Lithium aluminium hydride

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Lithium aluminium hydride
Wireframe model of lithium aluminium hydride
Wireframe model of lithium aluminium hydride
Unit cell ball and stick model of lithium aluminium hydride
Unit cell ball and stick model of lithium aluminium hydride
Lithium aluminium hydride
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
)
Abbreviations LAH
ChEBI
ChemSpider
ECHA InfoCard
 100.037.146 Edit this at Wikidata
EC Number
  • 240-877-9
13167
RTECS number
  • BD0100000
UNII
UN number 1410
  • InChI=1S/Al.Li.4H/q-1;+1;;;; checkY
    Key: OCZDCIYGECBNKL-UHFFFAOYSA-N checkY
  • InChI=1S/Al.Li.4H/q-1;+1;;;;
  • Key: OCZDCIYGECBNKL-UHFFFAOYSA-N
  • [Li+].[AlH4-]
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 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)
87.9 J/(mol·K)
Std enthalpy of
formation
(ΔfH⦵298)
 −117 kJ/mol
−48.4 kJ/mol
Hazards[2]
GHS labelling:
GHS02: FlammableGHS05: Corrosive
Danger
H260, H314
P223, P231+P232, P280, P305+P351+P338, P370+P378, P422[1]
NFPA 704 (fire diamond)
[3]
NFPA 704 four-colored diamondHealth 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).
checkY verify (what is checkY☒N ?)

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.[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.

Properties, structure, preparation

Scanning Electron Microscope
image of LAH powder

LAH is a colourless solid but commercial samples are usually gray due to contamination.

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.

LAH crystallizes in the

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
.

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]
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 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].

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]
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°.

Applications

Use in organic chemistry

Lithium aluminium hydride (LAH) is widely used in organic chemistry as a

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

Epoxycyclohexanes are reduced to give axial alcohols preferentially.[33]

Partial reduction of

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]

alcoholEpoxidealcohol2alcohol3alcohol4AldehydeNitrileAmideAmineCarboxylic acidalcohol5azideAmineEsterKetone

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

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

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

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

2-methoxyethanol:[45]

See also

Wikimedia Commons has media related to lithium aluminium hydride.

References

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  2. ^ Index no. 001-002-00-4 of Annex VI, Part 3, to Regulation (EC) No 1272/2008 of the European Parliament and of the Council of 16 December 2008 on classification, labelling and packaging of substances and mixtures, amending and repealing Directives 67/548/EEC and 1999/45/EC, and amending Regulation (EC) No 1907/2006. OJEU L353, 31.12.2008, pp 1–1355 at p 472.
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  27. ^ "Red-Al, Sodium bis(2-methoxyethoxy)aluminumhydride". Organic Chemistry Portal.
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  30. ^ Koppenhoefer, B.; Schurig, V. (1988). "(R)-Alkyloxiranes of High Enantiomeric Purity from (S)-2-Chloroalkanoic Acids via (S)-2-Chloro-1-Alkanols: (R)-Methyloxirane". Organic Syntheses. 66: 160; Collected Volumes, vol. 8, p. 434.
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Further reading

External links

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