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Topic: Purification by sublimation  (Read 3039 times)

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Offline kamiyu

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Purification by sublimation
« on: October 11, 2014, 06:36:21 PM »
I have a compound to be purified. I tried column, recrystallization and activated charcoal but all fail. (it is always contaminated by broad peaks in the aromatic region which is like a background)

I plan to do sublimation. The MW of the compound is about 500.

Anyone with practical experience?? Anything to be alert???

Thanks

Offline gritch

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Re: Purification by sublimation
« Reply #1 on: October 12, 2014, 03:39:04 PM »
That's really not a lot of information to work on. I suggest finding of copy of a book like "Purification of Laboratory Chemicals" and looking up suggested purification methods for your compound or similar such compounds.

For my personal advice:
Make sure to double check the impurity isn't in the NMR solvent you're using. Run a quick blank (just solvent) and see if those broad aromatic peaks are still there. If they are try a different bottle/solvent and see if the peaks go away. If you end up doing the sublimation, test a small amount of your sample first to make sure it's actually stable at temperatures you'd need to get it to sublime it. Nothing's worst than heating your entire sample only to have it decompose on you.

Offline TheUnassuming

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Re: Purification by sublimation
« Reply #2 on: October 14, 2014, 02:29:58 PM »
Broad peaks in the aromatic region?  What does the carbon look like?  Can you see the impurity by TLC? 

Sublimation can be tricky depending on your compound, but when it works it gives you some of the prettiest crystals you have ever seen.  A mass of 500 seems high for this, what makes you think it can be purified by sublimation? 

I second that you need to heat it up to see if it decomposes, or just try it on a small scale depending on how precious your material is.  The biggest and only real major concern when doing sublimation/distillation is if your compound will get roached before flying. 
When in doubt, avoid the Stille coupling.

Offline MOTOBALL

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Re: Purification by sublimation
« Reply #3 on: October 15, 2014, 02:37:46 PM »
Because of the potential issue of decomposition, referred to above, you could use sublimation under reduced pressure.

I have purified a compound of MW ~300 with m.p. 55 C, under reduced pressure (75 C/ 0.03 mm) to obtain, as previously indicated, beautiful crystals deposited on the cold finger.

Set a steady vacuum BEFORE applying any heat to the crude material; use an oil-bath to warm up the sample SLOWLY.

You would do very well to purchase a reference book on organic techniques, such as that described above; another one would be "A Textbook of Practical Organic Chemistry, by A.I. Vogel, pub. by Longmans

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