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 | |||
| Names | |||
|---|---|---|---|
| Preferred IUPAC name Lithium tetrahydridoaluminate(III) | |||
| Systematic IUPAC name Lithium alumanuide | |||
Other names
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| Identifiers | |||
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3D model (JSmol) | |||
| Abbreviations | LAH | ||
| ChEBI | |||
| ChemSpider |
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| ECHA InfoCard | 100.037.146 | ||
| EC Number |
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| 13167 | |||
| RTECS number |
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| UNII | |||
| UN number | 1410 | ||
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| Properties | |||
| Li[AlH4] | |||
| Molar mass | 37.95 g·mol−1 | ||
| Appearance | white crystals (pure samples) grey powder (commercial material) hygroscopic | ||
| Odor | odorless | ||
| Density | 0.917 g/cm3, solid | ||
| Melting point | 150 °C (302 °F; 423 K) (decomposes) | ||
| Reacts | |||
| Solubility intetrahydrofuran | 112.332 g/L | ||
| Solubility indiethyl ether | 39.5 g/(100 mL) | ||
| Structure | |||
| monoclinic | |||
| P21/c | |||
| Thermochemistry | |||
| 86.4 J/(mol·K) | |||
Std molar entropy(S⦵298) | 87.9 J/(mol·K) | ||
Std enthalpy of formation(ΔfH⦵298) | −117 kJ/mol | ||
Gibbs free energy(ΔfG⦵) | −48.4 kJ/mol | ||
| Hazards[2] | |||
| GHS labelling: | |||
  | |||
| Danger | |||
| H260,H314 | |||
| P223,P231+P232,P280,P305+P351+P338,P370+P378,P422[1] | |||
| NFPA 704 (fire diamond) | |||
| Flash point | 125 °C (257 °F; 398 K) | ||
| Safety data sheet (SDS) | Lithium aluminium hydride | ||
| Related compounds | |||
Relatedhydride | aluminium hydride sodium borohydride sodium hydride Sodium aluminium hydride | ||
Except where otherwise noted, data are given for materials in theirstandard state (at 25 °C [77 °F], 100 kPa). | |||
Lithium aluminium hydride, commonly abbreviated toLAH, is aninorganic compound with thechemical formulaLi[AlH4] orLiAlH4. It is a white solid, discovered by Finholt, Bond andSchlesinger in 1947.[4] This compound is used as areducing agent inorganic synthesis, especially for the reduction ofesters,carboxylic acids, andamides. The solid is dangerously reactive toward water, releasing gaseoushydrogen (H2). Some related derivatives were once discussed forhydrogen storage.
LAH is a colourless solid, but commercial samples are usually gray due to contamination.[5] This material can be purified by recrystallization fromdiethyl ether. Large-scale purifications employ aSoxhlet extractor. Commonly, the impure gray material is used in synthesis, since the impurities are innocuous and can be easily separated from the organic products. The pure powdered material ispyrophoric but not its large crystals.[6] Some commercial materials containmineral oil to inhibit reactions with atmospheric moisture, but more commonly it is packed in moisture-proof plastic sacks.[7]
LAH violently reacts with water to liberate hydrogen gas. The reaction proceeds according to the following idealized equation:[5]
This reaction could be used to generate hydrogen in the laboratory. Aged, air-exposed samples often appear white because they have absorbed sufficient moisture to generate a mixture of the white compoundslithium hydroxide andaluminium hydroxide.[8]
LAH crystallizes in themonoclinicspace groupP21/c. Theunit cell has the dimensions:a = 4.82,b = 7.81, andc = 7.92 Å, α = γ = 90° and β = 112°. In the structure,Li+cations are surrounded by five[AlH4]−anions, which havetetrahedral molecular geometry. TheLi+ cations are bonded to onehydrogen atom from each of the surrounding tetrahedral[AlH4]− anion creating abipyramid arrangement. At high pressures (>2.2 GPa) a phase transition may occur to give β-LAH.[9]
Li[AlH4] was first prepared from the reaction betweenlithium hydride (LiH) andaluminium chloride:[4][5]
In addition to this method, the industrial synthesis entails the initial preparation ofsodium aluminium hydride from the elements under high pressure and temperature:[10]
Li[AlH4] is then prepared by asalt metathesis reaction according to:
LiCl is removed byfiltration from anethereal solution of LAH, with subsequent precipitation of LAH to yield a product containing around 1 wt% LiCl.[10]
An alternative preparation starts from LiH, and metallic Al instead ofAlCl3. Catalyzed by a small quantity ofTiCl3 (0.2%), the reaction proceeds well usingdimethylether as solvent. This method avoids the cogeneration of salt.[11]
| Solvent | Temperature (°C) | ||||
|---|---|---|---|---|---|
| 0 | 25 | 50 | 75 | 100 | |
| Diethyl ether | – | 5.92 | – | – | – |
| THF | – | 2.96 | – | – | – |
| Monoglyme | 1.29 | 1.80 | 2.57 | 3.09 | 3.34 |
| Diglyme | 0.26 | 1.29 | 1.54 | 2.06 | 2.06 |
| Triglyme | 0.56 | 0.77 | 1.29 | 1.80 | 2.06 |
| Tetraglyme | 0.77 | 1.54 | 2.06 | 2.06 | 1.54 |
| Dioxane | – | 0.03 | – | – | – |
| Dibutyl ether | – | 0.56 | – | – | – |
LAH is soluble in manyethereal solutions. However, it may spontaneously decompose due to the presence of catalytic impurities, though, it appears to be more stable intetrahydrofuran (THF). Thus, THF is preferred over, e.g.,diethyl ether, despite the lower solubility.[12]
LAH ismetastable at room temperature. During prolonged storage it slowly decomposes toLi3[AlH6] (lithium hexahydridoaluminate) andLiH.[13] This process can be accelerated by the presence ofcatalytic elements, such astitanium,iron orvanadium.
