Rich_Insane
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Difficult contaminant
Alright, so I've isolated a product. Basically I was forming amides from oximes using copper acetate in xylene as a solvent. Now I have some product
sitting in a beaker which I isolated as best I could using boiling water. I have some sitting in a flask which crystallized out of cool xylene. The
problem is that there's this strange brown gunk in there. Basically the stuff carries a lot of the product in it (you can see crystals protruding out
of it). It is insoluble in water and melts at around 50-70oC, forming a thick oily material. The problem is that I want to isolate the product and not
this goo (my guess is that it is a combination of oxidized aldehyde, decomposition products and copper (II)chelates). It also smells strongly of
xylene, but that could be because xylene forms an azeotrope with water (so the water contains a small but detectable amount of xylene).
How can I do so? I've tried hot filtration but when this stuff melts, it can pass through the filter bit by bit (eventually the coffee filter clogs up
and I have to discard it). It's very hard to work with this brown goo because it sticks to and discolors everything; I've had to throw out glassware
before because it was beyond repair from the tar.
Edit: Pictures!
These are taken on my iPod so the quality is not good.
[Edited on 20-3-2013 by Rich_Insane]
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Metacelsus
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Hmmm... I have a few ideas:
1) If you can't filter, try decanting.
2) Redissolve the whole mess in hot xylene, or another solvent. Then chill the solvent as much as you can. One compound will be less soluble and
precipitate more.
3) Find a solvent that dissolves one but not the other.
4) Pick out the crystals with tweezers, Pasteur-style. 
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bananaman
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1) My personal experience was to test the solubility of your compound with various solvents, e.g. CH2Cl2, CHCl3, MeOH to determine which would
dissolve your desired product. After the selection of the solvent, consider sonicating for about 30 mins. Filter it if possible.
2) Else, I would try it some celite into the container, and wash the product out with the desired solvent.
3) Throw in DMSO to dissolve most of the crap, followed by extraction with CH2Cl2.
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AndersHoveland
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you might have to go a different route to produce the amine.
for example, reduce the ketone to an alcohol, form the bromo- ester, then bubble in NH3 gas
I'm not saying let's go kill all the stupid people...I'm just saying lets remove all the warning labels and let the problem sort itself out.
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Magpie
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Don't throw out your glassware! For tar try the classic alcoholic KOH soak.
This sounds like a typical oil-out. Oil-outs typically can occur when you are trying to crystallize a compound with a low melting point. Also, the
oil is an excellent solvent for tarry contaminants so it is hard to clean up or even crystallize.
If possible I recommend you use distillation instead of crystallization to isolate your product. This worked well for me when my 1-napthol was giving
an oil-out.
The single most important condition for a successful synthesis is good mixing - Nicodem
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madscientist
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Aside from the other ideas for cleaning your glass, one thing that's worked great for me is a mixture of DCM and MeOH and a little scrubbing. DMSO and
DCM will dish out heavy damage to gloves, and DMSO can pass things straight through the gloves into your bloodstream, so use a spatula if you scrub
with Kimwipes.
Of course, you can also try to burn it off with something oxidizing (stay the hell away from piranha solution though).
Quote: Originally posted by AndersHoveland  | you might have to go a different route to produce the amine.
for example, reduce the ketone to an alcohol, form the bromo- ester, then bubble in NH3 gas |
He's trying to make an amide, not an amine, and he's probably starting with an aldehyde, not a ketone.
What is this about bromoesters and bubbling in ammonia? I'm not seeing how any of that could lead to the desired product. Also, when you're trying to
alkylate ammonia, the order of addition is substrate to excess of ammonia, not vice versa. Otherwise polyalkylation will dominate.
| Quote: | | If the reaction was run with excess tert-butyl bromoacetate (>3 equivalents), overalkylation gave a quaternary ammonium salt.
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See page 5.
http://www.jmpharmaservices.com/da1d0dee-067f-4241-bc86-e604...
If you had any real synthetic experience whatsoever you would know this.
I weep at the sight of flaming acetic anhydride.
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Hockeydemon
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This is a bit out of my league, but you said it seeps through the filter bit by bit.. Would diatomaceous earth do anything to make filtering feasible?
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Rich_Insane
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What I've been doing is essentially boiling this "goo" with water. The aq. layer is siphoned off with a turkey baster while still boiling
hot.
The water will turn milky white if the temperature goes below say 70oC. Crystals form spontaneously upon cooling, and the recrystallized product is a
clean white color. The problem with this method is that it takes forever to carefully siphon off the water layer. And then at the end, I'm left with
an oily mess (that still contains a little product) on the bottom of my glassware. Treatment with concentrated NaOH helps and causes the "goo" to
become brittle and flake off... Isopropyl alcohol seems to slightly dissolve the brownish material.
I think Celite would be great if this stuff didn't crash out of water so easily. I'd just boil the product with water and filter it. Unfortunately, a
lot of the desired amide would crystallize before passing all the way through the filter: A problem I have faced with hot filtration (gravity
filtration) as well.
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