Aluminium hydride

Aluminium hydride
Preferred IUPAC name Aluminium hydride
Systematic name Alumane
Other names Alane Aluminic hydride Aluminium(III) hydride Aluminium trihydride Trihydridoaluminium
Identifiers
CAS number	7784-21-6 Y
PubChem	14488 Y, 14399066 (2H3) Y, 16721258 (3H3) Y
ChemSpider	13833 Y, 17625618 (3H3)
ChEBI	CHEBI:30136 Y
Gmelin Reference	245
Jmol-3D images	Image 1
SMILES [AlH3]
InChI InChI=1S/Al.3H Y Key: AZDRQVAHHNSJOQ-UHFFFAOYSA-N Y InChI=1S/Al.3H Key: AZDRQVAHHNSJOQ-UHFFFAOYSA-N InChI=1/Al.3H/rAlH3/h1H3 Key: AZDRQVAHHNSJOQ-FSBNLZEDAV
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 aluminium hydride
Y (verify) (what is: Y/N?) Except where noted otherwise, data are given for materials in their standard state (at 25 °C, 100 kPa)
Infobox references

Aluminium hydride is an inorganic compound with the formula AlH₃. It is a colourless solid that is pyrophoric. Although rarely encountered outside of research laboratory, alane and its derivatives are used as reducing agents in organic synthesis.^[1]

Structure

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-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

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

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 precipitate 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

Alane can be produced electrochemically.^{[11][12][13][14]} 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.

Reactions

Formation of adducts with Lewis bases

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,^[15] but in the solid phase it is dimeric with bridging hydrogen centres, (NMe₃Al(μ-H))₂.^[16] The 1:2 complex adopts a trigonal bipyramidal structure.^[15] Some adducts (e.g. dimethylethylamine alane, NMe₂Et · AlH₃) thermally decompose to give aluminium metal and may have use in MOCVD applications.^[17]

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

In organic chemistry, aluminium hydride is mainly used for the reduction of functional groups.^[18] 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.^[19]

Alane enables the hydroxymethylation of certain ketones, that is the replacement of C-H by C-CH₂OH).^[20] 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.^{[21][citation needed]}

Nitro groups are not reduced by aluminium hydride. Likewise, aluminium hydride can accomplish the reduction of an ester in the presence of nitro groups.^[22]

Aluminium hydride can be used in the reduction of acetals to half protected diols.^[23]

Aluminium hydride can also be used in epoxide ring opening reaction as shown below.^[24]

The allylic rearrangement reaction carried out using aluminium hydride is a S_N2 reaction, and it is not sterically demanding.^[25]

Aluminium hydride even reduces carbon dioxide to methane under heating:

4 AlH₃ + 3 CO₂ → 3 CH₄ + 2 Al₂O₃

Hydroalumination

Aluminium hydride has been shown to add to propargylic alcohols.^[26] Used together with titanium tetrachloride, aluminium hydride can add across double bonds.^[27] Hydroboration is a similar reaction.

Fuel

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

Precautions

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]

References

External links

• Aluminium Hydride on EnvironmentalChemistry.com Chemical Database
• Hydrogen Storage from Brookhaven National Laboratory
• Aluminum Trihydride on WebElements