Lithium Aluminium Hydride
Lithium aluminium hydride is an inorganic compound with CAS No 16853-85-3 and chemical formula LiAlH4. The name of the substance is commonly abbreviated as LAH. Besides, this chemical is also known as lithal, lithium alanate, lithium aluminohydride, lithium tetrahydridoaluminate, lithium alumanuide, and lithium tetrahydridoaluminate(III). LAH was discovered in 1947 by Finholt, Bond, and Schlesinger.
Properties and Structure
Pure lithium aluminium hydride is usually presented as colorless and odorless solid crystals. However, commercial material is powder colored grey due to contamination. This material is typically used in synthesis, since the impurities are harmless and easy to separate from organic products. Besides, the chemical can be purified easily by recrystallization from diethyl ether.
The LAH is also characterized by the following properties:
- Molar mass: 37.95 g/mol;
- Density: 0.917 g/cm3;
- Melting point: 150 °C;
- Flash point: 125 °C
Lithium aluminium hydride reactions with water and even atmospheric moisture are violent. They result in the release of gaseous hydrogen (H2) and are often used in the laboratories to generate it. That is why some commercial LAH may contain mineral oil to restrain the reaction with atmospheric moisture, but in most cases, the chemical is simply placed in moisture-proof pack.
The crystal structure of lithium aluminium hydride was difficult to determine due to the small size of its hydrogen atoms. It was eventually solved using a combination of powder diffraction and neutron diffraction. In the structure, the chemical belongs to the monoclinic crystal system. The Li+ centers are bounded to one hydrogen atom from the surrounding tetrahedra creating thus a bipyramid arrangement.
This inorganic compound was first prepared in the course of the reaction between aluminium chloride and lithium hydride. This reaction takes about an hour and requires constant stirring to prevent to prevent LAH forming as a sediment or a crust on top of the solution.
The typical industrial preparation of lithium aluminium hydride consists of two steps:
- The sodium aluminium hydride is prepared from elements under high temperature and pressure.
- LAH is prepared in the course of a salt metathesis reaction.
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Applications and Uses
Lithium aluminium hydride is mainly applied as a reducing agent in an organic synthesis. It is often preferred to the related reagent sodium borohydride due to the weaker Al-H bond in comparison with the B-H bond. The solution of LAH in diethyl ether is often used to convert acyl chlorides, esters, ketones, aldehydes, and carboxylic acids into the corresponding alcohols.
In inorganic chemistry, this compound is used to prepare transition and main group metal hydrides from the metal halides. It can react with some inorganic ligands to form coordinated alumina complexes. Besides, the chemical is often discussed as a potential hydrogen storage medium for fuel cell-powered vehicles.