Lithium aluminium hydride

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Lithium aluminium hydride
Wireframe model of lithium aluminium hydride Lithium aluminium hydride.svg
Wireframe model of lithium aluminium hydride
Unit cell ball and stick model of lithium aluminium hydride Lithium-aluminium-hydride-layer-3D-balls.png
Unit cell ball and stick model of lithium aluminium hydride
Lithium aluminium hydride.jpg
Names
Preferred IUPAC name
Lithium tetrahydridoaluminate(III)
Systematic IUPAC name
Lithium alumanuide
Other names
  • Lithium aluminium hydride
  • Lithal
  • Lithium alanate
  • Lithium aluminohydride
  • Lithium tetrahydridoaluminate
Identifiers
3D model (JSmol)
AbbreviationsLAH
ChEBI
ChemSpider
ECHA InfoCard 100.037.146 OOjs UI icon edit-ltr-progressive.svg
EC Number
  • 240-877-9
13167
PubChem CID
RTECS number
  • BD0100000
UNII
UN number 1410
  • InChI=1S/Al.Li.4H/q-1;+1;;;; Yes check.svgY
    Key: OCZDCIYGECBNKL-UHFFFAOYSA-N Yes check.svgY
  • InChI=1S/Al.Li.4H/q-1;+1;;;;
  • Key: OCZDCIYGECBNKL-UHFFFAOYSA-N
  • [Li+].[AlH4-]
Properties
Li[AlH4]
Molar mass 37.95 g·mol−1
Appearancewhite 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 in tetrahydrofuran 112.332 g/L
Solubility in diethyl 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)
−117 kJ/mol
−48.4 kJ/mol
Hazards [1]
GHS labelling:
GHS-pictogram-flamme.svg GHS-pictogram-acid.svg
Danger
H260, H314
P223, P231+P232, P280, P305+P351+P338, P370+P378, P422 [2]
NFPA 704 (fire diamond)
[3]
NFPA 704.svgHealth 3: Short exposure could cause serious temporary or residual injury. E.g. chlorine gasFlammability 2: Must be moderately heated or exposed to relatively high ambient temperature before ignition can occur. Flash point between 38 and 93 °C (100 and 200 °F). E.g. diesel fuelInstability 2: Undergoes violent chemical change at elevated temperatures and pressures, reacts violently with water, or may form explosive mixtures with water. E.g. white phosphorusSpecial hazard W: Reacts with water in an unusual or dangerous manner. E.g. sodium, sulfuric acid
3
2
2
W
Flash point 125 °C (257 °F; 398 K)
Safety data sheet (SDS) Lithium aluminium hydride
Related compounds
Related hydride
aluminium hydride
sodium borohydride
sodium hydride
Sodium aluminium hydride
Except where otherwise noted, data are given for materials in their standard state (at 25 °C [77 °F], 100 kPa).
Yes check.svgY  verify  (what is  Yes check.svgYX mark.svgN ?)

Lithium aluminium hydride, commonly abbreviated to LAH, is an inorganic compound with the chemical formula Li [ Al H 4] or LiAlH4. It is a white solid, discovered by Finholt, Bond and Schlesinger in 1947. [4] 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.

Contents

Properties, structure, preparation

Scanning Electron Microscope image of LAH powder Lialh4 sem.png
Scanning Electron Microscope image of LAH powder

LAH is a colourless solid but commercial samples are usually gray due to contamination. [5] This material can be purified by recrystallization from diethyl ether. Large-scale purifications employ a Soxhlet 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 is pyrophoric, but not its large crystals. [6] Some commercial materials contain mineral oil to inhibit reactions with atmospheric moisture, but more commonly it is packed in moisture-proof plastic sacks. [7]

LAH violently reacts with water, including atmospheric moisture, to liberate dihydrogen gas. The reaction proceeds according to the following idealized equation: [5]

Li[AlH4] + 4 H2O → LiOH + Al(OH)3 + 4 H2

This reaction provides a useful method to generate hydrogen in the laboratory. Aged, air-exposed samples often appear white because they have absorbed enough moisture to generate a mixture of the white compounds lithium hydroxide and aluminium hydroxide. [8]

Structure

The crystal structure of LAH; Li atoms are purple and AlH4 tetrahedra are tan. Lithium-aluminium-hydride-unit-cell-3D-polyhedra.png
The crystal structure of LAH; Li atoms are purple and AlH4 tetrahedra are tan.

