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Klute
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Drying ethers over P2O5
Can ethers (diethyl ether, THF, etc) be dried by reflux and distn over P2O5 under inert atmospher? Most of the time benzophenone-ketyl or straight Na
wire is sued for these solvants, but unfortunaly my supply of sodium is too limited for this use. On the othe rhand, I ahev a kilo of P2O5. I know
this dessicant is usually used with chlorinated solvants, esters and hydrocarbons, but I've haven't any explicity notice on not using it for ethers.
I've was told that after a while dealkylation can happen yielding corresponding phosphate esters (there's a preparation of trimethyphosphate using
dimethylether and P2O5 IIRC), but to what extent that would happen during 2 hours reflux and distn is unclear. Any formed phosphate esters would stay
in the still anyhow.
Has anyone already used such a dessicant for ethers?
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smuv
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http://www.mallbaker.com/techlib/documents/americas/3045.htm...
Apparently it is ok...but without looking I would have said no. The reason being, upon hydration P2O5 yeilds phosphoric acid (as you probably know).
Phosphoric acid on coke was and still probably is the catalyst of choice for hydration of ethylene; this reaction is of course reversible. I would
not be surprised if some ethylene or ethanol are produced by refluxing wet ether over P2O5. Based on this, I think if you want super dry ether it
might be worthwhile considering alternatives.
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DJF90
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You could distill through a column of alumina, as this would help fractionation between the ether and any water present plus is a drying agent. the
alumina needs to be preheated to 175C for a long time to ensure it is anhydrous. I also read about flame drying apparatus when dealing with water
sensitive reagents, and maybe someone else on the forum can share som knowledge regarding this.
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leu
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Phosphorus pentoxide has been used as a preservative for diethyl ether, not as a drying agent 
Chemistry is our Covalent Bond
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MagicJigPipe
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"I also read about flame drying apparatus when dealing with water sensitive reagents"
I honestly don't think flame drying would be necessary here. Usually, flame drying is reserved for drying reaction vessels that will be used for
reactions in which the reagent(s) can react with trace amounts of water to form undesirable byproducts which could be detrimental to the reaction. An
example IIRC is Grignard reactions.
"There must be no barriers to freedom of inquiry ... There is no place for dogma in science. The scientist is free, and must be free to ask any
question, to doubt any assertion, to seek for any evidence, to correct any errors. ... We know that the only way to avoid error is to detect it and
that the only way to detect it is to be free to inquire. And we know that as long as men are free to ask what they must, free to say what they think,
free to think what they will, freedom can never be lost, and science can never regress." -J. Robert Oppenheimer
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DJF90
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Yes I think that was where I have seen it... I guess it depends how "dry" Klute wants his ether, and how badly he wants it. If it needs to be
anhydrous and its needed badly then maybe it is worth sacrificing some sodium. If you can recover some of the sodium from the distillation flask then
maybe you could keep that portion of sodium aside for drying another batch of ether, provided there is still enough metallic sodium still present. But
in that case you are probably using too much if theres that much excess...
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chemrox
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The mehtod of choice is Na ketyl. Why not use it?
"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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smuv
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because he has a limited supply of sodium....as stated in the initial post.
Klute I assume you will be using this ether for organometalics and want super dry ether. Is this correct?
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Klute
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| Quote: | Originally posted by smuv
Klute I assume you will be using this ether for organometalics and want super dry ether. Is this correct? |
Yes, indeed. LiAlH4 and grignards. But perhaps not up to a butyllithium-suited extent. For the LiAlH4 reductions, Maybe I pull it off simply by
leaving the THF over 3A molecular seives for a few days, and use a little excess of the hydride. That might suffice for the grignard, but as I will
have to distill the solvant anyway to get it completly desoxygenated, I was thinking of a quick reflux followed by distn over P2O5. Any formed ethanol
would be problematic though. If using THF, could 1,4-butandiol possibly form? I guess that would happen less easily than with ether. I've noticed a
few more citations where P2O5 is said to be compatible with ethers, and only one claiming the opposite. I was hoping someone had first hand experience
with such a method, as I don't have any means of titrating residual water at home, and don't really want to bother colleagues at work with that.
I have a very small amount of sodium, like a couple of grams, that I would really want to keep for other uses. My supply of LiAlH4 is also
somewhat limited, and considering the price I prefer avoiding wastes. What would be a efficient and simple way of detecting/titrating any alcohol in
the distd ether? Mg? Can't really use any oxidants as they would attack the THF to some extent.
