Lithium aluminium hydride, commonly abbreviated to LAH, is an inorganic compound with the chemical formula or . It is a white solid, discovered by Finholt, Bond and Schlesinger in 1947. This compound is used as a reducing agent in organic synthesis, especially for the reduction of esters, carboxylic acids, and amides. The solid is dangerously reactive toward water, releasing gaseous hydrogen (H2). Some related derivatives have been discussed for hydrogen storage.
Properties, structure, preparation
LAH is a colourless solid but commercial samples are usually gray due to contamination. This material can be purified by recrystallization from diethyl ether. Large-scale purifications employ a Soxhlet extractor. Commonly, the impure gray material is used in synthesis, since the impurities are innocuous and can be easily separated from the organic products. The pure powdered material is pyrophoric, but not its large crystals. Some commercial materials contain mineral oil to inhibit reactions with atmospheric moisture, but more commonly it is packed in moisture-proof plastic sacks.
LAH violently reacts with water, including atmospheric moisture, to liberate dihydrogen gas. The reaction proceeds according to the following idealized equation:
This reaction provides a useful method to generate hydrogen in the laboratory. Aged, air-exposed samples often appear white because they have absorbed enough moisture to generate a mixture of the white compounds lithium hydroxide and aluminium hydroxide.
Structure
LAH crystallizes in the monoclinic space group P21/c. The unit cell has the dimensions: a = 4.82, b = 7.81, and c = 7.92 Å, α = γ = 90° and β = 112°. In the structure, cations are surrounded by five anions, which have tetrahedral molecular geometry. The cations are bonded to one hydrogen atom from each of the surrounding tetrahedral anion creating a bipyramid arrangement. At high pressures (>2.2 GPa) a phase transition may occur to give β-LAH.
Preparation
was first prepared from the reaction between lithium hydride (LiH) and aluminium chloride:
In addition to this method, the industrial synthesis entails the initial preparation of sodium aluminium hydride from the elements under high pressure and temperature:
is then prepared by a salt metathesis reaction according to:
which proceeds in a high yield. LiCl is removed by filtration from an ethereal solution of LAH, with subsequent precipitation of LAH to yield a product containing around 1% w/w LiCl.
An alternative preparation starts from LiH, and metallic Al instead of . Catalyzed by a small quantity of (0.2%), the reaction proceeds well using dimethylether as solvent. This method avoids the cogeneration of salt.
Solubility data
LAH is soluble in many ethereal solutions. However, it may spontaneously decompose due to the presence of catalytic impurities, though, it appears to be more stable in tetrahydrofuran (THF). Thus, THF is preferred over, e.g., diethyl ether, despite the lower solubility.
Thermal decomposition
LAH is metastable at room temperature. During prolonged storage it slowly decomposes to (lithium hexahydridoaluminate) and LiH. This process can be accelerated by the presence of catalytic elements, such as titanium, iron or vanadium.
When heated LAH decomposes in a three-step reaction mechanism:
is usually initiated by the melting of LAH in the temperature range 150–170 °C, immediately followed by decomposition into solid , although is known to proceed below the melting point of as well. At about 200 °C, decomposes into LiH () and Al which subsequently convert into LiAl above 400 °C (). Reaction R1 is effectively irreversible. is reversible with an equilibrium pressure of about 0.25 bar at 500 °C. and can occur at room temperature with suitable catalysts.
Thermodynamic data
The table summarizes thermodynamic data for LAH and reactions involving LAH, in the form of standard enthalpy, entropy, and Gibbs free energy change, respectively.