GhostAliensFromMars
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Problem when trying to vacuum distil a compound.
I was attempting to purify an organic liquid I synthesised at my university using fractional distillation under a storng vacuum, approx 1mm hg. I
started stirring and gradually applied the vacuum. The liquid starting bubbling vigorously and nothing was coming over or condensing, so I assumed
this was just air trapped in the organic liquid escaping. This bubbling went on for 3 HOURS so I decided to increase the temperature, and the bubbling
got so violent that I had to cut the vacuum and stop the distillation. Now i have approx 75g of my crude undistilled product and I need suggestions on
how I could fix the offgassing issue. Would it be ok to take a small sample of my crude product, maybe 15g and plug it into a reaction to see if the
impurities will not affect the reaction much? I believe my crude product is at least 80% pure.
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bariumbromate
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Did you add boiling chips? I'd think that would help.
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GhostAliensFromMars
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No but I did use magnetic stirring, which I believe is a better option than boiling chips for vacuum distilation
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MrDoctor
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use both, sand is good. when its possible to i think cracked pelletized activated carbon is really useful because of the insanely high surface area.
otherwise, hop on ebay/ali or a dollar store and buy a bag of little 1-2mm glass beads, then etch them somehow, like stirring them with hot sodium
hydroxide and/or aluminum oxide or other abrasive oxide/grit, because glass beads are too smooth to nucleate in many liquids except when they tumble
and basically its just that same effect that makes fresh thermometers only nucleate when you scrape it good against a surface or ping it. this with
glass beads or other media that only nucleates when it generates, i assume sound shock waves, can cause a flash boil, which i did with sulfuric
acid/ethyl sulfate solution while making ether, it instantly filled the receiver and died just before it sprayed out the vacuum takeoff. long story
short smooth glass beads that tumble about in boiling solution can cause a chain reaction.
Old silica gel would actually be really good but not re-usable, its porous not just on the surface, but it actually does catalyze some reactions and
will release adsorbed water so be mindful there. i think even sodium carbonate will eat away at it if silicates are a problem.
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Sulaiman
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IF pressure is reduced so low that a liquid boils at ambient temperatures
a condenser at a temperature much lower than ambient will be required
it is possible that you are boiling away your product.
CAUTION : Hobby Chemist, not Professional or even Amateur
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fx-991ex
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Quote: Originally posted by Sulaiman  | IF pressure is reduced so low that a liquid boils at ambient temperatures
a condenser at a temperature much lower than ambient will be required
it is possible that you are boiling away your product. |
Would like to know if after 3 hour the level dropped in the flask or stayed the same.
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Dr.Bob
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That is quite possible, maybe try dry ice. Or test the crude product, as purifying oils is tough, so if you can get the MW up, it might be easier to
purify the final product. Good luck.
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GhostAliensFromMars
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| Quote: Originally posted by fx-991ex | | Quote: Originally posted by Sulaiman | IF pressure is reduced so low that a liquid boils at ambient temperatures
a condenser at a temperature much lower than ambient will be required
it is possible that you are boiling away your product. |
Would like to know if after 3 hour the level dropped in the flask or stayed the same. |
No it didnt change significantly, I was left with maybe 1-2 mls less product than the start, but this is likely due to the viscous oil sticking to the
flask. Based on my own research I calculated the boiling point of my product to be around 105c at my vacuum strength, far above ambient temperature.
Due to the viscosity of the crude product I had to warm it to 40c in order for stiring to work effectively, it had a viscosity similar to glycerine. I
assume this high viscosity was responsible for all the trapped air inside my product, similar to how cream traps air when whipped and thickens. I will
take 10mls of my crude product and plug it into my reaction and inform you on the results. Whether this small scale test reaction goes well or not, I
will attempt to redistill my product.
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greenlight
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Is it definitely offgassing or just violently boiling under the vacuum.
I have done fractional vacuum distillations before and needed to insulate the entire column and still head with foil just to get the heat to move up
and the condensation front to rise up the column. Prior to insulating the liquid was just boiling and the vapours not moving anywhere.
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BromicAcid
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My starting questions would involve your setup. What size glassware? I.e., 14/20 is more likely to flood and/or constrict, 24/40 wouldn't have that
issue. You mentioned a fractional distillation but no mention of the column if you had one. Was it silvered, vacuum jacketed, length, packing, etc?
Did you see any material in the column condensing, assuming there was no flooding as it seems you would treat that as a positive sign. Make sure you
have plenty of volume in the pot for this bubbling you saw so you're not afraid to give it some more heat to start pushing things over.
Did you vacuum check your setup before running, were there places for potential air ingress, i.e., non-ground glass connections like compression
fittings for thermometers and the like.
Did you insulate your setup with fiberglass or the like? I've had distillations where the material would distill/condense and repeat before reaching
the condenser because of the relative temperature of the lab vs the pot.
Did you have traps inline to catch the volatiles, this helps maintain your vacuum without killing your pump oil. There is also the potential that
there is something in your pot decomposing leading to the volatiles.
Basically, there are a lot of unknowns here where things could go wrong. I mean, you say you're at a university and that's where you should have had
an adviser/TA/Teacher come over and critique your setup since this is what you're learning in the first place.
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bnull
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My first question would be what organic liquid is it.
