- Aluminium hydride
IUPACName = aluminium trihydride
OtherNames = Aluminium(III) hydride; alane; aluminum hydride, α-alane
Section1 = Chembox Identifiers
CASNo = 7784-21-6
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
Section8 = Chembox Related
Lithium aluminium hydride
chemical formulaAlH3, 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.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
pages = 1302
doi = 10.1039/C39930001302 ] , the dimer, Al2H6, has been isolated in solid hydrogen and is isostructural with
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 ] .
Aluminium hydride was reported as impurities,
amines and ethercomplexes 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 ]
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 with
trimethylamine. 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 in
MOCVDapplications. [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 ]
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, aluminium hydride is mostly used for the reduction of functional groups.
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.
It has selectivity different from other hydride reagents. For example, in the following cyclohexanone reduction,
lithium aluminium hydridegives a trans:cis ratio of 1.9 : 1, while aluminium hydride gives a trans:cis ratio of 7.3 : 1. [Ayres, D. C.; Sawdaye, R. " J. Chem. Soc.", Perkin Trans, 1967, 581.]
Corey "et al." have developed a procedure to hydroxymethylate certain ketones. [cite journal | title = Controlled hydroxymethylation of ketones | author =
E. J. Corey, David E. Cane | journal = J. Org. Chem.| year = 1971 | volume = 36 | issue = 20 | pages = 3070–3070 | doi = 10.1021/jo00819a047] (The ketone itself is not reduced as it is "protected" as its enolate.) Organohalides are reduced slowly or not at all by aluminium hydride. Therefore, reactive functional groups such as carboxylic acids can be reduced in the presence of halides. [Danishefsky, S.; Regan, J. " Tetrahedron", 1962, 559.] Nitro groups are not reduced by aluminium hydride. Likewise, aluminium hydride can accomplish the reduction of an esterin the presence of nitro groups. [S. Takano, M. Akiyama, S. Sato, K. Orgasawara, "Chem. Lett.", 1983, 1593.]
Aluminium hydride can be used in the reduction of acetals to half protected diols. [cite journal | author = W. J. Richter | journal =
J. Org. Chem.| year = 1981 | volume = 46 | pages = 5119–5124 | title = Asymmetric synthesis at prochiral centers: substituted 1, 3-dioxolanes | doi = 10.1021/jo00338a011]
Aluminium hydride can also be used in epoxide ring opening reaction as shown below. [K. Maruoka, S. Saito, T. Ooi, H. Yamamoto, "H. Synlett", 1991, 255.]
The allylic rearrangement reaction carried out using aluminium hydride is a SN2 reaction, and it is not sterically demanding. [cite journal | author = A. Claesson, L.-I. Olsson | journal =
J. Am. Chem. Soc.| year = 1979 | volume = 101 | pages = 7302–7311 | title = Allenes and acetylenes. 22. Mechanistic aspects of the allene-forming reductions (SN2' reaction) of chiral propargylic derivatives with hydride reagents | doi = 10.1021/ja00518a028]
Aluminium hydride has been shown to add to
propargylic alcohols. [Corey, E. J.; Katzenellenbogen, J. A.; Posner, G. H. " J. Am. Chem. Soc." 1967, "89", 4245.] Used together with titanium tetrachloride, aluminium hydride can add across double bonds. [Sato, F.; Sato, S.; Kodama, H.; Sato, M. " J. Organomet. Chem." 1977, "142", 71.] Hydroborationis a similar reaction.
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.
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