- Aluminium hydride
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IUPACName = aluminium trihydride
alumane
trihydridoaluminium
OtherNames = Aluminium(III) hydride; alane; aluminum hydride, α-alane
Section1 = Chembox Identifiers
CASNo = 7784-21-6
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Section2 = Chembox Properties
Formula = AlH3
MolarMass = 29.99 g/mol
Appearance = white crystalline solid, non-volatile, highly polymerized, needle-like crystals
Density = 1.486 g/cm3, solid
MeltingPt = 150 °C
BoilingPt = Decomposition
Solubility = Reactive
Section3 = Chembox Hazards
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FlashPt =
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Section8 = Chembox Related
OtherCpds =Lithium aluminium hydride Aluminium hydride,
chemical formula AlH3, is a chemical reagent used as a reducing agent. It is used in hydroalumination ofalkyne s, allylic rearrangements, and storing hydrogen in hydrogen-fueled vehicles.Galatsis, P. In "Encyclopedia of Reagents for Organic Synthesis"; University of Guelph, Ontario, Canada] [Brown, H. C.; Krishnamurthy, S. "Tetrahedron" 1979, "35", 567. (Review)] . 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 [cite journal
title = Molecular aluminium trihydride, AlH3: generation in a solid noble gas matrix and characterisation by its infrared spectrum and Ab initio calculations
author = Kurth F A, Eberlein R A, Schnöckel H, Downs A J, Pulham C R,
journal = J. Chem. Soc., Chem. Commun
year = 1993
volume =
issue =
pages = 1302
doi = 10.1039/C39930001302 ] , the dimer, Al2H6, has been isolated in solid hydrogen and is isostructural withdiborane [cite journal
title = The Infrared Spectrum of Al2H6 in Solid Hydrogen
author = Andrews, Lester; Wang, Xuefeng
journal = Science
year = 2003
volume = 299
issue = 5615
pages = 2049–2052
doi = 10.1126/science.1082456 ] .History
Aluminium hydride was reported as impurities,
amine s andether complexes throughout the history, [cite journal | author = F. M. Brower, N. E. Matzek, P. F. Reigler, H. W. Rinn, C. B. Roberts, D. L. Schmidt, J. A. Snover, K. Terada | publisher =J. Am. Chem. Soc. | year = 1976 | volume = 98 | pages = 2450–2454 | doi = 10.1021/ja00425a011 | title = Preparation and properties of aluminum hydride | journal = Journal of the American Chemical Society] until its first synthesis published in 1947 by Finholt, Bond, and Schlesinger from the George Herbert Jones Laboratory at University of Chicago. [cite journal | author = A. E. Finholt, A. C. Bond, Jr., H. I. Schlesinger | journal =J. Am. Chem. Soc. | year = 1947 | volume = 69 | pages = 1199–1203 | doi = 10.1021/ja01197a061 | title = Lithium Aluminum Hydride, Aluminum Hydride and Lithium Gallium Hydride, and Some of their Applications in Organic and Inorganic Chemistry] 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.Lund, Gary K., Hanks, Jami M., Johnston, Harold E., US Patent and Trade Office, 2007, Pat. Application # 20070066839]
The structure of α-alane has been determined and contains 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°. [cite journal | title = The crystal structure of aluminum hydride| author =Turley J W,. Rinn H W | journal = Inorganic Chemistry| year = 1969 | volume = 8 | issue = 1 | pages = 18–22 | doi = 10.1021/ic50071a005 ]Chemical properties
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.
AlH3 readily forms adducts with strong Lewis bases e.g. 1:1 and 1:2 complexes withtrimethylamine . The 1:1 complex with trimethylamine is tetrahedral in the gas phase Greenwood&Earnshaw] , but in the solid phase it is dimeric with bridging hydrogens, (NMe3Al(μ-H))2. [cite journal
title = Tertiary amine stabilized dialane
author = Atwood JL , Bennett FR, Elms FM, Jones C, Raston CL, Robinson KD
journal = J. Am. Chem. Soc.
year = 1991
volume = 113
issue = 21
pages = 8183–8185
doi = 10.1021/ja00021a063] The 1:2 complex has a trigonal bipyramidal structure. Some adducts (e.g. dimethylethylamine alane, NMe2Et.AlH3) thermally decompose to give aluminium metal and may have use inMOCVD applications. [cite journal
title = Metal-organic chemical vapor deposition of aluminum from dimethylethylamine alane
author = Jong-Ho Yun, Byoung-Youp Kim and Shi-Woo Rhee
journal = Thin Solid Films
year = 1998
volume = 312
issue = 1-2
pages = 259–263
doi = 10.1016/S0040-6090(97)00333-7 ]Preparation
Aluminium hydride is generally prepared by treating an ether solution of
lithium aluminium hydride (LAH) withaluminium trichloride . An ether solution aluminium hydride is prepared after precipitation oflithium 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 [3] 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
Precautions
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