chemrox
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using LAH
Elswhere I mentioned having been away from wet chem for quite awhile. Am enjoying the experience of getting my chops back so to speak. I had lots of
experience with organometallic compounds, particularly with alkyl and aryl Mg Br's. I have no expewrience with LAH and need to do a workup with LAH
in ether (not THF). I'm asking for practical information on the precautions. Eg., how dry is dry enough? Is drying the ether with Na required or is
drying with drying less reactive agents effective enough? Should I plan on N2 atmosphere or is refluxing ether vapor enough air exclusion?
Any other advice and/or precautions? Much gratitude in advance.
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Fleaker
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There are threads that deal with the proper use and dangers of lithium aluminum hydride. If my advice is not good enough, search them out. LAH is
quite reactive, I'd go as far as saying pyrophoric in humid air (which means you breathing on it could light it up). It's much more aggressive than
sodium borohydride. How dry should your ether be? Sodium dry. What atmosphere? A nitrogen atmosphere. I say this not because a smidgen of water will
cause an explosion, but because it might, and it's better to be safe than sorry. Even a small little spark could light the ether, and then you'd have
a real problem on your hands.
As far as handling, handle it like you would potassium or sodium.
[Edited on 5-2-2007 by Fleaker]
Neither flask nor beaker.
"Kid, you don't even know just what you don't know. "
--The Dark Lord Sauron
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garage chemist
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As a rough guideline, if your ether fizzes considerably when you add the LAH it was not dry enough.
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chemrox
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Thanks to both of you!
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matei
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We perform LAH reductions without nitrogen atmosphere and it usually works.
[Edited on 5-2-2007 by matei]
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chemrox
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which solvents do you use..prefer and do you dry them over Na? always??
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Sandmeyer
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Your question can be answered by reading a documented LAH reduction (duh!). Shulgin is a LAH-freak, read his bible.
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Sergei_Eisenstein
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Ether/THF should be as dry as possible. I have stored ether over Na and THF over sieves. Both solvents are hygroscopic, which means that as soon as
you transfer the anhydrous solvent from your bottle to the reaction vessel, it will take up water from the air. It should be good enough if you
perform the reduction straight away. It may fizzle somewhat; just add more LAH to compensate. By bringing your reaction mixture to a mild reflux, it
will a protecting layer of solvent vapor is formed, making it harder (but not impossible) for air to penetrate the mixture.
damnant quod non intelligunt
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Sandmeyer
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I was wondering why Shulgin sometimes adds H2SO4 in some of the LAH reductions...
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Nicodem
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I would say it is to make AlH3 in situ when the substrate is base sensitive. AlCl3 can also be used if I remember corectly.
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Sergei_Eisenstein
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Yes, it is for in situ AlH3 preparation. I think Shulgin uses 100% sulfuric acid, though I've read papers where they used concentrated sulfuric acid
instead.
damnant quod non intelligunt
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