Leben
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Safely distilling THF?
I've been browsing over some solvents for a while and THF is just what I need for reaction I am to do. After I do the reaction I plan to distill the
THF out to store it for later usage.
Is the whole "explosive peroxides" thing, less of an issue than I am imagining? Sources say "never distill THF to dryness". Am I not able to distill
it out of the reaction mixture and then store it back in its bottle? If needed I could always store it under an inert gas. What is the risk of these
explosive peroxides forming during distillation, and how easily are they set off if formed?
Wikipedia also says "Commercial THF contains substantial water that must be removed for sensitive operations".... the THF I am looking at on a chem
suppliers website says it is 100% THF with 0.025% stabilizer. If I were to use this THF straight from purchasing, would residual water be an issue,
seeing as I need it for a dry reaction?
[Edited on 3-9-2014 by Leben]
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Pyro
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don't distill to dryness!
all above information is intellectual property of Pyro.  |
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Leben
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I am distilling it from a reaction to recover it for later reuse.... how can I distill it to dryness if there is, a) no water in the reaction to begin
with, and b) only THF coming over during the distillation?
I think I read in another post that these explosive peroxides only form when stored under air for periods of weeks. If they form this slowly then I
should just be able to store it under an inert gas.
Just wanted to get some second opinions on this.
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Amos
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I think by dryness he means don't distill for so long that your boiling flask is completely devoid of liquid. That or I've been interpreting the whole
"distill to dryness" concept all wrong. That may cause a whole mess of problems.
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smaerd
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Here's the thing. Explosive peroxides are very real. Lot's of lab fires have happened because of them. How easy are they to set off? Well, pretty damn
easy is what I have been told. I mean the bumping from rotovaps has caused explosions. I've even heard if THF dries with peroxides on the threads of a
solvent jug unscrewing the lid can lead to immediate disaster. Storing under inert atmosphere is almost always a good idea for any chemical.
You can perform tests for explosive peroxides, and there are ways to 'quench' them. I typically avoid distilling THF directly. I'd rather extract from
it after diluting it with say water(situational). If you're careful it can be distilled so that there is still a decent sized puddle of solvent in the
flask when the distillation is ceased(aka not distilling to 'dryness'). That means you watch the distillation, don't walk away and make cookies.
As much as I would love to say recycling THF is a great money saving idea for a home-chemist, it's probably not practical nor safe in a lot of
instances without precautions.
Is the residual water going to be an issue? Is an impossible question to answer, unless you ask the manufacturer and they confirm it is 'water-free'
and you know how to transfer solvents properly without moisture entering them. It can always be dried.
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Leben
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There were a number of threads I had seen where people have claimed to distill THF many times without issue, but there are many sources cautioning
against these "explosive peroxides". What about removing it under a vacuum? After recovering it, it could be quickly poured into the container, purged
with inert gas, and stored? How quickly do these peroxides form?
Unless you care to suggest another polar aprotic solvent that can be brought down to negative temperatures without freezing. DMSO is out of the
question, and Ethyl Acetate is too reactive. There were other 'common' ones I looked at, but none of them fit the bill. I need something that would be
relatively inert for multiple reaction steps and THF seems to fit the bill perfectly.
Quote: Originally posted by No Tears Only Dreams Now  | | I think by dryness he means don't distill for so long that your boiling flask is completely devoid of liquid. That or I've been interpreting the whole
"distill to dryness" concept all wrong. That may cause a whole mess of problems. |
Is that the case? My flask would definitely not ever be devoid of liquid.
| Quote: Originally posted by smaerd | Here's the thing. Explosive peroxides are very real. Lot's of lab fires have happened because of them. How easy are they to set off?
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What kind of "explosions" are we talking about here? Like "boom", or small fires starting? THF is flammable, but I don't think it is violently
explosive? Fumes may be.
| Quote: Originally posted by smaerd |
I'd rather extract from it after diluting it with say water(situational). If you're careful it can be distilled so that there is still a decent sized
puddle of solvent in the flask when the distillation is ceased(aka not distilling to 'dryness'). That means you watch the distillation, don't walk
away and make cookies.
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Well I wouldn't go to dryness since the reactants are far heavier than the THF. I just can't further work with the organics until the THF is removed.
| Quote: Originally posted by smaerd |
As much as I would love to say recycling THF is a great money saving idea for a home-chemist, it's probably not practical nor safe in a lot of
instances without precautions. |
Home-Chemist is quite a broad term. The lab I use is quite well equipped and I have no problem working with potentially dangerous reactions, as I am a
very careful and attentive person. As long as there is a way to control the situation with the right approach, I am comfortable in my ability to
perform it. I like to do my research before trying things (hence the thread). If there is a proper procedure for what I am hoping to attempt, I would
gladly like to hear it.
| Quote: Originally posted by smaerd |
Is the residual water going to be an issue? Is an impossible question to answer, unless you ask the manufacturer and they confirm it is 'water-free'
and you know how to transfer solvents properly without moisture entering them. It can always be dried. |
I'm not too worried about transfer water-contamination. The humidity here is well controlled, and the amount of water that could possibly get in the
solvent during transfer is so little that it would barely have any effect on the reaction.
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paulr1234
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@Leben
There's a couple of things here that you need to be a little better acquainted with.
