamination)
- Primary use
- Reducing agent in organic synthesis
- Hydrogen content
- 10.6 wt%
- Hazard
- Pyrophoric; violently reacts with water to release H2
Lore & Background
Lithium aluminium hydride was first prepared from the reaction between lithium hydride (LiH) and aluminium chloride: 4 LiH + AlCl3 → Li[AlH4] + 3 LiCl. The industrial synthesis entails the initial preparation of sodium aluminium hydride from the elements under high pressure and temperature, followed by a salt metathesis reaction with LiCl. An alternative preparation starts from LiH and metallic Al, catalyzed by a small quantity of TiCl3 (0.2%) using dimethylether as solvent, avoiding the cogeneration of salt.
LAH crystallizes in the monoclinic space group P21/c. In the structure, Li+ cations are surrounded by five [AlH4]− anions, which have tetrahedral molecular geometry. At high pressures (>2.2 GPa) a phase transition may occur to give β-LAH. LAH is metastable at room temperature and slowly decomposes to Li3[AlH6] and LiH; this process can be accelerated by catalytic elements such as titanium, iron or vanadium. When heated, LAH decomposes in a three-step reaction mechanism, with the final step reversible at high temperature.
LAH is widely used in organic chemistry as a reducing agent, more powerful than sodium borohydride. It converts esters, carboxylic acids, acyl chlorides, aldehydes, and ketones into the corresponding alcohols, and amides, nitro, nitrile, imine, oxime, and organic azides into amines. It also reduces alkyl halides to alkanes, with alkyl iodides reacting fastest. Due to its pyrophoric nature, instability, toxicity, low shelf life and handling problems, it has been replaced in many applications by sodium bis(2-methoxyethoxy)aluminium hydride.
Reader's Guide
Lithium aluminium hydride is significant as a powerful reducing agent that revolutionized organic synthesis, particularly for converting esters and carboxylic acids to primary alcohols—a difficult transformation before its discovery. Its high hydrogen content (10.6 wt%) also made it a candidate for hydrogen storage research, though practical challenges remain due to the high pressure required for recycling. The compound's reactivity with water and air necessitates careful handling, often in inert atmospheres or with mineral oil coatings. Its legacy includes enabling reductions that were previously impractical, though it has been partially supplanted by safer alternatives. The discovery of reversible hydrogen storage in Ti-doped NaAlH4 sparked further research into LAH and related alanates for fuel cell applications. Despite its hazards, LAH remains a staple in many organic chemistry laboratories for specific reductions where its strength is required.
Did You Know?
- LAH was discovered in 1947 by Finholt, Bond and Schlesinger.
- It violently reacts with water to liberate hydrogen gas, following the equation: Li[AlH4] + 4 H2O → LiOH + Al(OH)3 + 4 H2.
- Pure powdered LAH is pyrophoric, but its large crystals are not.
- LAH contains 10.6 wt% hydrogen, making it a potential hydrogen storage medium.
More in Inorganic Compounds & Materials 1-20
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