LiAlH4 + AlCl3 → 4 AlH3 + 3 LiCl
"ether solution aluminium hydride is prepared after precipitation of lithium chloride"
but one would think that it would be the AlH3, not the LiCl that would precipitate from ether, maybe they got it switched around? So if you do this,
you might check first to see whether the precipitate if flammable before dumping it. Wikipedia then goes on to say that the solution requires
immediate use, otherwise the AlH3 will precipitate. Does this means that the LiCl will precipitate first, and then the AlH3 will slowly also start to
precipitate?
Or more likely, wikipedia made a mistake, and it is only the AlH3 that will do any precipitation, the LiCl will stay perfectly dissolved in the ether?
When AlH3 is dissolved in ether, H3Al•O(C2H5)2 is the complex that has formed. I read that there has to be more than 10% by weight of AlH3 dissolved
in ether for it to be able to precipitate from the solvent.
ZnCl2 will work instead of AlCl3, and has the advantage of being more easily dried, but the yield (relative to the expensive) is lower, and ZnH2 would
also form with the AlH3.
Concentrated sulfuric acid works, apparently. It probably has low yields, I would think some of the LiAlH4 reduces some of the acid before the main
reaction can go to completion. This is likely a very violent dangerous reaction, so AlCl3 is probably best.
2 LiAlH4 + H2SO4 → 2 AlH3 + Li2SO4 + 2 H2
"Reaction of sulfuric acid with NaAlH4 at 92–100°C in pure toluene leaves undissolved pure crystals of aluminum hydride"
Electrolytic Method
Molten sodium aluminum hydride as the electrolyte, an aluminum anode, and an iron wire submerged in mercury (which acted as the cathode since mercury
does not dissolve iron) The sodium Hg amalgam cathode prevents side reactions and the hydrogen produced can be captured and reacted back with the
sodium mercury amalgam to produce sodium hydride.
Industrially, it may now be possible to prepare AlH3 directly from the elements: aluminum was hydrogenated to trihydride at 8.9 GPa and 600 °C. (Too
much pressure to be done in the laboratory)
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