|PubChem||, (2H3) , (3H3)|
|Jmol-3D images||Image 1|
|Molar mass||29.99 g/mol|
|Appearance||white crystalline solid, non-volatile, highly polymerized, needle-like crystals|
|Density||1.477 g/cm3, solid|
|Melting point||150 °C|
|Solubility in water||Reactive|
|Related compounds||Lithium aluminium hydride|
|Except where noted otherwise, data are given for materials in their standard state (at 25 °C (77 °F), 100 kPa)|
|(what is: / ?)|
Aluminium hydride (also known as alane) is an inorganic compound with the formula AlH3. It is a colourless pyrophoric solid. Although rarely encountered outside of research laboratories, alane and its derivatives are used as reducing agents in organic synthesis.1
Alane is a polymer. Its formula is sometimes represented with the formula (AlH3)n. Aluminium hydride forms numerous polymorphs, which are named α-alane, α’-alane, β-alane, δ-alane, ε-alane, θ-alane, and γ-alane. α-Alane has a cubic or rhombohedral morphology, whereas 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.2
The structure of α-alane has been determined and features aluminium atoms surrounded by 6 hydrogen atoms that bridge to 6 other aluminium atoms. The Al-H distances are all equivalent (172pm) and the Al-H-Al angle is 141°.3
α-Alane is the most thermally stable polymorph. β-alane and γ-alane are produced together, and convert to α-alane upon heating. δ, ε, and θ-alane are produced in different crystallization condition. Although they are less thermally stable, they do not convert into α-alane upon heating.2
Monomeric AlH3 has been isolated at low temperature in a solid noble gas matrix and shown to be planar.4 The dimer Al2H6 has been isolated in solid hydrogen and is isostructural with diborane,B2H6, and digallane, Ga2H6.56
Aluminium hydride impurities and related amines and ether complexes have long been reported.7 Its first synthesis published in 1947 and a U.S. patent for the synthesis was assigned to Petrie et al. in 1999.89 Aluminium hydride is prepared by treating lithium aluminium hydride with aluminium trichloride.10 The procedure is intricate, attention must be given to the removal of lithium chloride.
- 3 LiAlH4 + AlCl3 → 4 AlH3 + 3 LiCl
The ether solution of alane requires immediate use, because polymeric material precipitates otherwise. Aluminium hydride solutions are known to degrade after 3 days. Aluminium hydride is more reactive than LiAlH4, but their handling properties are similar.2
Several other methods exist for the preparation of aluminium hydride:
- 2 LiAlH4 + BeCl2 → 2 AlH3 + Li2BeH2Cl2
- 2 LiAlH4 + H2SO4 → 2 AlH3 + Li2SO4 + 2 H2
- 2 LiAlH4 + ZnCl2 → 2 AlH3 + 2 LiCl + ZnH2
Alane can be produced electrochemically.11121314 As described in the 1962 patent, the method avoids chloride impurities. Two possible mechanisms are discussed for the formation of alane in Clasen's electrochemical cell containing THF as the solvent, sodium aluminium hydride as the electrolyte, an aluminium anode, and an iron (Fe) wire submerged in mercury (Hg) as the cathode. The sodium forms an amalgam with the Hg cathode preventing side reactions and the hydrogen produced in the first reaction could be captured and reacted back with the sodium mercury amalgam to produce sodium hydride. Clasen's system results in no loss of starting material. For an insoluble anode see reaction 1.
1. AlH4- - e- → AlH3 · nTHF + ½H2 For soluble anodes, anodic dissolution is expected according to reaction 2,
2. 3AlH4- + Al - 3e- → 4AlH3 · nTHF In reaction 2, the aluminium anode is consumed, limiting the production of aluminium hydride for a given electrochemical cell.
α-AlH3 can be produced by hydrogenation of aluminium metal at 10GPa and 600 °C. The reaction between the liquified hydrogen produces α-AlH3 which could be recovered under ambient conditions.15
AlH3 readily forms adducts with strong Lewis bases. For example, both 1:1 and 1:2 complexes form with trimethylamine. The 1:1 complex is tetrahedral in the gas phase,16 but in the solid phase it is dimeric with bridging hydrogen centres, (NMe3Al(μ-H))2.17 The 1:2 complex adopts a trigonal bipyramidal structure.16 Some adducts (e.g. dimethylethylamine alane, NMe2Et · AlH3) thermally decompose to give aluminium metal and may have use in MOCVD applications.18
Its complex with diethyl ether forms according to the following stoichiometry:
- AlH3 + (C2H5)2O → H3Al · O(C2H5)2
- AlH3 + LiH → LiAlH4
In organic chemistry, aluminium hydride is mainly used for the reduction of functional groups.19 In many ways, the reactivity of aluminium hydride is similar to that of lithium aluminium hydride. Aluminium 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.
In terms of functional group selectivity, alane differs from other hydride reagents. For example, in the following cyclohexanone reduction, lithium aluminium hydride gives a trans:cis ratio of 1.9 : 1, whereas aluminium hydride gives a trans:cis ratio of 7.3 : 1.20
Alane enables the hydroxymethylation of certain ketones, that is the replacement of C-H by C-CH2OH).21 The ketone itself is not reduced as it is "protected" as its enolate.
