Sciencemadness Discussion Board

Drying ethers over P2O5

Klute - 11-2-2008 at 10:37

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?

smuv - 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.

DJF90 - 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.

leu - 11-2-2008 at 17:38

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

MagicJigPipe - 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.

DJF90 - 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...

chemrox - 12-2-2008 at 11:43

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

smuv - 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?

Klute - 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]

smuv - 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]

DJF90 - 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.

stoichiometric_steve - 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.

Nicodem - 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)

DJF90 - 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...

microcosmicus - 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]

DJF90 - 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?

Dr. Beaker - 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]

Klute - 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]

smuv - 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).

Klute - 14-2-2008 at 17:17

Will do so, when done :)

chemrox - 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.

Klute - 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.

Methyl.Magic - 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.

Klute - 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?

Drunkguy - 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]

Methyl.Magic - 19-3-2008 at 04:24

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.

Klute - 19-3-2008 at 09:48

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.

jizmaster - 24-3-2008 at 16:19

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]

Klute - 24-3-2008 at 22:29

Thanks for your remarks, they are appreciated!

Klute - 28-4-2008 at 07:35

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.

smuv - 16-6-2008 at 21:31

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 1060 times


smuv - 16-6-2008 at 21:42

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 694 times


Klute - 17-6-2008 at 06:00

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 :D

smuv - 17-6-2008 at 09:37

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.

F2Chemist - 30-6-2008 at 15:11

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.

F2Chemist - 30-6-2008 at 15:15

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.

Klute - 30-6-2008 at 16:16

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?

appetsbud - 2-11-2008 at 08:46

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?

Klute - 2-11-2008 at 16:54

indeed.