When heated LAH decomposes in a three-stepreaction mechanism:[13][14][15]
| 3 Li[AlH4] → Li3[AlH6] + 2 Al + 3 H2 | R1 |
| 2 Li3[AlH6] → 6 LiH + 2 Al + 3 H2 | R2 |
| 2 LiH + 2 Al → 2 LiAl + H2 | R3 |
R1 is usually initiated by themelting of LAH in the temperature range 150–170 °C,[16][17][18] immediately followed by decomposition into solidLi3[AlH6], althoughR1 is known to proceed below the melting point ofLi[AlH4] as well.[19] At about 200 °C,Li3[AlH6] decomposes into LiH (R2)[13][15][18] and Al which subsequently convert into LiAl above 400 °C (R3).[15] Reaction R1 is effectively irreversible.R3 is reversible with an equilibrium pressure of about 0.25 bar at 500 °C.R1 andR2 can occur at room temperature with suitable catalysts.[20]
The table summarizesthermodynamic data for LAH and reactions involving LAH,[21][22] in the form ofstandardenthalpy,entropy, andGibbs free energy change, respectively.
| Reaction | ΔH° (kJ/mol) | ΔS° (J/(mol·K)) | ΔG° (kJ/mol) | Comment |
|---|---|---|---|---|
| Li (s) + Al (s) + 2 H2 (g) → Li[AlH4] (s) | −116.3 | −240.1 | −44.7 | Standard formation from the elements. |
| LiH (s) + Al (s) +3⁄2 H2 (g) → LiAlH4 (s) | −95.6 | −180.2 | 237.6 | Using ΔH°f(LiH) = −90.579865, ΔS°f(LiH) = −679.9, and ΔG°f(LiH) = −67.31235744. |
| Li[AlH4] (s) → Li[AlH4] (l) | 22 | – | – | Heat of fusion. Value might be unreliable. |
| LiAlH4 (l) →1⁄3 Li3AlH6 (s) +2⁄3 Al (s) + H2 (g) | 3.46 | 104.5 | −27.68 | ΔS° calculated from reported values of ΔH° and ΔG°. |
Lithium aluminium hydride (LAH) is widely used in organic chemistry as areducing agent.[5] It is more powerful than the relatedreagentsodium borohydride owing to the weaker Al-H bond compared to the B-H bond.[23] Often as a solution indiethyl ether and followed by an acid workup, it will convertesters,carboxylic acids,acyl chlorides,aldehydes, andketones into the correspondingalcohols (see:carbonyl reduction). Similarly, it convertsamide,[24][25]nitro,nitrile,imine,oxime,[26] andorganic azides into theamines (see:amide reduction). It reducesquaternary ammonium cations into the corresponding tertiary amines. Reactivity can be tuned by replacing hydride groupsby alkoxy groups. Due to its pyrophoric nature, instability, toxicity, low shelf life and handling problems associated with its reactivity, it has been replaced in the last decade, both at the small-industrial scale and for large-scale reductions by the more convenient related reagentsodium bis (2-methoxyethoxy)aluminium hydride, which exhibits similar reactivity but with higher safety, easier handling and better economics.[27]
LAH is most commonly used for the reduction ofesters[28][29] andcarboxylic acids[30] to primary alcohols; prior to the advent of LAH this was a difficult conversion involvingsodium metal in boilingethanol (theBouveault-Blanc reduction).Aldehydes andketones[31] can also be reduced to alcohols by LAH, but this is usually done using milder reagents such asNa[BH4]; α, β-unsaturated ketones are reduced to allylic alcohols.[32] Whenepoxides are reduced using LAH, the reagent attacks the lesshindered end of the epoxide, usually producing a secondary or tertiary alcohol.Epoxycyclohexanes are reduced to give axial alcohols preferentially.[33]
Partial reduction ofacid chlorides to give the corresponding aldehyde product cannot proceed via LAH, since the latter reduces all the way to the primary alcohol. Instead, the milderlithium tri-tert-butoxyaluminum hydride, which reacts significantly faster with the acid chloride than with the aldehyde, must be used. For example, whenisovaleric acid is treated withthionyl chloride to give isovaleroyl chloride, it can then be reduced via lithium tri-tert-butoxyaluminum hydride to give isovaleraldehyde in 65% yield.[34][35]
