Lithium Aluminum Hydride
LiAIH4
I. 4 LiH + A1C18 = LiAlH4 + 3LiCl
For the reaction between LiH and A1C13 to occur to a noticeable
extent, the thick, hard lumps of lithium hydride must be ground to
a very fine powder and then sieved. This treatment is performed
under nitrogen. A very reactive ether suspension of LiH is obtained
if the material is wet-ground with anhydrous ether in a special ball
mill (K. Ziegler et al.) and used immediately.
A fresh suspension of 23.5 g. (2.96 moles) of LiH in 200 ml. of
ether is introduced (with exclusion of moisture) into a threeneck
flask provided with a dropping funnel, a reflux condenser and
a stirrer. A solution of 71.2 g. (0.534 mole) of A1C13 in 300 ml. of
ether is then added in drops with vigorous stirring. The reaction
starts immediately, as shown by the boiling of the ether. The rate
is kept uniform by adjustment of the addition rate. When all the
A1C13 solution has been added, stirring is continued until the reaction
has subsided, and the mixture is left to stand for some
time. The contents of the flask are then filtered, using nitrogen
pressure, through a fritted glass filter, and the clear filtrate is
concentrated at atmospheric pressure until it has a syrupy consistency.
The residual ether may be evaporated in vacuum at
70°C. The yield of LiAlH4, based on A1C13, is about 85%.
Insufficient grinding of the LiH may prevent the reaction from
starting spontaneously, or from going to completion, and lead to
sudden, explosive bursts of renewed activity; in such cases the
reaction is started by adding 200 ml. of ether, followed by a solution
of 3 g. of LiAlH4 in 30 ml. of ether, to the finely divided LiH.
The subsequent procedure is the same as described above. According
to Wiberg, a slight quantity of iodine may be used as an
initiator instead of the LiAlH4.
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