LAH crystallizes in the monoclinic space group P21/c. The unit cell has the dimensions: a = 4.82, b = 7.81, and c = 7.92 Å, α = γ = 90° and β = 112°. In the structure, Li+ cations are surrounded by five [AlH4]− anions, which have tetrahedral molecular geometry. The Li+ cations are bonded to one hydrogen atom from each of the surrounding tetrahedral [AlH4]− anion creating a bipyramid arrangement. At high pressures (>2.2 GPa) a phase transition may occur to give β-LAH. [9]

X-ray powder diffraction pattern of as-received Li[AlH4]. The asterisk designates an impurity, possibly LiCl. Lialh4 xrpd.svg
X-ray powder diffraction pattern of as-received Li[AlH4]. The asterisk designates an impurity, possibly LiCl.

Preparation

Li[AlH4] was first prepared from the reaction between lithium hydride (LiH) and aluminium chloride: [4] [5]

4 LiH + AlCl3 → Li[AlH4] + 3 LiCl

In addition to this method, the industrial synthesis entails the initial preparation of sodium aluminium hydride from the elements under high pressure and temperature: [10]

Na + Al + 2 H2 → Na[AlH4]

Li[AlH4] is then prepared by a salt metathesis reaction according to:

Na[AlH4] + LiCl → Li[AlH4] + NaCl

which proceeds in a high yield. LiCl is removed by filtration from an ethereal solution of LAH, with subsequent precipitation of LAH to yield a product containing around 1% w/w LiCl. [10]

An alternative preparation starts from LiH, and metallic Al instead of AlCl3. Catalyzed by a small quantity of TiCl3 (0.2%), the reaction proceeds well using dimethylether as solvent. This method avoids the cogeneration of salt. [11]

Solubility data

Solubility of Li[AlH4] (mol/L) [12]
SolventTemperature (°C)
0255075100
Diethyl ether –5.92–––
THF –2.96–––
Monoglyme 1.291.802.573.093.34
Diglyme 0.261.291.542.062.06
Triglyme 0.560.771.291.802.06
Tetraglyme 0.771.542.062.061.54
Dioxane –0.03–––
Dibutyl ether –0.56–––

LAH is soluble in many ethereal solutions. However, it may spontaneously decompose due to the presence of catalytic impurities, though, it appears to be more stable in tetrahydrofuran (THF). Thus, THF is preferred over, e.g., diethyl ether, despite the lower solubility. [12]

Thermal decomposition

LAH is metastable at room temperature. During prolonged storage it slowly decomposes to Li3[AlH6] (lithium hexahydridoaluminate) and LiH. [13] This process can be accelerated by the presence of catalytic elements, such as titanium, iron or vanadium.

Differential scanning calorimetry of as-received Li[AlH4]. Lialh4 dsc.svg
Differential scanning calorimetry of as-received Li[AlH4].

When heated LAH decomposes in a three-step reaction 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 the melting of LAH in the temperature range 150–170 °C, [16] [17] [18] immediately followed by decomposition into solid Li3[AlH6], although R1 is known to proceed below the melting point of Li[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 and R2 can occur at room temperature with suitable catalysts. [20]

Thermodynamic data

The table summarizes thermodynamic data for LAH and reactions involving LAH, [21] [22] in the form of standard enthalpy, entropy, and Gibbs free energy change, respectively.

Thermodynamic data for reactions involving Li[AlH4]
ReactionΔH°
(kJ/mol)		ΔS°
(J/(mol·K))		ΔG°
(kJ/mol)		Comment
Li (s) + Al (s) + 2 H2 (g) → Li[AlH4] (s)−116.3−240.1−44.7Standard formation from the elements.
LiH (s) + Al (s) + 3⁄2 H2 (g) → LiAlH4 (s)−95.6−180.2237.6Using 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)3.46104.5−27.68ΔS° calculated from reported values of ΔH° and ΔG°.