Unstabilized THF like Diethyl Ether will, when stored in the presence of oxygen, form explosive peroxides that can give you a very bad day. Even if
the explosion is small, the danger posed by flying glass, and subsequent fires caused by a heated, highly flammable solvent are very real. Plastic
surgery is painful and expensive. Your starting material appears to be stabilized, what you distill over will not be.
The other thing that isn't being properly covered here is that there is a difference between distilling to simply remove [most] of the water from a
solvent and creating a truly anhydrous batch that can be used for highly water sensitive reactions like Grinards. To do the latter you have to distill
the solvent by boiling it in the presence of freshly extruded sodium wire with Benzophenone and under a dry, inert atmosphere.
This mixture in the boiling flask eventually turns a deep blue color, showing that the solvent is truly water free (and oxygen free also for those
reactions where that matters). The solvent is distilled over into a special receptacle called a Schlenk flask that has an outlet for the inert gas
that is passing through the system. After a batch is distilled, the flask is sealed to preserve the precious water and oxygen free solvent.
This is a frequent activity for skilled chemists in well equipped organic labs but it is not for the inexperienced home chemist. Don't do it. Boiling
flasks of highly flammable and potentially explosive solvent mixed with sodium metal is dangerous.
Unless you need the oxygen-free part, you would be better of investigating the use of a 4a molecular sieve to remove the moisture. This is often the
preferred method in many labs these days. It should be fine for THF but it is not suitable for all solvents like Acetone.
Your mileage may vary, so do your homework. Be safe.
[Edited on 4-9-2014 by paulr1234]
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Leben
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Based on some of the threads I have been reading, it seems the peroxides are only really a danger when the solvent is being distilled, by iteself, out
of a flask, which when gone to dryness would cause the peroxides to concentrate in the flask. From what I've been reading, I haven't found any reports
of any major reports of THF peroxides during its use as a solvent. Correct me if I am wrong.
| Quote: Originally posted by paulr1234 | @Leben
Unstabilized THF like Diethyl Ether will, when stored in the presence of oxygen, form explosive peroxides that can give you a very bad day.
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"stored" = I have no problem with storing under an inert gas if that is what is required.
You seem to be misunderstanding me about my concerns of water. I am not concerned with moisture in the slightest. I am not trying to dry my THF by
distilling it, I am just wanting to remove it from the (liquid) reaction products by distillation so I can precede to the next steps. Throwing it away
would be wasteful if I could just reuse it.
Is there a reason I cannot distill it over, and then add in my own stabilizer? BHT seems to be the popular choice for commercial THF. At 0.025%
required concentration, what reason is there not to just add that small amount to the receiving flask when distilling the THF, and stirring while
receiving? Applying a medium vacuum should be suitable to rid most oxygen from the system for the period of the distillation.
I'm just trying to get a realistic idea of this. THF can't be that dangerous to distill, it is a very commonly used solvent.
As per your recommendations about drying solvents with sodium wire - this is actually a reaction I am extremely well familiarized in practice and one
I have had to perform in research studies many times, luckily my reactions are not that moisture sensitive. Tiny amounts of moisture will have a
negligible effect any variation of the reaction I am researching.
Furthering:
I've found several threads of people stating that the risk of these peroxides is blown out of proportion, I can see how distilling to dryness in a
flask could pose a problem, but beyond that??? :
http://www.sciencemadness.org/talk/viewthread.php?tid=12852&...
- Stated that IPA more readily forms peroxides, than THF, which I've never known, and I've never had problems with that.
http://www.sciencemadness.org/talk/viewthread.php?tid=24211&...
This person worked with a 30 year old bottle heavily contaminated with peroxides, without anything bad happening. I'm sure that bottle may have been
rocked a few times in its days too.
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forgottenpassword
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If you are concerned, surely you could test it for peroxides -- and if there are none, then why not distill it to dryness? If you have an inhibitor
dissolved in your THF, it inhibits the formation of peroxides, so just test it before you use it.
[Edited on 4-9-2014 by forgottenpassword]
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Leben
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I have no plans to distill to dryness. The most distilling I will be doing of this THF is to distill it out of a reaction mixture that is composed
primarily of liquids. I'll probably get some inhibitors and store it under argon of nitrogen for safe measure, and probably vacuum distill it instead
of normal distillation to prevent oxygen contact in doing so. Most people don't seem to have a problem with distilling it normally so I don't think
there will be many problems. Thanks for the input guys!
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Dr.Bob
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THF can be safe to distill from a reaction, especially if you just purchased the THF recently. The material distilled will be free of peroxides, as
they are higher MW, but the residue left over in the pot will be highly enriched in peroxides, so that material should be quenched with water to
dilute it, and the flask should be handled carefully. Doing this under a low vacuum is best, again, just don't boil it dry. And the distilled THF
will be more prone to peroxide formation, so try to add the BHT or whatever back quickly to it, and try to keep it away from light, heat and air.
But THF with inhibitors should be good for months if keep sealed, but once the bottle is open, it will form some peroxides within 6-12 months, most
likely, based on what I have seen. The real hazard is when you concentrate the peroxides in the remaining solvent in a still pot and then let the
mixture sit or heat it/ Once there is sufficient water mixed with the pot residue, it will not be concentrated enough to create a dangerous mix.
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