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Author: Subject: Difficult contaminant
Rich_Insane
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[*] posted on 20-3-2013 at 13:25
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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[*] posted on 20-3-2013 at 16:36


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. :D
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bananaman
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[*] posted on 20-3-2013 at 16:52


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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[*] posted on 20-3-2013 at 17:22


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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[*] posted on 20-3-2013 at 17:57


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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[*] posted on 26-3-2013 at 17:15


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.


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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[*] posted on 27-3-2013 at 01:19


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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[*] posted on 1-4-2013 at 08:13


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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