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Aluminium hydride

Aluminium hydride
IUPAC name	Aluminum hydride
Other names	Aluminium(III) hydride; alane; aluminium trihydride; aluminum hydride, α-alane
Identifiers
CAS number	7784-21-6
Properties
Molecular formula	AlH₃
Molar mass	29.99 g/mol
Appearance	white crystalline solid, non-volatile, highly polymerized, needle-like crystals
Density	1.486 g/cm³, solid
Melting point	150 °C
Boiling point	Decomposition
Solubility in water	Reactive
Related Compounds
Related compounds	Lithium aluminum hydride
Except where noted otherwise, data are given for materials in their standard state (at 25 °C, 100 kPa) Infobox disclaimer and references

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.^[1]^[2]. 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^[3], the dimer, Al₂H₆, has been isolated in solid hydrogen and is isostructural with diborane^[4].

Additional recommended knowledge

Essential Laboratory Skills Guide

What is the Correct Way to Check Repeatability in Balances?

History

Aluminium hydride was reported as impurities, amines and ether complexes throughout the history,^[5] until its first synthesis published in 1947 by Finholt, Bond, and Schlesinger from the George Herbert Jones Laboratory at University of Chicago.^[6] 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.^[7]
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°.^[8]

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.^[7]
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^[9], but in the solid phase it is dimeric with bridging hydrogens, (NMe₃Al(μ-H))₂.^[10] The 1:2 complex has a trigonal bipyramidal structure.^[9] Some adducts (e.g. dimethylethylamine alane, NMe₂Et.AlH₃) thermally decompose to give aluminum metal and may have use in MOCVD applications.^[11]

Preparation

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.^[1]

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.^[7]

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

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.^[7]

Uses

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.^[12]

Corey et al. have developed a procedure to hydroxymethylate certain ketones.^[13] (The ketone itself is not reduced as it is "protected" as its enolate.)

Organohalides are reduced slowly or not at all by aluminum hydride. Therefore, reactive functional groups such as carboxylic acids can be reduced in the presence of halides.^[14]

Nitro groups are not reduced by aluminum hydride. Likewise, aluminum hydride can accomplish the reduction of an ester in the presence of nitro groups.^[15]

Aluminum hydride can be used in the reduction of acetals to half protected diols.^[16]

Aluminum hydride can also be used in epoxide ring opening reaction as shown below.^[17]

The allylic rearrangement reaction carried out using aluminum hydride is a S_N2 reaction, and it is not sterically demanding.^[18]

Hydroalumination

Aluminum hydride has been shown to add to propargylic alcohols.^[19] Used together with titanium tetrachloride, aluminum hydride can add across double bonds.^[20] Hydroboration is a similar reaction.

Hydrogen storage

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.

References

^ ^a ^b Galatsis, P. In Encyclopedia of Reagents for Organic Synthesis; University of Guelph, Ontario, Canada
^ Brown, H. C.; Krishnamurthy, S. Tetrahedron 1979, 35, 567. (Review)
^ Kurth F A, Eberlein R A, Schnöckel H, 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". J. Chem. Soc., Chem. Commun: 1302. doi:10.1039/C39930001302.
^ Andrews, Lester; Wang, Xuefeng (2003). "The Infrared Spectrum of Al₂H₆ in Solid Hydrogen". Science 299 (5615): 2049 - 2052. doi:10.1126/science.1082456.
^ F. M. Brower, N. E. Matzek, P. F. Reigler, H. W. Rinn, C. B. Roberts, D. L. Schmidt, J. A. Snover, K. Terada (1976). "Preparation and properties of aluminum hydride" 98: 2450-2454. J. Am. Chem. Soc.. doi:10.1021/ja00425a011.
^ A. E. Finholt, A. C. Bond, Jr., H. I. Schlesinger (1947). "Lithium Aluminum Hydride, Aluminum Hydride and Lithium Gallium Hydride, and Some of their Applications in Organic and Inorganic Chemistry". J. Am. Chem. Soc. 69: 1199-1203. doi:10.1021/ja01197a061.
^ ^a ^b ^c ^d Lund, Gary K., Hanks, Jami M., Johnston, Harold E., US Patent and Trade Office, 2007, Pat. Application # 20070066839
^ Turley J W,. Rinn H W (1969). "The crystal structure of aluminum hydride". Inorganic Chemistry 8 (1): 18-22. doi:10.1021/ic50071a005.
^ ^a ^b Greenwood, N. N.; Earnshaw, A. (1997). Chemistry of the Elements, 2nd Edition, Oxford:Butterworth-Heinemann. ISBN 0-7506-3365-4.
^ Atwood JL , Bennett FR, Elms FM, Jones C, Raston CL, Robinson KD (1991). "Tertiary amine stabilized dialane". J. Am. Chem. Soc. 113 (21): 8183 - 8185. doi:10.1021/ja00021a063.
^ Jong-Ho Yun, Byoung-Youp Kim and Shi-Woo Rhee (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.
^ Ayres, D. C.; Sawdaye, R. J. Chem. Soc., Perkin Trans, 1967, 581.
^ E. J. Corey, David E. Cane (1971). "Controlled hydroxymethylation of ketones". J. Org. Chem. 36 (20): 3070-3070. doi:10.1021/jo00819a047.
^ Danishefsky, S.; Regan, J. Tetrahedron, 1962, 559.
^ S. Takano, M. Akiyama, S. Sato, K. Orgasawara, Chem. Lett., 1983, 1593.
^ W. J. Richter (1981). "Asymmetric synthesis at prochiral centers: substituted 1, 3-dioxolanes". J. Org. Chem. 46: 5119 - 5124. doi:10.1021/jo00338a011.
^ K. Maruoka, S. Saito, T. Ooi, H. Yamamoto, H. Synlett, 1991, 255.
^ A. Claesson, L.-I. Olsson (1979). "Allenes and acetylenes. 22. Mechanistic aspects of the allene-forming reductions (SN2' reaction) of chiral propargylic derivatives with hydride reagents". J. Am. Chem. Soc. 101: 7302 - 7311. doi:10.1021/ja00518a028.
^ Corey, E. J.; Katzenellenbogen, J. A.; Posner, G. H. J. Am. Chem. Soc. 1967, 89, 4245.
^ Sato, F.; Sato, S.; Kodama, H.; Sato, M. J. Organomet. Chem. 1977, 142, 71.

Categories: Aluminium compounds | Inorganic compounds | Reducing agents | Metal hydrides

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