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Methyl.Magic
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| Quote: | Message original : Klute
And do you dry your THF on 3A MS too? Is that sufficent to avoid excesive LiAlH4 consumption?
Diethylether can be practical for more sensible compounds, or when usinga modified soxhlet extractor. On the other hand, THF often ables shorter
reaction times....
Do you perform LiAlH4 reduction without inert atmosphere? |
No I never dried THF because I use the pure THF bought from fluka. I rarely distill my THF because of the danger of explosive peroxide formation. But
I always redistill diethylether, then I dry it over MgSO4 and preserve it over 3A MS.
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Klute
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Good point. I plan on distilling the bisulfite or FeSO4-treated ethers over P2O5 directly before reaction, and keep the recylced ethers (after
reaction) over solid bisulfite until new distillation. Won't recycle it more than a few times though, it isn't THAT expesnive after all. I think
p-methoxyphenol can be used a preservative for ethers, which is a good thing as I still have some at hand.
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jizmaster
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That's one good thing about benzophenone ketal with ethers, no peroxides!
Phosphorous pentoxide kinda sucks because it turns to goo, polyphosphoric acid. Sodium wire is fairly useless too, we used it for preliminary drying
of THF. It gets covered with NaOH after not that many refills and doesn't work any more, but when you quench it there's still most of the sodium left
underneath. Calcium hydride is a pretty good general one if you can get it, not too crazy, easy to handle, reacts vigorously with water but not much
else. Even MeOH can be dried over CaH.
IME the only time it's really necessary to use really dry solvents is when doing a reaction on a miniscule amount of compound that will react further
if an excess of the moisture sensitive reagent is used. Or maybe reactions with metal complexes, i haven't done much with those. Drying over sieves
should be more than enough for grignard or LAH reactions.
I've put THF (HPLC grade i think, 0.01% water) straight from the bottle onto LAH before with no problems, wouldn't reccommend that but reasonably dry
is fine. Be sure to add the solvent to the LAH though, not the other way around! I always used a good flow of inert gas when adding solvent and
quenching to make sure it didn't catch fire and also wash away all the hydrogen. Btw, even Na/benzophenone distilled THF fizzes when added to LAH, so
you can't use that as a test for dryness.
I once got a grignard going by adding a few little pieces of lithium to take out any water that might have been in there. One particularly stubbon
aryl bromide refused to go with dry solvent, I2, iPrMgCl, Li, BuLi, dibromoethane, everything i could think to throw in there. Eventually figured out
how to do it, added all the halide to 2 eq magnesium in THF followed by 0.5 eq dibromoethane in one go, then nuked it with a heat gun to start it
boiling hard then kept going for an hour in an oil bath. The higher boiling point of THF often comes in handy.
[Edited on 25-3-2008 by jizmaster]
[Edited on 25-3-2008 by jizmaster]
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Klute
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Thanks for your remarks, they are appreciated!
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Klute
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Finally, i have abandoned the idea of using P2O5 to dry ethers, well at least small amounts of THF. It seems pretty obvious now that any P2O5/HPO3
present will react and form phosphate esters and polymers. I was hoping that the neutralization of water would be a quick enough process for the other
side reactions to not be a problem.
With dioxane, it seems that it takes a long time to achieve ultra dry conditions. By analogy, a certain amount of the P2O5 should have reacted with
THF by the time acceptable levels of water would be attained.
Thank you to the people from ChemKnowHow that were very helpfull: http://chemknowhow.com/forum/viewtopic.php?p=1214#1214
The process could be more adequate for drying ether, more resistant to electrophilic attack, and lower reflux temperatures. But traces of ethanol
could be possible if small amounts of P2O5 are used.
For the close futur, I will prepare small amounts of dry THF by predrying over KOH after testing for peroxides and eventual neutralization of
theses, followed by reflux and distn over NaH, under argon.
I might try the P2O5 drying procedure out for ether and THF once I have had reproducible results with the reactions, and see if it affects
yields/reactions.
DMF seems to be efficiently dried by vac distn over P2O5, on the other hand.