I think I will try stirring at room temp or refluxing predried THF over P2O5 for a while, discard the forerun (for recycling) and use only the
middle fraction for the grignards. I hope this will minimize the amount of H3PO3 formed, and would favor formation of phosphate esters with any
eventuall alcohol present. Would decanting the residual P2O5 sludge/mess reduce the possibilty of dealkyaltion, without risking too much exposure to
atmospher/handling?
I take it LiAlH4 can be a little foregiving with a few extra ppm of water (that is very relative of course, I wouldn't try dumping some hydride
directly even into ether from a freshly opened bottle). Has anyone tried using ethers dried only with freshly activated 3A MS for such reactions?
For the apparatus, a few hours at 130°C in a oven, and cooling in a slow flow of argon as always been fine. Even in the garage where humidity is
rarely less than 70%.... Obviously the setup is cooled inside first 
PS: from Smuv's link:
Lithium Aluminum Hydride P403-05
100 g
[suitable for drying:] Aldehydes, ketones, esters, carboxylic acids, peroxides, acid anhydrides, acid chlorides, ethers
WTF? I'm I missing something here? They will dry alright, but reduced!
[Edited on 13-2-2008 by Klute]
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smuv
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Ha! That slipped past me....
I was thinking about P2O5; it is very possible that the temp of refluxing ether or THF would be low enough that the ether would not get chewed up by
formed phosphoric acids. After all the lab prep of ethylene from ethanol works best ca 160 degrees C.
Your mention of magnesium reminds me, why not just use magnesium to dry the solvent. I think I have heard the use of magnesium to dry sovents
mentioned around this forum. Anyone who has tried to get a stuck grignard going knows it works (magnesium + activator, usually iodine or
1,2-dibromoethane). To dry a somewhat wet solvent it might not be so practical, but after sitting over molecular sieves, it might be just what is
necessary to remove the last traces of water. Search the forum and literature and I think you will find something.
[Edited on 13-2-2008 by smuv]
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DJF90
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I would have thought that the use of P2O5 would have dehydrated the ether to but-2-ene (or other products?), regardless of temperature. Since ether
can be produced from ethanol and conc. sulphuric acid then maybe refluxing with a quantity of conc. sulphuric acid would do the job? It may not dry
the ether sufficiently for your purposes but perhaps its worth a shot.
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stoichiometric_steve
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| Quote: | | Originally posted by DJF90 Since ether can be produced from ethanol and conc. sulphuric acid then maybe refluxing with a quantity of conc.
sulphuric acid would do the job? It may not dry the ether sufficiently for your purposes but perhaps its worth a shot. |
arrrh...think before you speak. ever heard of something called law of mass action...
what kinda bullshit are you posting anyways.
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Nicodem
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| Quote: | Originally posted by Klute
If using THF, could 1,4-butandiol possibly form? I guess that would happen less easily than with ether. I've noticed a few more citations where P2O5
is said to be compatible with ethers, and only one claiming the opposite. |
THF is quite sensitive toward protic acids and electrophiles in general. Electrophilic ring opening of THF is quite easy, while diethyl ether is
nearly not as reactive (for example, dry HCl/Et2O solutions are stable for many months, while not the same can be said for such solutions in THF).
(Some reactions of THF with electrophiles were briefly mentioned in this thread)
…there is a human touch of the cultist “believer” in every theorist that he must struggle against as being
unworthy of the scientist. Some of the greatest men of science have publicly repudiated a theory which earlier they hotly defended. In this lies their
scientific temper, not in the scientific defense of the theory. - Weston La Barre (Ghost Dance, 1972)
Read the The ScienceMadness Guidelines!
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DJF90
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@ stoichiometric_steve: havent heard of the law of mass action... possibly because I'm still doing my A-levels. My preferences tend toward organic
chemistry, and that sounds like something physical. I'm just suggesting (with the best of my knowledge) possible ideas that might work. But as you
obviously know what you are talking about why don't you make a suggestion instead of grilling me...
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microcosmicus
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Whatever type of chemistry you may be interested in, the law of mass action
is fundamental. Basically, it states that the equilibrium concentrations of
reactants and products are governed by an equilibrium constant and that
the equilibrium constant can be related to the reaction rates. Whether or
not this term "mass action" (and the related term "detailed balance")
rings a bell, you should at least have a practical working knowledge of
what an equilibrium constant is and how to use one.
