Lithium Tétrahydruroaluminate
The lithium tétrahydruroaluminate, also called hydride of aluminum and lithium (LiAlH4) which we will call LAH, is powerful a Réducteur used in Organic chemistry. It is more powerful than the borohydrure of sodium, another reagent of reduction, because of the bond Al-H which is weaker than connection B-H. It transforms the Ester S, the carboxylic acid and the Cétone S in alcohol S and the compounds nitrated in amine S. LAH reacts violently with water and the pure product is pyrophoric. The commercial product is preserved in mineral oil to protect it from the air. Pure and crystallized in the diethylic ether, it is a white solid. The commercial samples are always gray because of contaminations with metal aluminum traces. Commercial samples which absorbed sufficient moisture a mixture of Hydroxyde of lithium and of Hydroxyde of aluminum.
Preparation
The LAH is normally synthesized by reaction between the Hydrure of lithium (LiH) and the Chlorure of aluminum (AlCl3)
4LiH + AlCl3 - > LiAlH4 + 3LiCl
who has an high rate of conversion (97%). LiH in excess is eliminated by a filtration from the solution from LAH in ether followed by a precipitation to obtain a product containing approximately more than 1% of LiCl.
Inorganic reactions
The LAH and NaH can be used to produce the Tétrahydruroaluminate of sodium by Métathèse in THF with an output of 90,7%.LiAlH4 + NaH - > NaAlH4 + LiH
The Tétrahydruroaluminate of potassium can be prepared same manner by reaction in the Diglyme with an output of 90%.
LiAlH4 + KH - > KAlH4 + LiH
The reverse, i.e. the production of LAH starting from tétrahydruroaluminate of sodium or aluminum and potassium hydride, can be obtained by reaction with LiCl in the diethylic ether and the THF with an output of 93,5 and 91%.
NaAlH4 + LiCl - > LiAlH4 + NaCl
KAlH4 + LiCl LiAlH
The Alanate of magnesium can be synthesized starting from LAH and of magnesium bromide (MgBr2)
2LiAlH4 + MgBr2-> Mg (AlH4) 2 + 2LiBr
Uses in organic chemistry
The lithium and aluminum hydride is largely used in organic chemistry like powerful reducing agent. Considering the difficulties of handling because of its reactivity, it is especially used with small scales; for larger quantities, one rather uses the Hydrure of sodium (a) (2-méthoxyéthoxy) aluminum. Like reducer, the LAH is normally used in ether diethylic and a washing with water is carried out after the reduction to withdraw the inorganic by-products. It is especially used for the reduction of ethyl oxide S, Ester S and the carboxylic acid with alcohol S primary educations; before the appearance of the LAH, one used the metal Sodium in the ebullient ethanol (the Réduction of Bouveault-White). Aldehyde S and Cétone S can also be reduced out of alcohol by the LAH but, in general, one uses a softer reagent like the Borohydrure of sodium (NaBH4). When the epoxy S are reduced with the LAH, the reagent tackles the least encumbered part epoxy and in general the product is a secondary or tertiary alcohol. The Amines can be prepared by the reduction with the LAH of Amide S, of Nitrile S, of nitrated compounds or Alkylazide S. the LAH is also able to reduce the halides of alkyl in Alcane S. the hydride of aluminum and of lithium is not able to reduce simple Alcène S or benzene cores and the Alcyne S are reduced only in the presence of one grouping alcohol.
Thermal decomposition
With the room temperature, LAH is normally stable, though, during a prolonged storage, it can break up slowly into Li3AlH6. This degradation can be accelerated by the presence of a Catalyseur like one of metals: Ti, Fe, V. When it is heated, the LAH breaks up by the reaction into three stages:
LiAlH4 - > 1/3 Li3AlH6 + 2/3 Al + H2 (R1)
1/3 Li3AlH6 - > LiH + 1/3 Al + ½ H2 (R2)
LiH - > Li + 1/2H2 (R3)
The R1 reaction is in general initiated by the fusion of the LAH with approximately 150-170ºC and immediately followed by its decomposition in Li3AlH6. Starting from 200-250ºC, Li3AlH6 se breaks up into LiH (R2 reaction) and above 400ºC, it also breaks up into Li (R3 reaction). Considering the metal presence of aluminum, the solid reaction can produce alloys Li-Al. Unless being catalyzed, the R1 reactions and R2 are really irreversible.
Within sight of the R1 reactions in R3, LiAlH4 contains hydrogen 10,6% in weight and, for that, could be a good hydrogen tank for the future cells of combustible batteries intended to equip the vehicles.
Solubility
The LAH is soluble in many ethers. However, it can break up spontaneously in the presence of impurities. For that, it seems to be more stable in the THF. Thus, this last is preferred, for example, with the diéthyl ether in spite of its least solubilité.
Crystalline structure
The crystalline structure of LAH is monoclinical. This structure consists of Li atoms surrounded by five AlH4 tetrahedrons. The lithium atoms are surrounded by a hydrogen atom for each close tetrahedron creating a pyramid. With high pressure, (>2.2 GPa), there is transition from phase towards the phase βLAH.
Thermodynamic data
The table summarizes the thermodynamic data for LAH and the reactions where LAH intervient.
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