Aluminium hydride

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Aluminium hydride
Unit cell spacefill model of aluminium hydride
Names
Preferred IUPAC name
Aluminium hydride
Systematic IUPAC name
Alumane
Other names
  • Alane
  • Aluminic hydride
  • Aluminium(III) hydride
  • Aluminium trihydride
  • Trihydridoaluminium
Identifiers
3D model (
JSmol
)
ChEBI
ChemSpider
ECHA InfoCard
 100.029.139 Edit this at Wikidata
245
UNII
  • InChI=1S/Al.3H checkY
    Key: AZDRQVAHHNSJOQ-UHFFFAOYSA-N checkY
  • InChI=1S/Al.3H
    Key: AZDRQVAHHNSJOQ-UHFFFAOYSA-N
  • InChI=1/Al.3H/rAlH3/h1H3
    Key: AZDRQVAHHNSJOQ-FSBNLZEDAV
  • [AlH3]
Properties
AlH3
Molar mass 30.006 g·mol−1
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 (221 °F)
reacts
Solubility soluble in ether
reacts in ethanol
Thermochemistry
40.2 J/(mol·K)
30 J/(mol·K)
Std enthalpy of
formation
(ΔfH⦵298)
 −11.4 kJ/mol
46.4 kJ/mol
Related compounds
Related compounds
 Lithium aluminium hydride, diborane
Except where otherwise noted, data are given for materials in their standard state (at 25 °C [77 °F], 100 kPa).
checkY verify (what is checkY☒N ?)

Aluminium hydride (also known as alane and alumane) is an

nitro groups), and although it is not a reagent of choice, it can react with carbon-carbon multiple bonds (i.e., through hydroalumination). Given its density, and with hydrogen content on the order of 10% by weight,[2] some forms of alane are, as of 2016,[3] active candidates for storing hydrogen and so for power generation in fuel cell applications, including electric vehicles.[not verified in body
] As of 2006 it was noted that further research was required to identify an efficient, economical way to reverse the process, regenerating alane from spent aluminium product.

Solid aluminium hydride, or alane, is colorless and nonvolatile, and in its most common reagent form it is a highly

metal hydrides, and must be handled and stored with the active exclusion of moisture. Alane decomposes on exposure to air (principally because of adventitious moisture), though passivation — here, allowing for development of an inert surface coating — greatly diminishes the rate of decomposition of alane preparations.[not verified in body
]

Form and structure

Alane is a colorless and nonvolatile solid that melts with decomposition at 110 °C.[4] The solid form, however, often presents as a white solid that may be tinted grey (with decreasing reagent particle size or increasing impurity levels).[citation needed] This coloration arises from a thin surface passivation layer of aluminium oxide or hydroxide.[citation needed]

Under common laboratory conditions, alane is "highly polymeric", structurally.[4] This is sometimes indicated with the formula (AlH3)n, where n is left unspecified.[5][non-primary source needed] Preparations of alane dissolve in tetrahydrofuran (THF) or diethyl ether (ether),[4] from which pure allotropes precipitate.[6][non-primary source needed]

Structurally, alane can adopt numerous

polymorphic forms. By 2006, "at least 7 non-solvated AlH3 phases" were known: α-, α’-, β-, γ-, ε-, and ζ-alanes;[2] the δ- and θ-alanes have subsequently been discovered.[citation needed] Each has a different structure, with α-alane being the most thermally stable polymorph.[citation needed] For instance, crystallographically, α-alane adopts a cubic or rhombohedral morphology, while α’-alane forms needle-like crystals and γ-alane forms bundles of fused needles.[citation needed] The crystal structure of α-alane has been determined, and features aluminium atoms surrounded by six octahedrally oriented hydrogen atoms that bridge to six other aluminium atoms (see table), where the Al-H distances are all equivalent (172 pm) and the Al-H-Al angle is 141°.[7]

Crystallographic Structure of α-AlH3[8]
The α-AlH3 unit cell Aluminium coordination Hydrogen coordination

When β- and γ-alanes are produced together, they convert to α-alane upon heating, while δ-, ε-, and θ-alanes are produced in still other crystallization conditions; although they are less thermally stable, the δ-, ε-, and θ-alane polymorphs do not convert to α-alane upon heating.

better source needed
]

Under special conditions, non-polymeric alanes (i.e., molecular forms of it) can be prepared and studied. Monomeric AlH3 has been isolated at low temperature in a solid noble gas matrix where it was shown to be planar.[9] The dimeric form, Al2H6, has been isolated in solid hydrogen, and it is isostructural with diborane (B2H6) and digallane (Ga2H6).[10][11][non-primary source needed]

Handling

Alane is not spontaneously flammable.

dry box".[13] After use, solution containers are typically sealed tightly with concomitant flushing with inert gas to exclude the oxygen and moisture of ambient air.[13]

Passivation[clarification needed] greatly diminishes the decomposition rate associated with alane preparations.[citation needed] Passivated alane nevertheless retains a hazard classification of 4.3 (chemicals which in contact with water, emit flammable gases).[14]

Reported accidents

Alane reductions are believed to proceed via an intermediate

fluorinated, the intermediate may instead explode if exposed to a hot spot above 60°C.[15]

Preparation

Aluminium hydrides and various complexes thereof have long been known.

aluminium trichloride.[19] The procedure is intricate: attention must be given to the removal of lithium chloride
.

