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Offline 23SigmaTropic

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Purification woes
« on: March 25, 2016, 09:59:38 PM »
Hey

So I have a bicyclic 2-sulfonyl pyridine with an CH2- dimethyl amide attached. The reaction appears high yielding by crude NMR but the TLC profile has a more or less a continuous streak of trace impurities all the way up the plate which are only visible by UV. I've tried many different solvent systems and I can't get any resolution. I don't think its my desired compound streaking or anything like that because triethylamine in the eluent didn't help.  I'm working on a small scale so I tried multiple elutions on prep TLC after an initial silica plug, but there is still roughly 10 % junk. I'm thinking I should try recrystalization, but since I haven't gotten the stuff pure I don't know if its a solid or not (MW>400). Any ideas on how to approach this? I have plenty of the crude to mess around with, but I don't want to waste my time trying to recrystallize if its an oil in pure form.  Thanks!

Offline phth

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Re: Purification woes
« Reply #1 on: March 26, 2016, 01:49:05 AM »
Streaking means it's decomposing, or changing its form i.e. partially ionized.  This probably means your plate didn't work. Did you run the 5% TEA solvent choice up the TLC plate, let dry, and then run the tlc?  Not doing this would be equivalent to dryloading a column. Also be aware that the way your chemicals behave is not the way they will behave on the column because silica on a plate is different than the gel.  Have you tried using benzene or toluene as an eluent?  How long did you allow for equilibration time?

Offline Dan

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Re: Purification woes
« Reply #2 on: March 26, 2016, 04:29:40 AM »
The reaction appears high yielding by crude NMR but the TLC profile has a more or less a continuous streak of trace impurities all the way up the plate which are only visible by UV. I've tried many different solvent systems and I can't get any resolution.

This sounds like it could be a decomposition problem. I'd recommend running a 2D TLC to check.

If the compound decomposes on silica, you can try a different stationary phase, e.g. alumina, for purification.

If this is an intermediate, it might be worth submitting the crude to the next reaction directly.
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Offline 23SigmaTropic

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Re: Purification woes
« Reply #3 on: March 26, 2016, 06:33:29 AM »
The reaction appears high yielding by crude NMR but the TLC profile has a more or less a continuous streak of trace impurities all the way up the plate which are only visible by UV. I've tried many different solvent systems and I can't get any resolution.

This sounds like it could be a decomposition problem. I'd recommend running a 2D TLC to check.

If the compound decomposes on silica, you can try a different stationary phase, e.g. alumina, for purification.

If this is an intermediate, it might be worth submitting the crude to the next reaction directly.

2D TLC didn't cross my mind but its worth trying, thanks! Unfortunately its a methodology paper so I need it pure.

Offline Dan

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Re: Purification woes
« Reply #4 on: March 26, 2016, 06:57:34 AM »
Unfortunately its a methodology paper so I need it pure.

You could process the crude and isolate a derivative. We have done this before - we had an alcohol product in a methodology project that was inseparable from another byproduct, so we isolated it after acetylation of the crude. It's not a problem to have a couple of entries like that.

Is this a general problem with your products or just this one?
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Offline curiouscat

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Re: Purification woes
« Reply #5 on: March 26, 2016, 07:46:01 AM »
I have plenty of the crude to mess around with, but I don't want to waste my time trying to recrystallize if its an oil in pure form.

I'm no expert at this at all but just a thought: Can you predict from the structure & one of the standard algos. what the MP could be? Even if not accurate that may give you a hint if it is likely solid or liq?

Maybe more a naive question than a suggestion.

Offline 23SigmaTropic

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Re: Purification woes
« Reply #6 on: March 26, 2016, 07:59:12 PM »
So I tried just triturating the crude oil with ether and a few drops of methanol and decanting the supernatant solution. This left a solid chunk of brown gunk and a solution which upon concentration gave a much cleaner looking solid. NMR of the solid was more or less equivalent to what I got after an initial plug then prep TLC. Now I'm sure it's a solid and it looks like it's recrystallizing from ether. Wow. 4 hours for the plug and prep TLC versus a half hour to do what I did just now. Sometimes you have to just laugh.

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