Applications

Use in organic chemistry

Lithium aluminium hydride (LAH) is widely used in organic chemistry as a reducing agent. [5] It is more powerful than the related reagent sodium borohydride owing to the weaker Al-H bond compared to the B-H bond. [23] Often as a solution in diethyl ether and followed by an acid workup, it will convert esters, carboxylic acids, acyl chlorides, aldehydes, and ketones into the corresponding alcohols (see: carbonyl reduction). Similarly, it converts amide, [24] [25] nitro, nitrile, imine, oxime, [26] and organic azides into the amines (see: amide reduction). It reduces quaternary ammonium cations into the corresponding tertiary amines. Reactivity can be tuned by replacing hydride groups by 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 reagent sodium 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 of esters [28] [29] and carboxylic acids [30] to primary alcohols; prior to the advent of LAH this was a difficult conversion involving sodium metal in boiling ethanol (the Bouveault-Blanc reduction). Aldehydes and ketones [31] can also be reduced to alcohols by LAH, but this is usually done using milder reagents such as Na[BH4]; α, β-unsaturated ketones are reduced to allylic alcohols. [32] When epoxides are reduced using LAH, the reagent attacks the less hindered end of the epoxide, usually producing a secondary or tertiary alcohol. Epoxycyclohexanes are reduced to give axial alcohols preferentially. [33]

Partial reduction of acid chlorides to give the corresponding aldehyde product cannot proceed via LAH, since the latter reduces all the way to the primary alcohol. Instead, the milder lithium tri-tert-butoxyaluminum hydride, which reacts significantly faster with the acid chloride than with the aldehyde, must be used. For example, when isovaleric acid is treated with thionyl chloride to give isovaleroyl chloride, it can then be reduced via lithium tri-tert-butoxyaluminum hydride to give isovaleraldehyde in 65% yield. [34] [35]

LAH rxns.pngalcoholalcohol2alcohol3alcohol4alcohol5azide

Lithium aluminium hydride also reduces alkyl halides to alkanes. [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 simple alkenes or arenes. Alkynes are reduced only if an alcohol group is nearby. [39] It was observed that the LiAlH4 reduces the double bond in the N-allylamides. [40]

Inorganic chemistry

LAH is widely used to prepare main group and transition metal hydrides from the corresponding metal halides.

LAH also reacts with many inorganic ligands to form coordinated alumina complexes associated with lithium ions. [21]

LiAlH4 + 4NH3 → Li[Al(NH2)4] + 4H2

Hydrogen storage

Volumetric and gravimetric hydrogen storage densities of different hydrogen storage methods. Metal hydrides are represented with squares and complex hydrides with triangles (including LiAlH4). Reported values for hydrides are excluding tank weight. DOE FreedomCAR targets are including tank weight. Volvsgrav.png
Volumetric and gravimetric hydrogen storage densities of different hydrogen storage methods. Metal hydrides are represented with squares and complex hydrides with triangles (including LiAlH4). Reported values for hydrides are excluding tank weight. DOE FreedomCAR targets are including tank weight.

LiAlH4 contains 10.6 wt% hydrogen, thereby making LAH a potential hydrogen storage medium for future fuel cell-powered vehicles. The high hydrogen content, as well as the discovery of reversible hydrogen storage in Ti-doped NaAlH4, [41] 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 by ball milling. [42] In order to take advantage of the total hydrogen capacity, the intermediate compound LiH 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. [42] 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 ]

Other tetrahydridoaluminiumates

A variety of salts analogous to LAH are known. NaH can be used to efficiently produce sodium aluminium hydride (NaAlH4) by metathesis in THF:

LiAlH4 + NaH → NaAlH4 + LiH

Potassium aluminium hydride (KAlH4) can be produced similarly in diglyme as a solvent: [43]

LiAlH4 + KH → KAlH4 + LiH

The reverse, i.e., production of LAH from either sodium aluminium hydride or potassium aluminium hydride can be achieved by reaction with LiCl or lithium hydride in diethyl ether or THF: [43]