Aluminium hydride can be used in the reduction of acetals to half protected diols.24
Aluminium hydride can also be used in epoxide ring opening reaction as shown below.25
The allylic rearrangement reaction carried out using aluminium hydride is a SN2 reaction, and it is not sterically demanding.26
- 4 AlH3 + 3 CO2 → 3 CH4 + 2 Al2O3
Aluminium hydride have been discussed for storing hydrogen in hydrogen-fueled vehicles. AlH3 contains up to 10% hydrogen by weight, corresponding to 148g/L, twice the density of liquid H2. Unfortunately, AlH3 is not a reversible carrier of hydrogen.29 It is a potential additive to rocket fuel and in explosive and pyrotechnic compositions.
Aluminium hydride is not spontaneously flammable, but it is highly reactive, similar to lithium aluminium hydride. Aluminium hydride decomposes in air and water. Violent reactions occur with both.2 With care AlH3 can be handled safely in air, thought to be a result of a protective layer of aluminium oxide29
- Brown, H. C.; Krishnamurthy, S. (1979). "Forty Years of Hydride Reductions". Tetrahedron 35 (5): 567–607. doi:10.1016/0040-4020(79)87003-9.
- US application 2007066839, Lund, G. K.; Hanks, J. M.; Johnston, H. E., "Method for the Production of α-Alane"
- Turley, J. W.; Rinn, H. W. (1969). "The Crystal Structure of Aluminum Hydride". Inorganic Chemistry 8 (1): 18–22. doi:10.1021/ic50071a005.
- Kurth, F. A.; Eberlein, R. A.; Schnöckel, H.-G.; Downs, A. J.; Pulham, C. R. (1993). "Molecular Aluminium Trihydride, AlH3: Generation in a Solid Noble Gas Matrix and Characterisation by its Infrared Spectrum and ab initio Calculations". Journal of the Chemical Society, Chemical Communications 1993 (16): 1302–1304. doi:10.1039/C39930001302.
- Andrews, L.; Wang, X. (2003). "The Infrared Spectrum of Al2H6 in Solid Hydrogen". Science 299 (5615): 2049–2052. doi:10.1126/science.1082456. PMID 12663923.
- Pulham, C. R.; Downs, A. J.; Goode, M. J.; Rankin D. W. H.; Robertson, H. E. (1991). "Gallane: Synthesis, Physical and Chemical Properties, and Structure of the Gaseous Molecule Ga2H6 as Determined by Electron Diffraction". Journal of the American Chemical Society 113 (14): 5149–5162. doi:10.1021/ja00014a003.
- Brower, F. M.; Matzek, N. E.; Reigler, P. F.; Rinn, H. W.; Roberts, C. B.; Schmidt, D. L.; Snover, J. A.; Terada, K. (1976). "Preparation and Properties of Aluminum Hydride". Journal of the American Chemical Society 98 (9): 2450–2454. doi:10.1021/ja00425a011.
- Finholt, A. E.; Bond, A. C. Jr.; Schlesinger, H. I. (1947). "Lithium Aluminum Hydride, Aluminum Hydride and Lithium Gallium Hydride, and Some of their Applications in Organic and Inorganic Chemistry". Journal of the American Chemical Society 69 (5): 1199–1203. doi:10.1021/ja01197a061.
- US patent 6228338, Petrie, M. A.; Bottaro, J. C.; Schmitt, R. J.; Penwell, P. E.; Bomberger, D. C., "Preparation of Aluminum Hydride Polymorphs, Particularly Stabilized α-AlH3", issued 2001-05-08
- Schmidt, D. L.; Roberts, C. B.; Reigler, P. F.; Lemanski, M. F. Jr.; Schram, E. P. (1973). "Aluminum Trihydride-Diethyl Etherate: (Etherated Alane)". Inorganic Syntheses 14: 47–52. doi:10.1002/9780470132456.ch10.
- Alpatova, N. M.; Dymova, T. N.; Kessler, Yu. M.; Osipov, O. R. (1968). "Physicochemical Properties and Structure of Complex Compounds of Aluminium Hydride". Russian Chemical Reviews 37 (2): 99–114. doi:10.1070/RC1968v037n02ABEH001617.
- Semenenko, K. N.; Bulychev, B. M.; Shevlyagina, E. A. (1966). "Aluminium Hydride". Russian Chemical Reviews 35 (9): 649–658. doi:10.1070/RC1966v035n09ABEH001513.
- Osipov, O. R.; Alpatova, N. M.; Kessler, Yu. M. (1966). Elektrokhimiya 2: 984.
- DE patent 1141623, Clasen, H., "Verfahren zur Herstellung von Aluminiumhydrid bzw. aluminiumwasserstoffreicher komplexer Hydride", issued 1962-12-27, assigned to Metallgesellschaft
- Saitoh, H; Sakurai, Y; Machida, A; Katayama, Y; Aoki, K (2010). "In situX-ray diffraction measurement of the hydrogenation and dehydrogenation of aluminum and characterization of the recovered AlH3". Journal of Physics: Conference Series 215: 012127. doi:10.1088/1742-6596/215/1/012127. ISSN 1742-6596.
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- Yun, J.-H.; Kim, B.-Y.; Rhee, S.-W. (1998). "Metal-Organic Chemical Vapor Deposition of Aluminum from Dimethylethylamine Alane". Thin Solid Films 312 (1–2): 259–263. doi:10.1016/S0040-6090(97)00333-7.
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- Aluminium Hydride on EnvironmentalChemistry.com Chemical Database
- Hydrogen Storage from Brookhaven National Laboratory
- Aluminum Trihydride on WebElements