soma
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drying thf
I was thinking to dry thf by 1st using calcium chloride, then molecular sieve, and then lialh4. The reaction uses lialh4.
The question is, would it be better to distill the thf from the lithium, or to just go ahead and use it without distilling?
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smuv
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Best avoid CaCl2 for drying THF. It can be used, but things get complicated because calcium chloride forms a complex with it. Pick another desiccant
to do the rough drying of THF (or just use mol sieves).
In general (since you aren't telling me what you are trying to reduce) After drying with mol sieve, just use it in the reaction as is. Just be
careful with the initial addition of LAH, and maybe use a smidge in excess of what is proscribed.
Even then, I think people get a little carried away with their fears of LAH. I quench LAH reactions with water all the time; its prudent to be
careful, but careful addition to LAH even to very wet THF can be done without huge problems.
Also, make sure you wear a dust mask when working with LAH. It has a great tendency to reduce you sense of smell for a week or more, after inhaling
even the smallest amounts of its dust.
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SmashGlass
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Commonly the way to dry THF is to distill it fresh, in an
atmosphere of dry Nitrogen or Argon, using sodium metal and
having a small amount of benzophenone to see that is it free
from water.
Yellow (contains water); Green (OK but you can boil for longer);
Blue (good to go); Purple tinge (unbelievably dry). Seeing the
Purple colour is rare. You also need a very good glassware
setup for distilling THF as one can generate peroxides and this
is explosive! Ground glass joints should be minimized or
avoided if possible. And Teflon slips that fit between the joints
can also be purchased. Investing in a few peroxide testing kits
could be of use before you start distilling. It just depends on
how much you like your hands, fingers, arms or life...
Mol sieves usually wont keep THF dry as THF is relatively
hygroscopic.
Also of note THF often contains a stabilizer which
will kill the benzophenone reaction. Check the label on the
bottle (tank/drum...). If it does contain the stabilizer dry the THF
using sodium wire. It wont be absolutely dry but it will be
better. If you cant get your hands on sodium either, then the
mol sieves will have to suffice but it wont be anywhere near dry
enough for a LiAlH reduction.
Happy hunting.
If it ain't broke don't fix it....
Now where are my screwdrivers?  |
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DJF90
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Distil the THF from a small amount of LiAlH4 after preliminary drying with sodium metal. This ensures the solvent is free from peroxides and dry.
Remember not to distill to dryness.
[Edited on 11-2-2011 by DJF90]
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antibody
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New wisdom on the subject
According to this JOC article Na is actually a poor and obsolete way of drying THF. They have several methods that result is much drier THF at
considerably less risk, cost and labour.
Here is the PDF
Attachment: Drying-of-Organic-Solvents-Quantitative-Evaluation-of-the-Efficiency-of-Several-Desiccants-Williams-DBG-Lawton-M-J-Org-C (735kB)
This file has been downloaded 1502 times
[Edited on 22-2-2011 by antibody]
[Edited on 22-2-2011 by antibody]
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