soma
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dangers of LAH
The Wikipedia article on LAH states: "LAH is soluble in many ethereal solutions. However, it may spontaneously decompose due to the presence of
catalytic impurities, though, it appears to be more stable in tetrahydrofuran (THF). Thus, THF is preferred over, e.g., diethyl ether, despite the
lower solubility"
I'm wondering what "catalytic impurities" they're talking about and why ethyl ether would cause this?
I've used LAH for years with pretty much no problems. One time it ignited in the weigh pan on a very humid night. The flames were pretty small. I
dumped it on the front lawn where not much else happened.
I keep the reaction in THF at around 50C with stirring -- quench with wet THF and then water at around 15-20C -- and make sure that no crust forms on
top of the liquid.
Are the dire warnings over-kill? Is LAH dangerous in ethyl ether?
Thanks.
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macckone
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I think they are referring to the spontaneous release of hydrogen. Depending on how it is made, excess aluminum chloride can cause decomposition. It
isn't super stable and of course free hydrogen can explode over a wide range of mixtures with air.
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DraconicAcid
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Quote: Originally posted by soma  |
I've used LAH for years with pretty much no problems. One time it ignited in the weigh pan on a very humid night. The flames were pretty small. I
dumped it on the front lawn where not much else happened. |
Now imagine if that had happened while you were adding it to the ether.
Please remember: "Filtrate" is not a verb.
Write up your lab reports the way your instructor wants them, not the way your ex-instructor wants them.
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soma
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| Quote: Originally posted by DraconicAcid | | Quote: Originally posted by soma |
I've used LAH for years with pretty much no problems. One time it ignited in the weigh pan on a very humid night. The flames were pretty small. I
dumped it on the front lawn where not much else happened. |
Now imagine if that had happened while you were adding it to the ether. |
I now have a humidity gauge and wait til the humidity is around 30-40% to do LAH reactions. (I don't want it to be too dry to prevent static).
[Edited on 18-12-2013 by soma]
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soma
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| Quote: Originally posted by macckone | | I think they are referring to the spontaneous release of hydrogen. Depending on how it is made, excess aluminum chloride can cause decomposition. It
isn't super stable and of course free hydrogen can explode over a wide range of mixtures with air. |
So, the AlCl3 could cause some decomposition which would heat up the ether? If the addition is done with cooling, (which is the method used for adding
LAH to ether or THF), why would it be a problem? I have never seen the temp get out of control when adding LAH to THF - I would think it wouldn't be
hard to keep the temp at around 25C.
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zed
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At one time, a preferred form of LiAlH4, was LiAlH4...in oil. A thin coating of petroleum type oil, protected the product from exposure to
moisture/air. The product was kinda paste-like, and it was easy to work with.
Probably still available somewhere.
And, yeah. LiAlH4 is dangerous to work with. Especially so, in Diethyl-ether.
There are a plethora of sad, but true, stories....about LiAlH4 mishaps.
Try the search engine.
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chemrox
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I have two dehumidifiers running all the time (old bldg.) It's a struggle to maintain 30-40. Especially with damp make-up air. I could put an ac on
the intake but afraid the unit wouldn't handle the volume.
"When you let the dumbasses vote you end up with populism followed by autocracy and getting back is a bitch." Plato (sort of)
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soma
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I wait til the weather is right. That means not too much happens in the summer. Gives you an appreciation of winter.
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