Names | |
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Preferred IUPAC name Aluminium hydride | |
Systematic IUPAC name Alumane | |
Other names
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Identifiers | |
3D model (JSmol) | |
ChEBI | |
ChemSpider | |
ECHA InfoCard | 100.029.139 |
245 | |
PubChem CID | |
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). |
Aluminium hydride (also known as alane and alumane) is an inorganic compound with the formula Al H 3. 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.
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] sufficiently large samples may be further heated to complete decomposition at 150 °C. [5] 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. [6] [ non-primary source needed ] Preparations of alane dissolve in tetrahydrofuran (THF) or diethyl ether (ether), [4] from which pure allotropes precipitate. [7] [ 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°. [8]
Crystallographic Structure of α-AlH3 [9] | |||||
---|---|---|---|---|---|
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. [7] [ 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. [10] The dimeric form, Al2H6, has been isolated in solid hydrogen, and it is isostructural with diborane (B2H6) and digallane (Ga2H6). [11] [12] [13]
Alane is not spontaneously flammable. [14] 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". [15] 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". [15] After use, solution containers are typically sealed tightly with concomitant flushing with inert gas to exclude the oxygen and moisture of ambient air. [15]
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). [16]
This section needs expansionwith: 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. [17]
Aluminium hydrides and various complexes thereof have long been known. [18] Its first synthesis was published in 1947, and a patent for the synthesis was assigned in 1999. [19] [20] Aluminium hydride is prepared by treating lithium aluminium hydride with aluminium trichloride. [21] 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]. [7]
Several other methods exist for the preparation of aluminium hydride:
Several groups have shown that alane can be produced electrochemically. [22] [23] [24] [25] [26] Different electrochemical alane production methods have been patented. [27] [28] 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. [25] [26]
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. [29]
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, [30] but in the solid phase it is dimeric with bridging hydrogen centres, (N(CH3)3Al(μ-H))2. [31] The 1:2 complex adopts a trigonal bipyramidal structure. [30] Some adducts (e.g. dimethylethylamine alane, (CH3CH2)(CH3)2N·AlH3) thermally decompose to give aluminium and may have use in MOCVD applications. [32]
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. [33] 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. [34] 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. [35]
Alane enables the hydroxymethylation of certain ketones (that is the replacement of C−H by C−CH2OH at the alpha position). [36] 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. [37]
Nitro groups are not reduced by aluminium hydride. Likewise, aluminium hydride can accomplish the reduction of an ester in the presence of nitro groups. [38]
Aluminium hydride can be used in the reduction of acetals to half protected diols. [39]
Aluminium hydride can also be used in epoxide ring opening reaction as shown below. [40]
The allylic rearrangement reaction carried out using aluminium hydride is a SN2 reaction, and it is not sterically demanding. [41]
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. [42] [43] When the multiple bond in question is a propargylic alcohols, the results are Alkenylaluminium compounds. [44]
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%. [45]
Heated alane releases hydrogen gas and produces a very fine thin film of aluminum metal. [5]
In chemistry, a hydride is formally the anion of hydrogen (H−), a hydrogen atom with two electrons. The term is applied loosely. At one extreme, all compounds containing covalently bound H atoms are also called hydrides: water (H2O) is a hydride of oxygen, ammonia is a hydride of nitrogen, etc. For inorganic chemists, hydrides refer to compounds and ions in which hydrogen is covalently attached to a less electronegative element. In such cases, the H centre has nucleophilic character, which contrasts with the protic character of acids. The hydride anion is very rarely observed.
Diborane(6), commonly known as diborane, is the chemical compound with the formula B2H6. It is a highly toxic, colorless, and pyrophoric gas with a repulsively sweet odor. Given its simple formula, borane is a fundamental boron compound. It has attracted wide attention for its electronic structure. Several of its derivatives are useful reagents.
Lithium aluminium hydride, commonly abbreviated to LAH, is an inorganic compound with the chemical formula Li[AlH4] or LiAlH4. It is a white solid, discovered by Finholt, Bond and Schlesinger in 1947. This compound is used as a reducing agent in organic synthesis, especially for the reduction of esters, carboxylic acids, and amides. The solid is dangerously reactive toward water, releasing gaseous hydrogen (H2). Some related derivatives have been discussed for hydrogen storage.
In organometallic chemistry, organolithium reagents are chemical compounds that contain carbon–lithium (C–Li) bonds. These reagents are important in organic synthesis, and are frequently used to transfer the organic group or the lithium atom to the substrates in synthetic steps, through nucleophilic addition or simple deprotonation. Organolithium reagents are used in industry as an initiator for anionic polymerization, which leads to the production of various elastomers. They have also been applied in asymmetric synthesis in the pharmaceutical industry. Due to the large difference in electronegativity between the carbon atom and the lithium atom, the C−Li bond is highly ionic. Owing to the polar nature of the C−Li bond, organolithium reagents are good nucleophiles and strong bases. For laboratory organic synthesis, many organolithium reagents are commercially available in solution form. These reagents are highly reactive, and are sometimes pyrophoric.
Organic reductions or organic oxidations or organic redox reactions are redox reactions that take place with organic compounds. In organic chemistry oxidations and reductions are different from ordinary redox reactions, because many reactions carry the name but do not actually involve electron transfer. Instead the relevant criterion for organic oxidation is gain of oxygen and/or loss of hydrogen. Simple functional groups can be arranged in order of increasing oxidation state. The oxidation numbers are only an approximation:
Diisobutylaluminium hydride (DIBALH, DIBAL, DIBAL-H or DIBAH) is a reducing agent with the formula (i-Bu2AlH)2, where i-Bu represents isobutyl (-CH2CH(CH3)2). This organoaluminium compound is a reagent in organic synthesis.
