Aluminium hydride

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Aluminium hydride
Unit cell spacefill model of aluminium hydride
Identifiers
CAS number 7784-21-6 YesY
PubChem 14488 YesY, 14399066 (2H3YesY, 16721258 (3H3YesY
ChemSpider 13833 YesY, 17625618 (3H3YesY
ChEBI CHEBI:30136 YesY
Gmelin Reference 245
Jmol-3D images Image 1
Properties
Molecular formula AlH3
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 (302 °F; 423 K) starts decomposing at 105 °C
Solubility in water reacts
Solubility soluble in ether
reacts in ethanol
Thermochemistry
Specific
heat capacity
C
40.2 J/mol K
Std molar
entropy
So298
30 K/mol K
Std enthalpy of
formation
ΔfHo298
-11.4 kJ/mol
Gibbs free energy ΔG 46.4 kJ/mol
Related compounds
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)
 YesY (verify) (what is: YesY/N?)
Infobox references

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]

Structure[edit]

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]

Aluminium-hydride-unit-cell-3D-balls.png
Aluminium-hydride-Al-coordination-3D-balls.png
Aluminium-hydride-H-coordination-3D-balls.png
α-AlH3 unit cell
Al coordination
H coordination

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]

Molecular forms of alane[edit]

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.[5][6]

Preparation[edit]

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.[8][9] 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

Electrochemical synthesis[edit]

Several groups have shown that alane can be produced electrochemically.[11][12][13][14][15] Different electrochemical alane production methods have been patented.[16][17] Electrochemically generating alane 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.

The crystallization and recovery of aluminum hydride from electrochemically generated alane has been demonstrated.[18][19]

High pressure hydrogenation of aluminium metal[edit]

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.[20]

Reactions[edit]

Formation of adducts with Lewis bases[edit]

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,[21] but in the solid phase it is dimeric with bridging hydrogen centres, (NMe3Al(μ-H))2.[22] The 1:2 complex adopts a trigonal bipyramidal structure.[21] Some adducts (e.g. dimethylethylamine alane, NMe2Et · AlH3) thermally decompose to give aluminium metal and may have use in MOCVD applications.[23]

Its complex with diethyl ether forms according to the following stoichiometry:

AlH3 + (C2H5)2O → H3Al · O(C2H5)2

The reaction with lithium hydride in ether produces lithium aluminium hydride:

AlH3 + LiH → LiAlH4

Reduction of functional groups[edit]

In organic chemistry, aluminium hydride is mainly used for the reduction of functional groups.[24] 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.[25]

Stereoselective reduction of a substituted cyclohexanone using aluminium hydride

Alane enables the hydroxymethylation of certain ketones, that is the replacement of C-H by C-CH2OH).[26] The ketone itself is not reduced as it is "protected" as its enolate.

Functional Group Reduction using aluminium hydride

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.[27][citation needed]

Functional Group Reduction using aluminium hydride

Nitro groups are not reduced by aluminium hydride. Likewise, aluminium hydride can accomplish the reduction of an ester in the presence of nitro groups.[28]

Ester reduction using aluminium hydride

Aluminium hydride can be used in the reduction of acetals to half protected diols.[29]

Acetal reduction using aluminium hydride

Aluminium hydride can also be used in epoxide ring opening reaction as shown below.[30]

Epoxide reduction using aluminium hydride

The allylic rearrangement reaction carried out using aluminium hydride is a SN2 reaction, and it is not sterically demanding.[31]

Phosphine reduction using aluminium hydride

Aluminium hydride even reduces carbon dioxide to methane under heating:

4 AlH3 + 3 CO2 → 3 CH4 + 2 Al2O3

Hydroalumination[edit]

Aluminium hydride has been shown to add to propargylic alcohols.[32] Used together with titanium tetrachloride, aluminium hydride can add across double bonds.[33] Hydroboration is a similar reaction.

Hydroalumination of 1-hexene

Fuel[edit]

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.[34] It is a potential additive to rocket fuel and in explosive and pyrotechnic compositions.

Precautions[edit]

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 oxide[34]

References[edit]

  1. ^ Brown, H. C.; Krishnamurthy, S. (1979). "Forty Years of Hydride Reductions". Tetrahedron 35 (5): 567–607. doi:10.1016/0040-4020(79)87003-9. 
  2. ^ a b c d US application 2007066839, Lund, G. K.; Hanks, J. M.; Johnston, H. E., "Method for the Production of α-Alane" 
  3. ^ Turley, J. W.; Rinn, H. W. (1969). "The Crystal Structure of Aluminum Hydride". Inorganic Chemistry 8 (1): 18–22. doi:10.1021/ic50071a005. 
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  17. ^ US patent 8470156, Zidan, R., "Electrochemical process and production of novel complex hydrides", issued 2013-06-25, assigned to Savannah River Nuclear Solutions, LLC 
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External links[edit]