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Chemical compound From Wikipedia, the free encyclopedia
Aluminium hydride (also known as alane and alumane) is an inorganic compound with the formula AlH3. Alane and its derivatives are part of a family of common reducing reagents in organic synthesis based around group 13 hydrides.[1] In solution—typically in ethereal solvents such tetrahydrofuran or diethyl ether—aluminium hydride forms complexes with Lewis bases, and reacts selectively with particular organic functional groups (e.g., with carboxylic acids and esters over organic halides and 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.
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Names | |
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Preferred IUPAC name
Aluminium hydride | |
Systematic IUPAC name
Alumane | |
Other names
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Identifiers | |
3D model (JSmol) |
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ChEBI | |
ChemSpider | |
ECHA InfoCard | 100.029.139 |
245 | |
PubChem CID |
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UNII | |
CompTox Dashboard (EPA) |
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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 | |
Heat capacity (C) |
40.2 J/(mol·K) |
Std molar entropy (S⦵298) |
30 J/(mol·K) |
Std enthalpy of formation (ΔfH⦵298) |
−11.4 kJ/mol |
Gibbs free energy (ΔfG⦵) |
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).
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Solid aluminium hydride, or alane, is colorless and nonvolatile, and in its most common reagent form it is a highly polymerized species (i.e., has multiple AlH3 units that are self-associated); it melts with decomposition at 110 °C.[4] While not spontaneously flammable, alane solids and solutions require precautions in use akin to other highly flammable 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]
This section needs additional citations for verification. (July 2022) |
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]
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.[6][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][12]
Alane is not spontaneously flammable.[13] Even so, "similar handling and precautions as... exercised for Li[AlH4]" (the chemical reagent, lithium aluminium hydride) are recommended, as its "reactivity [is] comparable" to this related reducing reagent.[4] For these reagents, both preparations in solutions and isolated solids are "highly flammable and must be stored in the absence of moisture".[14] Laboratory guides recommend alane use inside a fume hood.[4][why?] Solids of this reagent type carry recommendations of handling "in a glove bag or dry box".[14] After use, solution containers are typically sealed tightly with concomitant flushing with inert gas to exclude the oxygen and moisture of ambient air.[14]
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).[15]
This section needs expansion with: a careful, source-derived presentation of accidents known to be associated with use of this agent, at small and large scale. You can help by adding to it. (July 2022) |
Alane reductions are believed to proceed via an intermediate coordination complex, with aluminum attached to the partially reduced functional group, and liberated when the reaction undergoes protic quenching. If the substrate is also fluorinated, the intermediate may instead explode if exposed to a hot spot above 60°C.[16]
Aluminium hydrides and various complexes thereof have long been known.[17] Its first synthesis was published in 1947, and a patent for the synthesis was assigned in 1999.[18][19] Aluminium hydride is prepared by treating lithium aluminium hydride with aluminium trichloride.[20] The procedure is intricate: attention must be given to the removal of lithium chloride.
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:
Several groups have shown that alane can be produced electrochemically.[21][22][23][24][25] Different electrochemical alane production methods have been patented.[26][27] 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:
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.[24][25]
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.[28]
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,[29] but in the solid phase it is dimeric with bridging hydrogen centres, (N(CH3)3Al(μ-H))2.[30] The 1:2 complex adopts a trigonal bipyramidal structure.[29] Some adducts (e.g. dimethylethylamine alane, (CH3CH2)(CH3)2N·AlH3) thermally decompose to give aluminium and may have use in MOCVD applications.[31]
Its complex with diethyl ether forms according to the following stoichiometry:
The reaction with lithium hydride in ether produces lithium aluminium hydride (lithium alanate, lithium tetrahydridoaluminate):
Analogous alanates (e.g. Na
3AlH
6, Ca(AlH
4))
2, SrAlH
5) exist with other alkali alkaline earth and some other metals.[32] Li
3AlH
6 is under investigation as a lithium ion cell anode material.
In organic chemistry, aluminium hydride is mainly used for the reduction of functional groups.[33] 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.[34]
Alane enables the hydroxymethylation of certain ketones (that is the replacement of C−H by C−CH2OH at the alpha position).[35] 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.[36]
Nitro groups are not reduced by aluminium hydride. Likewise, aluminium hydride can accomplish the reduction of an ester in the presence of nitro groups.[37]
Aluminium hydride can be used in the reduction of acetals to half protected diols.[38]
Aluminium hydride can also be used in epoxide ring opening reaction as shown below.[39]
The allylic rearrangement reaction carried out using aluminium hydride is a SN2 reaction, and it is not sterically demanding.[40]
Aluminium hydride will reduce carbon dioxide to methane with heating:[citation needed]
![]() | This section needs expansion. You can help by adding to it. (July 2022) |
Akin to hydroboration, aluminium hydride can, in the presence of titanium tetrachloride, add across multiple bonds.[41][42] When the multiple bond in question is a propargylic alcohols, the results are Alkenylaluminium compounds.[43]
![]() | This section needs to be updated. (July 2022) |
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]
Alane is also a potential additive to rocket fuel and in explosive and pyrotechnic compositions.[citation needed] In its unpassivated form, alane is also a promising rocket fuel additive, capable of delivering impulse efficiency gains of up to 10%.[44]