# Aluminium hydride

Aluminium hydride (also known as alane and alumane) refers to a collection of inorganic compounds with the formula AlH$_3$. As a gas, alane is a planar molecule. When generated in ether solutions, it exists as an ether adduct. Solutions of alane polymerizes to a solid, which exists in several crystallographically distinguishable forms.

## Structure

Alane can adopt 3-, 4-, or 6-coordination, depending on conditions.

### Gaseous alane

Monomeric AlH$_3$ has been isolated at low temperature in a solid noble gas matrix. It was shown to be planar. The dimeric form, Al$_2$H$_6$, has been isolated in solid hydrogen. It is isostructural with diborane (B$_2$H$_6$) and digallane (Ga$_2$H$_6$).

### Solid alane

Solid alane, which is colorless and nonvolatile, precipitates from etherial solutions over the course of hours at room temperature. Numerous polymorphs can be obtained, which have been labeled α-, α’-, β-, γ-, ε-, and ζ-alanes. The best characterized solid alane is α-alane. According to X-ray crystallography, adopts a cubic or rhombohedral morphology. It features octahedral AlH6 centers interconnected by Al-H-Al bridges. The Al-H distances are all equivalent (172 pm) and the Al-H-Al angles are 141°. α’-Alane forms needle-like crystals, and γ-alane forms bundles of fused needles.
Crystallographic Structure of α-AlH$_3$ :
| The α-AlH$_3$ unit cell | Aluminium coordination | Hydrogen coordination |

## Handling

Alane is not spontaneously flammable. Even so, "similar handling and precautions as... exercised for Li[AlH$_4$]" (the chemical reagent, lithium aluminium hydride) are recommended, as its "reactivity [is] comparable" to this related reducing reagent. For these reagents, both preparations in solutions and isolated solids are "highly flammable and must be stored in the absence of moisture". Laboratory guides recommend alane use inside a fume hood. Solids of this reagent type carry recommendations of handling "in a glove bag or dry box". After use, solution containers are typically sealed tightly with concomitant flushing with inert gas to exclude the oxygen and moisture of ambient air.
Passivation greatly diminishes the decomposition rate associated with alane preparations. Passivated alane nevertheless retains a hazard classification of 4.3 (chemicals which in contact with water, emit flammable gases).

### Reported accidents

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.

## Preparation

Aluminium hydrides and various complexes thereof have long been known. Its first synthesis was published in 1947, and a patent for the synthesis was assigned in 1999. Aluminium hydride is prepared by treating lithium aluminium hydride with aluminium trichloride. The procedure is intricate: attention must be given to the removal of lithium chloride.
    3 Li[AlH$_4$] + AlCl$_3$ → 4 AlH$_3$ + 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[AlH$_4$].
Several other methods exist for the preparation of aluminium hydride:
    2 Li[AlH$_4$] + BeCl$_2$ → 2 AlH$_3$ + Li$_2$[BeH$_2$Cl$_2$]
    2 Li[AlH$_4$] + H$_2$SO$_4$ → 2 AlH$_3$ + Li$_2$SO$_4$ + 2 H$_2$
    2 Li[AlH$_4$] + ZnCl$_2$ → 2 AlH$_3$ + 2 LiCl + ZnH$_2$
    2 Li[AlH$_4$] + I$_2$ → 2 AlH$_3$ + 2 LiI + H$_2$

### Electrochemical synthesis

Several groups have shown that alane can be produced electrochemically. Different electrochemical alane production methods have been patented. 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:
* [AlH$_4$]$^−$ − e$^−$ + n THF → AlH$_3$·nTHF + 1/$_2$ H$_2$
* 3 [AlH$_4$]$^−$ + Al − 3 e$^−$ + 4n THF → 4 AlH$_3$·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.

### High pressure hydrogenation of aluminium

α-AlH$_3$ can be produced by hydrogenation of aluminium at 10 GPa and 600 °C (1,112 °F). The reaction between the liquified hydrogen produces α-AlH$_3$ which could be recovered under ambient conditions.

## Reactions
### Formation of adducts with Lewis bases

AlH$_3$ 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, but in the solid phase it is dimeric with bridging hydrogen centres, [(CH$_3$)$_3$NAlH$_2$(μ-H)]$_2$. The 1:2 complex adopts a trigonal bipyramidal structure. Some adducts (e.g. dimethylethylamine alane, (CH$_3$CH$_2$)(CH$_3$)$_2$N·AlH$_3$) thermally decompose to give aluminium and may have use in MOCVD applications.
Its complex with diethyl ether forms according to the following stoichiometry:
    AlH$_3$ + (CH$_3$CH$_2$)$_2$O → (CH$_3$CH$_2$)$_2$O·AlH$_3$
Similar adducts are assumed to form when alane is generated in THF from lithium aluminium hydride.
The reaction with lithium hydride in ether produces lithium aluminium hydride:
    AlH$_3$ + LiH → Li[AlH$_4$]
Various alanates have been characterized beyond lithium aluminium hydride. They tend to feature five- and six-coordinate Al centers: Na $_3$AlH $_6$, Ca(AlH $_{4)}$) $_2$, SrAlH $_5$).

### Reduction of functional groups

Alane and its derivatives are reducing reagents in organic synthesis based around group 13 hydrides. 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, some forms of alane are, as of 2016, active candidates for storing hydrogen and so for power generation in fuel cell applications, including electric vehicles. 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.
In organic chemistry, aluminium hydride is mainly used for the reduction of functional groups. 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.
Alane enables the hydroxymethylation of certain ketones (that is the replacement of C−H by C−CH$_2$OH at the alpha position). 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.
Aluminium hydride reduces ester in the presence of nitro groups.
Aluminium hydride reduces acetals to half protected diols.
Aluminium hydride reduces epoxide to the corresponding alcohol:
The allylic rearrangement reaction carried out using aluminium hydride is a S$_N$2 reaction, and it is not sterically demanding:
Aluminium hydride will reduce carbon dioxide to methane with heating:
    4 AlH$_3$ + 3 CO$_2$ → 3 CH$_4$ + 2 Al$_2$O$_3$

### Hydroalumination

Akin to hydroboration, aluminium hydride can, in the presence of titanium tetrachloride, add across multiple bonds. When the multiple bond in question is a propargylic alcohols, the results are Alkenylaluminium compounds.

### 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. AlH$_3$ contains up 10.1% hydrogen by weight (at a density of 1.48 grams per milliliter), or twice the hydrogen density of liquid H$_2$. As of 2006, AlH$_3$ was 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".
Alane is also a potential additive to solid rocket fuel and to explosive and pyrotechnic compositions due to its high hydrogen content and low dehydrogenation temperature. In its unpassivated form, alane is also a promising rocket fuel additive, capable of delivering impulse efficiency gains of up to 10%. However, AlH$_3$ can degrade when stored at room temperature, and some of its crystal forms have "poor compatibility" with some fuel components.

### Deposition

Heated alane releases hydrogen gas and produces a very fine thin film of aluminum metal.