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smuv
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Use Of Magnesium Sulfate in the Preparation of Anhydrous Ethyl Ether
Journal of Chemical education, 1962 (39) p578.
Apparently suitable for Gignard reactions.
Attachment: dry_ether_MgSO4.pdf (980kB)
This file has been downloaded 1289 times
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smuv
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Readily Available Anhydrous Ether Solutions of Hydrogen Chloride
Journal of Chemical Education; 1967 (44) p108.
A solution of 50mL ether and 5mL conc. Hydrochloric acid is dried over 10-13g anhy. MgSO4. Can be adapted for HBr and HI; although HI reacts with the
ether (presumably with the formation of water!!).
I wonder how loosely the word anhydrous is used...
[Edited on 16-6-2008 by smuv]
Attachment: anhy_hcl_in_ether.pdf (486kB)
This file has been downloaded 868 times
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Klute
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Oh my! This is particular... I must admite that I really need to see this with my own eyes before beleive it... KOH, I can beleive, but MgSO4? I
always considered it to be a somewhat high-capacity drying agents, but certainly not a powerfull one.
I guess this isn't comparable, but even NaH-dried THF bubbles substancially when LAH is added. Ok, they didn't say th ether was compatible with LAH,
but usually if it doesn't react with sodium anymore, it's fit for LAH..
And I also though that when MgSO4 was added to a solution of ether and conc. HCl, it absorbed most of the acid with the water? They don't seems to
indicate any "yield", or molarity of the solution?
I might try a titration. As they suggest hydrogen bondign to be responsible for the absorption, maybe a solvent such as ethyl acetate would also work?
From when do these articles date?
I must admit I am a bit sceptic, but if it does work, it would greatly simplify life..
But in any case it's a good find! How many discussions on this subject (Ether + HCl+ MgSO4) had appeared before 
\"You can battle with a demon, you can embrace a demon; what the hell can you do with a fucking spiritual computer?\"
-Alice Parr
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smuv
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The first is from 1962; the second is from 1967.
I need to try it myself; the second article would be very convenient for the production of amine hydrochlorides without the need for a HCl
tank/generator.
Anyways, although I am skeptical myself; I'm keeping my fingers crossed. I've got a little ether lying around; once I can think of a reasonable test
I'll probably give the Anhy HCl in ether a try.
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F2Chemist
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To the original question...yes it can be done, I've done it and it yields quite dry ether (<50 ppm water via Karl Fischer). As long as you use
good vacuum techniques (vac line, etc), there is no problem.
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F2Chemist
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To the original question...yes it can be done, I've done it and it yields quite dry ether (<50 ppm water via Karl Fischer). As long as you use
good vacuum techniques (vac line, etc), there is no problem.
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Klute
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Is this refering to diethyl ether, or tetrahydrofuran? Nice to know a determined hydrometry, thanks.
Most of the time I distill the ethers in the reaction flask directly, under argon,and another fraction in a schlenk over MS if the substarte needs to
be added as a solution. In that case the solution is transfered via canula, so contact with any air is very minimal.
I was surprised at how much bubbling NaH-dried THF caused when LiAlH4 was added (6h reflux over 1g/100mL 60% NaH dispersion, and slow distillation
with 600mm vigreux). I have been told NaH is a pretty slow drying agent, maybe 12H reflux should be needed.
Next time I will need dry Et2O, I will try the P2O5, but for the moment it isn't possible as the ambient temp is often superior to diethyl ether's
bp....
Any further suggestions?
\"You can battle with a demon, you can embrace a demon; what the hell can you do with a fucking spiritual computer?\"
-Alice Parr
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appetsbud
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| Quote: | Originally posted by Klute
I think p-methoxyphenol can be used a preservative for ethers, which is a good thing as I still have some at hand. |
does this mean that it will help prevent the formation of ether peroxides?
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Klute
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indeed.
\"You can battle with a demon, you can embrace a demon; what the hell can you do with a fucking spiritual computer?\"
-Alice Parr
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