Germane is the chemical compound with the formula GeH4, and the germanium analogue of methane. It is the simplest germanium hydride and one of the most useful compounds of germanium. Like the related compounds silane and methane, germane is tetrahedral. It burns in air to produce GeO2 and water. Germane is a group 14 hydride.
The McMurry reaction is an organic reaction in which two ketone or aldehyde groups are coupled to form an alkene using a titanium chloride compound such as titanium(III) chloride and a reducing agent. The reaction is named after its co-discoverer, John E. McMurry. The McMurry reaction originally involved the use of a mixture TiCl3 and LiAlH4, which produces the active reagents. Related species have been developed involving the combination of TiCl3 or TiCl4 with various other reducing agents, including potassium, zinc, and magnesium. This reaction is related to the Pinacol coupling reaction which also proceeds by reductive coupling of carbonyl compounds.
Hydroperoxides or peroxols are compounds of the form ROOH, where R stands for any group, typically organic, which contain the hydroperoxy functional group. Hydroperoxide also refers to the hydroperoxide anion and its salts, and the neutral hydroperoxyl radical (•OOH) consist of an unbond hydroperoxy group. When R is organic, the compounds are called organic hydroperoxides. Such compounds are a subset of organic peroxides, which have the formula ROOR. Organic hydroperoxides can either intentionally or unintentionally initiate explosive polymerisation in materials with unsaturated chemical bonds.
Indium(III) chloride is the chemical compound with the formula InCl3 which forms a tetrahydrate. This salt is a white, flaky solid with applications in organic synthesis as a Lewis acid. It is also the most available soluble derivative of indium. This is one of three known indium chlorides.
Lithium borohydride (LiBH4) is a borohydride and known in organic synthesis as a reducing agent for esters. Although less common than the related sodium borohydride, the lithium salt offers some advantages, being a stronger reducing agent and highly soluble in ethers, whilst remaining safer to handle than lithium aluminium hydride.
Hydrosilanes are tetravalent silicon compounds containing one or more Si-H bond. The parent hydrosilane is silane (SiH4). Commonly, hydrosilane refers to organosilicon derivatives. Examples include phenylsilane (PhSiH3) and triethoxysilane ((C2H5O)3SiH). Polymers and oligomers terminated with hydrosilanes are resins that are used to make useful materials like caulks.
Lithium triethylborohydride is the organoboron compound with the formula LiEt3BH. Commonly referred to as LiTEBH or Superhydride, it is a powerful reducing agent used in organometallic and organic chemistry. It is a colorless or white liquid but is typically marketed and used as a THF solution. The related reducing agent sodium triethylborohydride is commercially available as toluene solutions.
Digallane is an inorganic compound with the chemical formula GaH2(H)2GaH2. It is the dimer of the monomeric compound gallane. The eventual preparation of the pure compound, reported in 1989, was hailed as a "tour de force." Digallane had been reported as early as 1941 by Wiberg; however, this claim could not be verified by later work by Greenwood and others. This compound is a colorless gas that decomposes above 0 °C.
Sodium aluminium hydride or sodium alumanuide is an inorganic compound with the chemical formula NaAlH4. It is a white pyrophoric solid that dissolves in tetrahydrofuran (THF), but not in diethyl ether or hydrocarbons. It has been evaluated as an agent for the reversible storage of hydrogen and it is used as a reagent for the chemical synthesis of organic compounds. Similar to lithium aluminium hydride, it is a salt consisting of separated sodium cations and tetrahedral AlH−
4 anions.
Beryllium hydride is an inorganic compound with the chemical formula n. This alkaline earth hydride is a colourless solid that is insoluble in solvents that do not decompose it. Unlike the ionically bonded hydrides of the heavier Group 2 elements, beryllium hydride is covalently bonded.
Zinc hydride is an inorganic compound with the chemical formula ZnH2. It is a white, odourless solid which slowly decomposes into its elements at room temperature; despite this it is the most stable of the binary first row transition metal hydrides. A variety of coordination compounds containing Zn–H bonds are used as reducing agents, but ZnH2 itself has no common applications.
Zirconocene dichloride is an organozirconium compound composed of a zirconium central atom, with two cyclopentadienyl and two chloro ligands. It is a colourless diamagnetic solid that is somewhat stable in air.
In organic chemistry, carbonyl reduction is the conversion of any carbonyl group, usually to an alcohol. It is a common transformation that is practiced in many ways. Ketones, aldehydes, carboxylic acids, esters, amides, and acid halides - some of the most pervasive functional groups, -comprise carbonyl compounds. Carboxylic acids, esters, and acid halides can be reduced to either aldehydes or a step further to primary alcohols, depending on the strength of the reducing agent. Aldehydes and ketones can be reduced respectively to primary and secondary alcohols. In deoxygenation, the alcohol group can be further reduced and removed altogether by replacement with H.
Indium trihydride is an inorganic compound with the chemical formula. It has been observed in matrix isolation and laser ablation experiments. Gas phase stability has been predicted. The infrared spectrum was obtained in the gas phase by laser ablation of indium in presence of hydrogen gas InH3 is of no practical importance.
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