Lithium aluminium hydride also reducesalkyl halides toalkanes.[36][37] Alkyl iodides react the fastest, followed by alkyl bromides and then alkyl chlorides. Primary halides are the most reactive followed by secondary halides. Tertiary halides react only in certain cases.[38]
Lithium aluminium hydride does not reduce simplealkenes orarenes.Alkynes are reduced only if an alcohol group is nearby,[39] and alkenes are reduced in the presence of catalyticTiCl4.[40] It was observed that theLiAlH4 reduces the double bond in theN-allylamides.[41]
LAH is widely used to prepare main group and transitionmetal hydrides from the corresponding metalhalides.
LAH also reacts with many inorganic ligands to form coordinated alumina complexes associated with lithium ions.[21]
LiAlH4 contains 10.6 wt% hydrogen, thereby making LAH a potentialhydrogen storage medium for futurefuel cell-poweredvehicles. The high hydrogen content, as well as the discovery of reversible hydrogen storage in Ti-doped NaAlH4,[42] have sparked renewed research into LiAlH4 during the last decade. A substantial research effort has been devoted to accelerating the decomposition kinetics by catalytic doping and byball milling.[43]In order to take advantage of the total hydrogen capacity, the intermediate compoundLiH must be dehydrogenated as well. Due to its high thermodynamic stability this requires temperatures in excess of 400 °C, which is not considered feasible for transportation purposes. Accepting LiH + Al as the final product, the hydrogen storage capacity is reduced to 7.96 wt%. Another problem related to hydrogen storage is the recycling back to LiAlH4 which, owing to its relatively low stability, requires an extremely high hydrogen pressure in excess of 10000 bar.[43] Cycling only reaction R2 — that is, using Li3AlH6 as starting material — would store 5.6 wt% hydrogen in a single step (vs. two steps for NaAlH4 which stores about the same amount of hydrogen). However, attempts at this process have not been successful so far.[citation needed]
A variety of salts analogous to LAH are known.NaH can be used to efficiently producesodium aluminium hydride (NaAlH4) bymetathesis in THF:
Potassium aluminium hydride (KAlH4) can be produced similarly indiglyme as a solvent:[44]
The reverse, i.e., production of LAH from either sodium aluminium hydride or potassium aluminium hydride can be achieved by reaction withLiCl or lithium hydride indiethyl ether orTHF:[44]
"Magnesium alanate" (Mg(AlH4)2) arises similarly usingMgBr2:[45]
Red-Al (or SMEAH, NaAlH2(OC2H4OCH3)2) is synthesized by reacting sodium aluminum tetrahydride (NaAlH4) and2-methoxyethanol:[46]
The highly reducing and pyrophoric nature of LAH requires special handling techniques to avoid its exposure to sources of ignition, moisture, and ambient oxygen. The use of a fume hood or dry box under an inert atmosphere is recommended for any work with large amounts of LAH. It is recommended that a class D fire extinguisher or dry sand is on standby in case of a fire, as other classes of extinguisher may intensify the fire if used.[47]
Due to the widespread use and hazardous character of LAH, it has been the cause of many lab accidents. Lab fires related to this compound have been the result of grinding,[48] runaway reactions,[49] improper storage,[50] and spontaneous ignition.[51] Often, these fires are made worse by the erroneous use of CO2 fire extinguishers, which can fuel LAH fires.[47]
The use of LAH to reduce fluorinated compounds has also caused multiple lab explosions. These explosions result from LAH creating a complex with the fluorinated compound, these complexes have been found to be heat and shock sensitive explosives.[52]
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