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Author: Subject: Drying ethers over P2O5
Klute
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[*] posted on 11-2-2008 at 10:37
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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[*] posted on 11-2-2008 at 13:19


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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[*] posted on 11-2-2008 at 14:48


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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[*] posted on 11-2-2008 at 17:38


Phosphorus pentoxide has been used as a preservative for diethyl ether, not as a drying agent :cool:



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[*] posted on 11-2-2008 at 20:42


"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.




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DJF90
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[*] posted on 12-2-2008 at 08:10


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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[*] posted on 12-2-2008 at 11:43


The mehtod of choice is Na ketyl. Why not use it?



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smuv
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[*] posted on 12-2-2008 at 17:23


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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[*] posted on 13-2-2008 at 08:07


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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[*] posted on 13-2-2008 at 16:59


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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[*] posted on 14-2-2008 at 01:01


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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[*] posted on 14-2-2008 at 03:17


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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[*] posted on 14-2-2008 at 07:42


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)




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DJF90
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[*] posted on 14-2-2008 at 07:56


@ 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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[*] posted on 14-2-2008 at 08:27


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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[*] posted on 14-2-2008 at 09:46


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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[*] posted on 14-2-2008 at 14:35


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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[*] posted on 14-2-2008 at 15:37


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 [...]


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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[*] posted on 14-2-2008 at 16:05


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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[*] posted on 14-2-2008 at 17:17


Will do so, when done :)
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chemrox
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[*] posted on 17-2-2008 at 00:05


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.




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Klute
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[*] posted on 17-3-2008 at 06:35


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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[*] posted on 17-3-2008 at 10:40


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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[*] posted on 17-3-2008 at 11:19


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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[*] posted on 17-3-2008 at 11:46


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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