                  Preparation of Lithium Aluminum Hydride

                      by Finholt, Bond & Schlesinger
                     J. Am. Chem. Soc. 69, 1199 (1947)


        When lithium hydride is treated with an ether solution of aluminum
chloride under the conditions described in the experimental part of this
paper, the new ether soluble compound, lithium aluminum hydride, LiAIH4, is
formed according to the equation:

                                   ether
                    4 LiH + AlCl3 =======> LiAlH4 + 3 LiCl

Addition of further quantities of aluminum chloride yields an etheral
solution of aluminum hydride:

                                   ether
                 3 LiAlH4 + AlCl3 =======> 4 AlH3 + 3 LiCl

        The latter solution is not stable; it soon deposits a white solid
in which the atomic ratio of aluminum to hydrogen still is 3:1, but from
which the ether cannot be completely removed without loss of hydrogen.
Lithium aluminum hydride, on the other hand, may be freed from the solvent
completely by evaporation of the latter under suitable conditions. Through
the use of lithium aluminum hydride, new methods, far simpler than any
hitherto available, have been developed for the preparation of hydrides
such as silane and stannane and of their partially alkylated derivatives.
The types of reaction by which these results have been achieved are
illustrated by the equations:
                                 ether
                LiAlH4 + SiCl4  =======> LiCl + AlCl3 + SiH4

                                 ether
        LiAlH4 + 2 (CH3)2SnCl2  =======> LiCl + AlCl3 + 2 (CH3)2SnH2

                                 ether
              LiAlH4 + (CH3)2Zn =======> LiAl(CH3)2H2 + ZnH2

        Reactions such as these usually proceed smoothly at room temperature,
and in general give excellent yields of products of high purity.

        In the field of organic chemistry, lithium aluminum hydride has
already proved extraordinarily useful as a reducing agent. One mole of
carbon dioxide is quantitatively absorbed at room temperature by an
ethereal solution containing two moles of the new compound; among the
products of the reaction are lithium and aluminum salts which yield
formaldehyde when they are treated with acids, but the course of the
reaction has not yet been completely elucidated. Aldehydes, ketones, acid
chlorides and esters are reduced to alcohols, nitrites to amines, aromatic
nitro compounds to azo compounds. In many cases in which the customary
reducing agents require high temperatures or pressures and give poor
yields, the new reagent reacts at room temperature and at an easily
regulated, convenient rate, and gives almost quantitative yields. Its
action is often highly specific. In the reduction of the nitrites so far
studied by us, only primary amines were produced. Its usefulness is
enhanced by the fact that olefinic double bonds are not attacked, except in
special cases, and it is thus possible to accomplish the selective
reduction of various functional groups in unsaturated compounds.

        It is of interest to compare the properties of lithium aluminum
hydride with those of lithium borohydride. Both compounds are white solids,
stable in air at room temperature. Thermal decomposition of lithium
borohydride becomes appreciable at from 250°C to 275°C, and seems to
involve an intermediate reversible step in which a compound LiBH2 is
formed. The decomposition of the aluminum compound becomes noticeable at a
considerably lower temperature (125°C to 150°C), and leads directly to the
formation of aluminum, hydrogen and lithium hydride. Both compounds are
soluble in diethyl ether, but the solubility of the aluminum compound is
from seven to eight times as great as that of the borohydride. The former
reacts completely and extremely rapidly when dropped into water; the
reaction of the latter is slower and less nearly complete (It is to be
noted, however, that lithium aluminum hydride may be safely handled, even
in very humid air, probably because of the formation of a protective
coating of aluminum hydroxide). Similar differences are observed in the
reactions of the two compounds toward alcohols. Finally, lithium
borohydride is inert toward liquid ammonia and toward amines, whereas the
aluminum compound reacts with both.

        The chemical properties of aluminum hydride are similar to those of
lithium aluminum hydride, although the instability of the ether solution of
the former limits its usefulness. Attention is here called to the
possibility that the unstable ether solution of the compound may afford a
means of following by molecular weight determinations the course of the
polymerization assumed to be responsible for the stable, insoluble solid
form of the substance.

Experimental Part
=================

The Preparation of Lithium Aluminum Hydride. 

A. Reagents. 

Unless the lithium hydride is finer than 100 mesh, its reaction with aluminum
chloride occurs very slowly. After weeks of refluxing 20 to 60 mesh hydride
with a diethyl ether solution of aluminum chloride, only a slight surface
reaction had occurred. The relatively coarse "crystalline lump" hydride,
which seems to be the only form now commercially available, must be ground
before use. The grinding and sifting of the material should be carried out
in an atmosphere of dry nitrogen.

We have used samples of purity varying from 87 to 94% without significant
differences in the results. The quantities of lithium hydride are stated in
the description of the preparative procedures in terms of pure compound.
The aluminum chloride was anhydrous and of reagent quality. Anhydrous diethyl
ether of the usual commercial grade proved satisfactory.