3 Li[AlH4] + AlCl3 → 4 AlH3 + 3 LiCl

The ether solution of alane requires immediate use, because polymeric material rapidly precipitates as a solid. Aluminium hydride solutions are known to degrade after 3 days. Aluminium hydride is more reactive than Li[AlH4].[6]

Several other methods exist for the preparation of aluminium hydride:

2 Li[AlH4] + BeCl2 → 2 AlH3 + Li2[BeH2Cl2]
2 Li[AlH4] + H2SO4 → 2 AlH3 + Li2SO4 + 2 H2
2 Li[AlH4] + ZnCl2 → 2 AlH3 + 2 LiCl + ZnH2
2 Li[AlH4] + I2 → 2 AlH3 + 2 LiI + H2

Electrochemical synthesis

Several groups have shown that alane can be produced electrochemically.[20][21][22][23][24] Different electrochemical alane production methods have been patented.[25][26] 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 insoluble anodes, reaction 1 occurs, while for soluble anodes, anodic dissolution is expected according to reaction 2:

  1. [AlH4]− − e− + n THF → AlH3·nTHF + 1/2 H2
  2. 3 [AlH4]− + Al − 3 e− + 4n THF → 4 AlH3·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 aluminium hydride from electrochemically generated alane has been demonstrated.[23][24]

High pressure hydrogenation of aluminium

α-AlH3 can be produced by hydrogenation of aluminium at 10

GPa and 600 °C (1,112 °F). The reaction between the liquified hydrogen produces α-AlH3 which could be recovered under ambient conditions.[27]

Reactions

Formation of adducts with Lewis bases

AlH3 readily forms adducts with strong

MOCVD applications.[30]

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

AlH3 + (CH3CH2)2O → (CH3CH2)2O·AlH3

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

AlH3 + LiH → Li[AlH4]

Reduction of functional groups

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

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.[32]

Stereoselective reduction of a substituted cyclohexanone using aluminium hydride
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 at the

alpha position).[33]
The ketone itself is not reduced as it is "protected" as its enolate.

Functional Group Reduction using aluminium hydride
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.[34]

Functional Group Reduction using aluminium hydride
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.[35]

Ester reduction using aluminium hydride
Ester reduction using aluminium hydride

Aluminium hydride can be used in the reduction of acetals to half protected diols.[36]

Acetal reduction using aluminium hydride
Acetal reduction using aluminium hydride

Aluminium hydride can also be used in epoxide ring opening reaction as shown below.[37]

Epoxide reduction using aluminium hydride
Epoxide reduction using aluminium hydride

The allylic rearrangement reaction carried out using aluminium hydride is a SN2 reaction, and it is not sterically demanding.[38]

Phosphine reduction using aluminium hydride
Phosphine reduction using aluminium hydride

Aluminium hydride will reduce carbon dioxide to methane with heating:[citation needed]

4 AlH3 + 3 CO2 → 3 CH4 + 2 Al2O3

Hydroalumination

See also: Carbometalation § Carboalumination

Akin to

propargylic alcohols, the results are Alkenylaluminium compounds.[41]

Hydroalumination of 1-hexene
Hydroalumination of 1-hexene

Fuel

In its passivated form, alane is an active candidate for storing hydrogen, and can be used for efficient power generation via fuel cell applications, including fuel cell and electric vehicles and other lightweight power applications.[citation needed] AlH3 contains up 10.1% hydrogen by weight (at a density of 1.48 grams per milliliter),[2] or twice the hydrogen density of liquid H2.[citation needed] As of 2006, AlH3 was being described as a candidate for which "further research w[ould] be required to develop an efficient and economical process to regenerate [it] from the spent Al powder".[2][needs update]

Allane is also a potential additive to

rocket fuel additive, capable of delivering impulse efficiency gains of up to 10%.[42]

References

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  5. ^ See, e.g., Andrews & Wang 2003.
  6. ^ a b c US application 2007066839, Lund, G. K.; Hanks, J. M.; Johnston, H. E., "Method for the Production of α-Alane." 
  7. ^ Turley & Rinn 1969. (Abstract) "The final Al⋯H distance of 1.72 Å, the participation of each Al in six bridges, and the equivalence of all Al⋯H distances suggest that 3c-2e bonding occurs." Angle is lasted as "Al(6)-H(5)-Al(4)" in Table IV.
  8. doi:10.1021/ic50071a005
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  9. doi:10.1039/C39930001302
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  18. ^ US patent 6228338, Petrie, M. A.; Bottaro, J. C.; Schmitt, R. J.; Penwell, P. E.; Bomberger, D. C., "Preparation of Aluminum Hydride Polymorphs, Particularly Stabilized α-AlH3", issued 2001-05-08 
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External links
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