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Topic: Problem with work up  (Read 2123 times)

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Offline vu_nam

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Problem with work up
« on: November 25, 2013, 03:49:40 AM »
Hi, I am a student who just begin work with organic synthesis. Lately, I did a lithiation reaction following by carboxylation with 4-methyl thiazole thiazole using nBuLi for lithilation, next added CO2 to make 4-methylthiazole 2-carboxylic acid. After finishing rnx, I added diluted HCl to adjust pH to ~5 and separated organic layer. However, TLC analysis supposed that the lowest spot (which I think that it is my product) presented only in the aqueous phase. I even tried to reduce pH to ~2 but still could not obtain product in organic phase. How can I separate my product? I'm really appreciated any kind of help.

Offline Babcock_Hall

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Re: Problem with work up
« Reply #1 on: November 25, 2013, 09:46:16 AM »
Is it possible that the thiazole ring got protonated and it is water-soluble?  I don't know the pKa value, so I am just thinking out loud.

Offline orgopete

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Re: Problem with work up
« Reply #2 on: November 25, 2013, 10:44:52 AM »
I am presuming 2-carboxy-4-methylthiazole is a known compound. Was this the literature procedure? If it was, describe it.
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Offline vu_nam

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Re: Problem with work up
« Reply #3 on: November 25, 2013, 09:02:42 PM »
As I know, pKa1=1.2, pKa2= 3.18. Procedure for lithiation for other compounds can be found, but it vary among substances. Now I am trying to add more solvent to aqueous phase and dry by Na2SO4, hopefully no inorganic impurities remain after filtration.

Offline vu_nam

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Re: Problem with work up
« Reply #4 on: November 28, 2013, 02:51:02 AM »
Hi, I have tried again with this reaction and tried to separate acidic product from aqueous phase but still unsuccessful. I used 1N HCl to add to the aqueous phase and adjust ph~ 1-2 to precipitate product but no solid product form. Please help me, I attach in here the procedure for similar compound  (page 694)

Offline orgopete

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Re: Problem with work up
« Reply #5 on: November 28, 2013, 03:35:29 AM »
Just by the looks of it, you added too much water. They used conc HCl, and you used 1N HCl. Their aqueous layer was more concentrated so when they acidified it after removing any neutrals, it precipitated. I suggest following the procedure exactly as described (but on your scale).
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Offline organicdave

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Re: Problem with work up
« Reply #6 on: December 05, 2013, 06:56:19 PM »
Just by the looks of it, you added too much water. They used conc HCl, and you used 1N HCl. Their aqueous layer was more concentrated so when they acidified it after removing any neutrals, it precipitated. I suggest following the procedure exactly as described (but on your scale).

I agree with orgopete, I've made this mistake before.  If you can, rotvap off some of the water, or put it in the freezer to induce crystallization.

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