In fact, the reaction of ethanol and sulfuric acid is used as a textbook
example --- both ethylene and ether can form and the proportion of
one to the other will depend upon the conditions under which the
reaction takes place. This is generally the case in organic chemistry,
where all sorts of products can result from a reaction. If you are
going to approach the subject intelligently as opposed to puttering
around cluelessly, then you will need to understand enough of
chemical equilibrium and kinetics to choose concentrations of
reactants, temperature, and other conditions which favor production
of your desired product over other possible outcomes.
[Edited on 14-2-2008 by microcosmicus]
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DJF90
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yes i know how to work with equilibrium constants, just never heard of the law of mass action. IIRC the temperature at which the ethylene (or ethene
as it is taught in the uk) becomes predominant in ether synthesis with conc. sulphuric acid is about 170C, so as long as the temperature is kept lower
than this (with a sufficient margin) then it should be ok? or am I missing something important here?
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Dr. Beaker
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to get super dried ethers:
reflux for several hours the ether with the apropriate drying agent + benzophenone, until deep purple color is developed.
for THF - K
for ether - NaK
from my experience P4O10 is good for drying organohalides and nitriles (uncompatible with alkali metals)
[Edited on 15-2-2008 by Dr. Beaker]
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Klute
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Yes, the benzophenone ketyl is the conventional means, but requires long reflux times and often a dedicated still, with a reflux-collector (or
whatever it's called in english..) As my need for dry ethers is relatively sporadic at home, I can't bother mounting such a setup.
Found this: From Chapter 12 of Migrdichian's Organic Synthesis
Preparation of Organomagnesium Compounds
| Quote: | Commercial alcohol-free ether distilled over phosphorus pentoxide is satisfactory [for grignard reactions]. [...]
The reaction often proceeds only when ether dried over phosphorus pentoxide is employed [...]
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Thanks Lugh for posting this a long time ago
That's enough of a respectable reference to make me want to give it a try. Refluxing and distn over coarse Mg powder/I2 would be second choice,
followed by 3A MS or KOH.
I guess a spatula-tip of LiALH4 in a small amount of freshly distilled solvant would be enough to tell me it's efficient enough or not. If I really
wanted to do this nicely, I would try P2O5 reflux/distn, followed by Mg/I2 reflux distn to get rid of any eventually formed alcohol, for LiALH4
reductions that is. For grignards, I would just use a little more exces Mg, and heat it up a bit before adding the alkyl halide.
Thank you everybody for your help.
[Edited on 15-2-2008 by Klute]
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smuv
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I hope you post your results...especially if you try the magnesium method (I will probably try that one in the future).
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Klute
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Will do so, when done
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chemrox
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I've run a lot of Grignards using ether that wasn't particularly fresh. The sensitivity to water is overstated a lot. If you exclude air by
refluxing ether in your apparatus before introducing the O-halide you should be OK. I don't have the same extensive experience with LAH and have also
asked here about it. What I've inferred is that the solvent matters. Rigorously dried ether or scrupulously dried THF. If the halide is bromine I
would distill technical grade ether under a drying tube and use it. If it's to be an organochloride it might take more drying of the ether. Those
are harder to start. Making sure the Mg is oxide free is more critical and the Mg has been the variable in most cases for me. Fresh Mg turnings are
the way to go if you can get them. I just bought five pounds very cheaply for a 'rainy day.'
The other day I was thinking about grinding up some mothballs and running them over Mg to see how hard it would be to make the double Grignard.
"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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Klute
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The article LSD25 posted (thanks!) in the phenolic aldehyde methylation thread has conforted my idea of using P2O5 for drying ethers, at least
diethyl ether:
| Quote: | Under the conditions
used by these authors any ether formed in the reaction
should have been recovered, since the reaction
of ether with phosphorus pentoxide at temperatures
attainable under atmospheric pressure is very slow.
For example, the preparation of ethyl metaphosphate
from phosphorus pentoxide and excess ether
at reflux temperature requires about 60 hours.4 |
Reference cited: K. Langheld, Ber., 44, 2076 (1911).
I don't plan on refluxing it more than a hour or two, so it should be OK. So I guess I will use Et2O for LAH reductions, and keep the THF for the
grignards, unless no special degradation of the THF can be noticed.
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Methyl.Magic
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Personally, I use THF for hydride reduction and diethylether for grignard...
I always dry my Et2O over 3A molecuar sieve and the organomagnesium formation starts easily with no iodine crystal.
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Klute
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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?
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Drunkguy
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Hmm this guy stated he already had 1kg of P2O5 so I guess he is entitled to ask that question.
I have molecular sieves though and will probably consider using them if I cant immediately do the Na/Ph2C=O idea.
[Edited on 17-3-2008 by Drunkguy]
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