Lithium aluminium hydride

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Lithium aluminium hydride
Missing image
Lialh4_sem.png
Scanning electron microscopy image of LiAlH4 powder

Other names LAH
Lithium tetrahydridoaluminate
Lithium alanate

"Lithal" (UK slang)

Molecular formula LiAlH4
Molar mass 37.95 g/mol
CAS number [16853-85-3]
Density 0.917 g/cm3
Solubility (water) Reactive!
Melting point 150 C
Boiling point  ? C
Heat capacity 83.2 J/(mol K)
Disclaimer and references
Missing image
Lithium_Aluminum_Hydride_Structure.png
Structure of lithium aluminium hydride

Lithium Aluminium Hydride (LiAlH4), commonly abbreviated to LAH, is a powerful reducing agent used in organic chemistry. It is more powerful than the related reducing agent Sodium tetrahydridoborate due to the weaker Al-H bond compared to the B-H bond. It will convert esters, carboxylic acids and ketones to alcohols; and nitro compounds into amines.

LAH reacts with water, including atmospheric moisture, and can spontaneously burst into flames. Fortunately, this process is slow enough that LAH normally doesn't require handling under an atmosphere of inert gases.

In an absolutely pure state, LAH is a white solid. Commercial samples are almost always grey due to trace contamination with aluminium metal. White air-exposed commercial samples of LAH have absorbed enough moisture to become a mixture of lithium hydroxide and aluminum hydroxide.

Contents

Preparation

LAH is usually produced by the reaction between lithium hydride (LiH) and aluminium chloride (AlCl3)

4LiH + AlCl3 <math>\rightarrow<math> LiAlH4 + 3LiCl

which proceeds with a high yield of LAH (97 w/w %). LiCl is removed by filtration from an ethereal solution of LAH, with subsequent precipitation of LAH to yield a product with around 1 w/w % LiCl.

Inorganic reactions

LAH and NaH can be used to produce sodium aluminium hydride (NaAlH4) by metathesis in THF with a yield of 90.7 w/w %

LiAlH4 + NaH <math>\rightarrow<math> NaAlH4 + LiH

Potassium aluminium hydride (KAlH4) can be produces with a yield of 90 w/w % by reaction in diglyme in a similar way

LiAlH4 + KH <math>\rightarrow<math> KAlH4 + LiH

The reverse i.e. production of LAH from either sodium aluminium hydride or potassium aluminium hydride can be obtained by reaction with LiCl in diethyl ether and THF with a yield of 93.5 and 91 w/w %, respectively.

NaAlH4 + LiCl <math>\rightarrow<math> LiAlH4 + NaCl
KAlH4 + LiCl <math>\rightarrow<math> LiAlH4 + KCl

Magnesium alanate (Mg(AlH4)2) can be synthezised from LAH and MgBr2

2LiAlH4 + MgBr2 <math>\rightarrow<math> Mg(AlH4)2 + 2LiBr

Use in organic chemistry

Missing image
LAH_rxns.png
Organic reactions of lithium aluminium hydride

Lithium aluminium hydride is widely used in organic chemistry as a very powerful reducing agent. Despite handling problems associated with its reactivity, it is even used at the small-industrial scale, although for large scale reductions the related reagent sodium bis(2-methoxyethoxy)aluminium hydride is more usual. For such purposes it is usually used in solution in diethyl ether, and an aqueous workup is usually performed after the reduction in order to remove inorganic by products. It is most commonly used for the reduction of esters and carboxylic acids to primary alcohols; prior to the advent of LiAlH4 this was a difficult conversion involving sodium metal in boiling ethanol (the Bouveault-Blanc reduction). Aldehydes and ketones can also be reduced to alcohols by LAH, but this is usually done using milder reagents such as NaBH4. When epoxides are reduced using LAH, the reagent attacks the less hindered end of the epoxide, usually producing a secondary or tertiary alcohol.

Amines can be prepared by the LAH reduction of amides, nitriles, nitro compounds or alkyl azides. LAH is also able to reduce primary alkyl halides to alkanes.

Lithium aluminium hydride is not able to reduce simple alkenes or benzene rings, and alkynes are only reduced if an alcohol group is nearby.

Thermal decomposition

At room temperature LAH is usually stable, although, during prolonged storage it may slowly decompose to Li3AlH6. This process can be accelerated by the presence of catalytic elements e.g. Ti, Fe, V.

When heated LAH decompose in a three step reaction mechanism.

LiAlH4 <math>\rightarrow<math> 1/3Li3AlH6 + 2/3Al + H2 (R1)
1/3Li3AlH6 <math>\rightarrow<math> LiH + 1/3Al + 1/2H2 (R2)
LiH <math>\rightarrow<math> Li + 1/2H2 (R3)

R1 is usually initiated by the melting of LAH around a temperature of 150-170oC immmediately followed by decomposition into solid Li3AlH6. From 200-250oC Li3AlH6 decompose into LiH (R2) which subsequently decompose into Li above 400oC (R3). Due to the presence of metallic aluminium the solid reaction product will be some sort of Li-Al alloy. Unless catalyzed R1 and R2 are effectively irreversible.

According to reactions R1-R3 LiAlH4 contains 10.6 w/w % hydrogen thereby making LAH a potential hydrogen storage medium for future fuel cell powered vehicles.

Solubility data

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

Solubility data for LiAlH4 (mol/l)
Temperature (oC)
Solvent 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 -- -- --

Crystal structure

Missing image
Lialh4_struct.png
The crystal structure of LAH. Li atoms are blue and AlH4 tetrahedra are red. The unit cell border is marked by a black line.

The crystal structure of LAH belongs to the monoclinic crystal system and the space group is P21c. The crystal structure of LAH is illustrated to the right. The structure consists of Li atoms surrounded by five AlH4 tetrahedra. The Li atoms are bonded to one hydrogen atom from each of the surrounding tetrahedra creating a bipyramid arrangement. The side lengths of the unit cell are approx. a=4.82, b=7.81 and c=7.92, and the β angle is approx. 112 °. At high pressures (>2.2GPa) a phase transition into β-LAH occurs.

Thermodynamic data

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

Thermodynamic data for reactions involving LiAlH4
Reaction ΔHo (kJ/mol) ΔSo (J/(mol K)) ΔGo (kJ/mol) Comment
Li(s) + Al(s) + 2H2(g) <math>\rightarrow<math> LiAlH4(s) -116.3 -240.1 -44.7 Standard formation from the elements.
LiH(s) + Al(s) + 3/2H2(g) <math>\rightarrow<math> LiAlH4(s) -25.6 -170.2 23.6 Using ΔHof(LiH) = -90.5, ΔSof(LiH) = -69.9, and ΔGof(LiH) = -68.3.
LiAlH4(s) <math>\rightarrow<math> LiAlH4(l) 22 -- -- Heat of fusion. Value is probably unreliable.
LiAlH4(l) <math>\rightarrow<math> 1/3Li3AlH6(s) + 2/3Al(s) + 1/2H2(g) 3.46 104.5 -27.68 ΔSo calculated from reported values of ΔHo and ΔGo.

See also

References and further reading

External links

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