Post by Anders Hoveland on Feb 1, 2011 17:59:05 GMT -8
Aluminum Hydride
3 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.
Also lithium is expensive. Save your LiCl, you can make LiClO4 from it, melt it to drive off water (keep the temperature under 400C), then it will be very soluble it ether and you can do displacement reactions with it on organic Iodides or Bromides.
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
If you are electrically inclined:
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.
Aluminium hydride does not spontaneously ignite, but it is a reactive reducing agent. Aluminum hydride decomposes in air and violently reacts with water, and even sealed containers of AlH3 are known to degrade within a few days.
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.
"Reaction of sulfuric acid with NaAlH4 at 92–100°C in pure toluene leaves undissolved pure crystals of aluminum hydride" (Russian researchers)
AlH3 is actually considered to only be a mildly reactive reducing agent. It will not reduce nitromethane, for example. Burning in oxygen, it is slightly more energetic than aluminum (about 10% more).
3 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.
Also lithium is expensive. Save your LiCl, you can make LiClO4 from it, melt it to drive off water (keep the temperature under 400C), then it will be very soluble it ether and you can do displacement reactions with it on organic Iodides or Bromides.
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
If you are electrically inclined:
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.
Aluminium hydride does not spontaneously ignite, but it is a reactive reducing agent. Aluminum hydride decomposes in air and violently reacts with water, and even sealed containers of AlH3 are known to degrade within a few days.
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.
"Reaction of sulfuric acid with NaAlH4 at 92–100°C in pure toluene leaves undissolved pure crystals of aluminum hydride" (Russian researchers)
AlH3 is actually considered to only be a mildly reactive reducing agent. It will not reduce nitromethane, for example. Burning in oxygen, it is slightly more energetic than aluminum (about 10% more).