NaAlH4 + LiCl → LiAlH4 + NaCl
KAlH4 + LiCl → LiAlH4 + KCl

"Magnesium alanate" (Mg(AlH4)2) arises similarly using MgBr2: [44]

2 LiAlH4 + MgBr2 → Mg(AlH4)2 + 2 LiBr

Red-Al (or SMEAH, NaAlH2(OC2H4OCH3)2) is synthesized by reacting sodium aluminum tetrahydride (NaAlH4) and 2-methoxyethanol: [45]

See also

Related Research Articles

<span class="mw-page-title-main">Hydride</span> Molecule with a hydrogen bound to a more electropositive element or group

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 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.

In chemistry, a reducing agent is a chemical species that "donates" an electron to an electron recipient . Examples of substances that are common reducing agents include hydrogen, the alkali metals, formic acid, oxalic acid, and sulfite compounds.

<span class="mw-page-title-main">Diborane</span> Chemical compound

Diborane(6), commonly known as diborane, is the chemical compound with the formula B2H6. It is a 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.

<span class="mw-page-title-main">Sodium borohydride</span> Chemical compound

Sodium borohydride, also known as sodium tetrahydridoborate and sodium tetrahydroborate, is an inorganic compound with the formula NaBH4. It is a white crystalline solid, usually encountered as an aqueous basic solution. Sodium borohydride is a reducing agent that finds application in papermaking and dye industries. It is also used as a reagent in organic synthesis.

The Bouveault–Blanc reduction is a chemical reaction in which an ester is reduced to primary alcohols using absolute ethanol and sodium metal. It was first reported by Louis Bouveault and Gustave Louis Blanc in 1903. Bouveault and Blanc demonstrated the reduction of ethyl oleate and n-butyl oleate to oleyl alcohol. Modified versions of which were subsequently refined and published in Organic Syntheses.

<span class="mw-page-title-main">McMurry reaction</span>

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.

<span class="mw-page-title-main">Sodium bis(2-methoxyethoxy)aluminium hydride</span> Chemical compound

Sodium bis(2-methoxyethoxy)aluminium hydride (SMEAH; trade names Red-Al, Synhydrid, Vitride) is a hydride reductant with the formula NaAlH2(OCH2CH2OCH3)2. The trade name Red-Al refers to its being a reducing aluminium compound. It is used predominantly as a reducing agent in organic synthesis. The compound features a tetrahedral aluminium center attached to two hydride and two alkoxide groups, the latter derived from 2-methoxyethanol. Commercial solutions are colorless/pale yellow and viscous. At low temperatures (below -60°C), the solution solidifies to a glassy pulverizable substance with no sharp melting point.

<span class="mw-page-title-main">Borohydride</span>

Borohydride refers to the anion [BH4]−, which is also called tetrahydridoborate, and its salts. Borohydride or hydroborate is also the term used for compounds containing [BH4−nXn]−, where n is an integer from 0 to 3, for example cyanoborohydride or cyanotrihydroborate [BH3(CN)]− and triethylborohydride or triethylhydroborate [BH(CH2CH3)3]−. Borohydrides find wide use as reducing agents in organic synthesis. The most important borohydrides are lithium borohydride and sodium borohydride, but other salts are well known. Tetrahydroborates are also of academic and industrial interest in inorganic chemistry.

<span class="mw-page-title-main">Aluminium hydride</span> Chemical compound

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<span class="mw-page-title-main">Sodium aluminium hydride</span> Chemical compound

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4 anions.

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<span class="mw-page-title-main">Carbonyl reduction</span> Organic reduction of any carbonyl group by a reducing agent

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<span class="mw-page-title-main">Lithium tetrahydridogallate</span> Chemical compound

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<span class="mw-page-title-main">Aluminium compounds</span>

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References

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  13. 1 2 3 Dymova T. N.; Aleksandrov, D. P.; Konoplev, V. N.; Silina, T. A.; Sizareva; A. S. (1994). Russian Journal of Coordination Chemistry. 20: 279.{{cite journal}}: Missing or empty |title= (help)
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