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GhostAliensFromMars
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Ok, so I performed the reaction on half the scale as the writeup, but used 15g of product instead of 12.5g to account for its lower purity. The
reaction went well, and looked exactly like the pics in the writeup. The writeup excluded an a/b extraction, but of course I did one to manage the
impurities. In the end i was left with 0.950g of product. It did work, and i did do an ir spec of the results and it was resonably pure, but the yeild
was terrible, it should have been closer to 10g.
I will reattempt to distil my oil and see if its storage in the freezer had any effect on the dissolved gases in it.
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GhostAliensFromMars
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On that note, yes I have started to think it could have been residual DCM in my product boiling out, this would explain the abscene of condensation
due to its low BP.
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macckone
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| Quote: Originally posted by GhostAliensFromMars | | On that note, yes I have started to think it could have been residual DCM in my product boiling out, this would explain the abscene of condensation
due to its low BP. |
If there is residual dcm or reactants, a 1mm vacuum will definitely cause them to boil.
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GhostAliensFromMars
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A very specific ketone with a color resembling that of a tennis ball.
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GhostAliensFromMars
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Update: I did redistilled my product and the same issue reoccured. I turned up the stirring to maximum which evened out the bubbling and turned the
heat up until it boiled. The liquid in the boiling flask was splashing around violent, but i was getting distillate coming over . I collected
everything between 118-122c and was left with 40g of product. The residue in the boiling flask on cooling became a very thick tar that was almost a
solid. The yield was less than expected (60g) but at least it worked. Interestingly, the temperature at which my product came over was much higher
than expected (80c). I blame the higher temperature on the offgassing which probably caused an increased pressure in the system.
[Edited on 18-12-2025 by GhostAliensFromMars]
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bnull
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Did you get this ketone after doing a Meyer-Schuster?
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GhostAliensFromMars
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Not entirely sure what a meyer schuster is, I have always known it as a Pinacol Rearangment.
And yes, I TOTALLY did this at my university...
[Edited on 19-12-2025 by GhostAliensFromMars]
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bnull
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| Quote: | | Not entirely sure what a meyer schuster is, I have always known it as a Pinacol Rearangment |
It's another rearrangement, from a propargyl alcohol to a ketone.
| Quote: | | And yes, I TOTALLY did this at my university... |
Then don't let them find it out.
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jackchem2001
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How is anybody supposed to help about a distillation if you wont even say what the components are? Dumbass if its illegal then why did you come post
about it here, nobody cares anyways and you arent impressing anyone
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GhostAliensFromMars
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| Quote: Originally posted by jackchem2001 | | How is anybody supposed to help about a distillation if you wont even say what the components are? Dumbass if its illegal then why did you come post
about it here, nobody cares anyways and you arent impressing anyone |
i figured that the exact identify of the compound wasnt important in solving my problem. But its fixed now, no need for you to say anything.
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bnull
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Take it easy, guys.
Knowing what substance is being handled is one of the most important aspects of laboratory practice. It doesn't matter if it is done for fun in the
shed in the backyard or to obtain a degree from Göttingen, you must have at least a vague idea of what is there when you make it. When you ask for
help, those offering suggestions need to know what is involved so as to find what is applicable and what is not (see the Mad Science FAQ, section 2.1
Board Topics). Bretherick's has several instances of reactions that were performed for years without accidents and then showed their violent nature in
some poor fool's lab.
This forum has strict rules when psychoactive substances and their precursors are involved. Mind you, this is not Erowid or The Vespiary (.org is the
forum, .com is a bookbinder's), so there are restrictions. The third item of the August 2023 Amendment of the Guidelines, for example, goes
| Quote: | | Discussions of synthesizing common, synthetically uninteresting drugs such as methamphetamine, MDMA, GHB, etc. are not permitted. There is already a
plethora of information about these substances on the internet, and there is no reason to discuss them on this site. Exceptions may be made at
moderator's discretion for theoretical discussions of unique synthetic approaches that have some intellectual merit. |
It doesn't mean that what you ask is inadequate; you're not asking or offering synthetic routes for whatever-bleeding-ketone-you-made-at-the-U. Hiding
it behind a puzzle (greenish-yellow ketone that may be illegal or restricted, psychoactive precursor or not) makes it very suspicious, though.
If you had "figured that the exact identify of the compound wasn't important," then chances are that the problem had already been solved in
literature.
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BromicAcid
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I think this thread shows exactly how difficult even a simple distillation can be. The OP didn't really answer my questions but I had so many
followup questions (location vacuum was being measured, type of manometer used, etc.) because even something that might be considered standard or
mundane is something that can easily be overlooked and can botch a distillation.
After distilling thousands of organic and inorganic compounds over my career I can say that even I get thrown a curve-ball distillation from time to
time but that at least with my experience pretty much anything can be overcome or at least the root problem identified (some compound due to their
nature or impurities present need additional workup or alternate purification methods).
I agree with bnull that the identity of the compound does come into play with a distillation. In general things with O-H bonds and N-H bonds tend to
have higher specific heat that boring old alkyl compounds and can require a bit more push to get them moving up the column.
Unfortunately we could not answer your question with the information or feedback provided. Perhaps a better question to ask would have been "What are
some important factors that contribute to a successful vacuum distillation?" Which doesn't rely on obfuscation to get the information you provided
and allows for more open discussion rather than sounding like dealing with a competitor concerned for their trade secret.
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GhostAliensFromMars
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I really appreciate both of your feedback, in future I will endeavour to word my questions in a way which makes them easier to answer.
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