B. Apparatus. 

Most of the reactions were carried out in three-necked flasks fitted with a
mercury-sealed stirrer, a reflux condenser, and a dropping funnel, such as
are used for Grignard reactions. Usually an atmosphere of dry, carbon
dioxide-free nitrogen was employed both during the reaction and in the
filtrations, but these precautions were found not to be absolutely essential.

C. Procedure.

In a typical preparation, an excess of lithium hydride (23.5g or 2.96 moles)
was added to a solution of 3.05g (0.08 mole) of lithium aluminum hydride in
30 ml of diethyl ether, and the mixture was stirred for a short time. After
the addition of a further quantity of ether (200 ml), a solution of 71.2g
(0.534 mole) of aluminum chloride in 300ml of ether was introduced at such
a rate that boiling of the liquid in the reaction vessel was continuous.
The mixture was stirred during this step, and for a short time after the
reaction had apparently ceased The precipitated lithium chloride and the
excess of lithium hydride were separated from the solution by passing the
latter through a coarse sintered glass disk under nitrogen pressure.

An aliquot (6.049g) of the 462.5g of combined filtrate and washings was
evaporated at atmospheric pressure until a thick sirup had formed The last
of the ether was then removed in vacuo at 70°C. The resulting solid weighed
0.280g and consisted of lithium aluminum hydride of 95.4% purity according
to an analysis carried out by dissolving an amount of the compound in
anhydrous dioxane, and adding water drop by drop until gas evolution ceased.
Lithium aluminum hydride hydrolyzes according to the following equation:

                 LiAlH4 + 4 H2O ===> LiOH + Al(OH)3 + 4 H2

In the resulting solution, aluminum was determined as its oxide, and lithium
as the sulfate. The quantity of the compound contained in the original
solution was, therefore, 20.42g, of which 3.05g had been contained in the
original reaction mixture. The net yield thus was 17.37g (0.458 mole) or
85.7% of the theoretical. It has been found that allowing the reaction to
stand for a longer time before filtration improves the yield and increases
the purity of the product (it may become as high as 99% without recrystall-
ization). The bulk of the reaction product was usually not taken to dryness,
since most of the reactions to be investigated were carried out in ethereal
solutions. For determination of the concentrations of the solutions, measure-
ment of the amount of hydrogen evolved by hydrolysis of an aliquot is adequate.

The addition of the small amount of previously prepared lithium aluminum
hydride to the reaction mixture prevents a phenomenon which otherwise may
be very troublesome. Without this addition, an initial reaction usually
manifests itself by a slight rise in temperature, but soon either ceases
entirely or becomes too slow to be appreciable. After an induction period,
which may last for only a few minutes or may persist for hours, the reaction
again sets in, this time with such vigor that it can usually not be
controlled by cooling of the mixture. In the presence of a small initial
quantity of lithium aluminum hydride, the reaction sets in at once at a rate
controlled by the rate of addition of aluminum chloride. To obtain lithium
aluminum hydride for this purpose when none is available from previous
preparations, two procedures are illustrated by the following examples:

1. A small quantity of aluminum chloride (2.7g) was mixed with a 6-fold
   excess of lithium hydride (4g) under dry nitrogen in a small round
   bottomed flask, which was then attached through a standard ground glass
   joint to a vacuum system, and evacuated. About 15 ml of ether was
   condensed on the reaction mixture at liquid nitrogen temperature.
   The reaction, which began as the flask was warmed slowly, was allowed
   to proceed vigorously, but was kept under control by cooling the flask
   with liquid nitrogen from time to time. The reaction was usually
   completed in about five minutes. Filtration of the resulting mixture and
   removal of the solvent from the filtrate as previously described yielded
   a sample of lithium aluminum hydride adequate for initiation of the
   reaction of larger batches.

2. Solid lithium hydride (7.0g) was mixed with solid anhydrous aluminum
   chloride (15.95g) in a 500 ml flask previously flushed with dry nitrogen.
   Subsequent addition of 150 ml of dioxane caused the temperature to rise
   to 50°C. The mixture was refluxed for one-half hour, after which it was
   cooled, diluted with 135 ml of diethyl ether and refluxed again for
   another three hours. The resulting mixture was filtered. The solid
   obtained from the filtrate by the procedure used in the preceding
   examples seemed to contain only about 30% of lithium aluminum hydride,
   but nevertheless served to initiate the desired reaction.

Solubility of lithium aluminum hydride

The solubility of lithium aluminum hydride in grams per 100g of various
ethers at 25°C are as follows: diethyl ether 25-30, tetrahydrofuran 13,
dibutyl ether 2 and dioxane 0.1. Because of the difficulty of handling the
extremely viscous solutions obtained in the first two solvents, the data
are of only approximate character.

