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Aluminium hydride, chemical formula AlH3, is a chemical reagent used as a reducing agent. It is used in hydroalumination of alkynes, allylic rearrangements, and storing hydrogen in hydrogen-fueled vehicles.. It is a colourless polymeric solid, (AlH3)n. The molecular AlH3 species are not stable. Monomeric AlH3 has been isolated at low temperature in a solid noble gas matrix and shown to be planar, the dimer, Al2H6, has been isolated in solid hydrogen and is isostructural with diborane.
Additional recommended knowledge
Aluminium hydride was reported as impurities, amines and ether complexes throughout the history, until its first synthesis published in 1947 by Finholt, Bond, and Schlesinger from the George Herbert Jones Laboratory at University of Chicago. A U.S. patent for the synthesis was assigned to Petrie et al. in 1999 with the U.S. Pat. No. 6228338.
Structure and physical properties
Aluminium hydride is formed as numerous polymorphs: α-alane, α’-alane, β-alane, δ-alane, ε-alane, θ-alane, and γ-alane. α-alane has a cubic or rhombohedral morphology, while α’-alane forms needle like crystals and γ-alane forms a bundle of fused needles. Alane is soluble in THF and ether, and its precipitation rate from ether depends on the preparation method.
α-Alane is the most thermally stable polymorph. β-alane and γ-alane are produced together, and will turn into α-alane upon heating. δ, ε, and θ-alane are produced in different crystallization condition. Though they are less thermally stable, they do not convert into α-alane upon heating.
Aluminium hydride is generally prepared by treating an ether solution of lithium aluminium hydride (LAH) with aluminium trichloride. An ether solution aluminium hydride is prepared after precipitation of lithium chloride. The dissolving process of aluminium trichloride requires the addition of 0.5-4 mol equivalents of borohydride salt, which is very expensive and not recovered. This makes the synthesis of aluminium hydride expensive.
3 LiAlH4 + AlCl3 → 4 AlH3 + 3 LiCl
The ether solution of aluminium hydride requires immediate use, because polymeric material with ether will precipitate with AlH otherwise. Aluminium hydride solutions are known to degrade after 3 days. Aluminium hydride is more reactive than LAH, but the procedure to handle aluminium hydride should be similar to that of LAH.
There are also several other methods to prepare aluminium hydride:
2 LiAlH4 + BeCl2 → 2 AlH3 + LiBeH2Cl2
2 LiAlH4 + H2SO4 → 2 AlH3 + Li2SO4 + 2 H2
2 LiAlH4 + ZnCl2 → 2 AlH3 + 2 LiCl + ZnH2
Aluminium hydride is not spontaneously flammable, but it is highly reactive. It is recommended to handle the chemical similar to the handling and precaution procedures for lithium aluminium hydride. It is known to degrade in a relatively short time, 3 days. It is required to be used in a fume hood.
Reduction of functional groups
In organic chemistry, aluminum hydride is mostly used for the reduction of functional groups.
In many ways, the reactivity of aluminum hydride is similar to that of lithium aluminum hydride. Aluminum hydride will reduce aldehydes, ketones, carboxylic acids, anhydrides, acid chlorides, esters, and lactones to their corresponding alcohols. Amides, nitriles, and oximes are reduced to their corresponding amines.
It has selectivity different from other hydride reagents. For example, in the following cyclohexanone reduction, lithium aluminum hydride gives a trans:cis ratio of 1.9 : 1, while aluminum hydride gives a trans:cis ratio of 7.3 : 1.
Aluminum hydride can be used in the reduction of acetals to half protected diols.
Aluminum hydride can also be used in epoxide ring opening reaction as shown below.
The allylic rearrangement reaction carried out using aluminum hydride is a SN2 reaction, and it is not sterically demanding.
Aluminium hydride may be a useful material for storing hydrogen in hydrogen-fueled vehicles. It contains up to 10% hydrogen by weight and can store up to 148g/L, twice the density of liquid H2. However, currently there are no ways to turn the aluminium byproduct back into AlH3. It also shows promise as an additive to rocket fuel. Aluminium hydride is used in explosive and pyrotechnic compositions.
|This article is licensed under the GNU Free Documentation License. It uses material from the Wikipedia article "Aluminium_hydride". A list of authors is available in Wikipedia.|