July 01, 2022, 07:51:09 PM
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Topic: Column Chromatography  (Read 454 times)

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Offline Anatolian

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Column Chromatography
« on: April 06, 2022, 10:19:29 PM »
Hi All,

I'm inquiring about a question regarding column chromatography. I would like some advice, or guidance, on how I should proceed. I am in the process of synthesizing a novel compound, and during the purification process, I always obtain mixed fractions.

Attached includes the TLC plate I obtained, as well as the reaction scheme. In regards to the TLC chamber eluent, I used 8:1 DCM/MeOH.

For my column, I typically go 200:1 (100 mL of DCM and 0.5 mL of MeOH) DCM/MeOH. In regards to how I prepare my column, I typically, add cotton, then my slurry. Afterwards, I dry-load my compound. In my column, I usually add 1% Et3N, and while preparing my dry-load, I also add Et3N.

The way I potentially wash might be an issue? The solvent is DMF. Once the reaction is over, I dilute with DCM (2x), then wash with water (2x), then brine.

The column is 250 mL, and fairly long and skinny. To clarify, the attachment was for 500 mg.

I am currently rerunning the reaction, and my theoretical yield is 1 gram. Any advice would be greatly appreciated.

The flow-rate is also not that fast. I am just confused on why I keep obtaining mixed fractions in my column. How can I prevent getting two fractions?

Could it be that my eluent for my column is too polar? When running the TLC, I can't get any movement using hexane/EA
« Last Edit: April 06, 2022, 10:30:34 PM by Anatolian »

Offline rolnor

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Re: Column Chromatography
« Reply #1 on: April 07, 2022, 08:09:21 AM »
You may have DMF left in you sample. Wash the reaction mixture 5times with water.  I would wet-load instead, use only DCMwhen you pack the column.

Offline Babcock_Hall

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Re: Column Chromatography
« Reply #2 on: April 07, 2022, 08:47:54 AM »
Some people advocate the use of 5% aqueous LiCl to remove DMF during the work-up.  I don't understand why there is so much more methanol in your TLC experiment (8:1) than in your column.  Or did I misunderstand?

Offline Anatolian

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Re: Column Chromatography
« Reply #3 on: April 07, 2022, 12:05:38 PM »
Some people advocate the use of 5% aqueous LiCl to remove DMF during the work-up.  I don't understand why there is so much more methanol in your TLC experiment (8:1) than in your column.  Or did I misunderstand?

Well, I thought that I should go with a less polar gradient, according to my labmate, in order to obtain impurities, or any byproducts. The (8:1) is 4 mL of DCM and 0.5 mL of MeOH

Offline Anatolian

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Re: Column Chromatography
« Reply #4 on: April 07, 2022, 12:08:01 PM »
You may have DMF left in you sample. Wash the reaction mixture 5times with water.  I would wet-load instead, use only DCMwhen you pack the column.


When you mean packing the column, so you mean when I create my slurry, etc. Not as a solvent system, correct?


Now, I'm unsure how much silica gel to add. I know if I add more than I should, it would impact diffusion, resulting in broadening my seperation, thereby not obtaining a lot of my product. So, I'm assuming 100 grams of silica should be enough for a slurry.

Offline clarkstill

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Re: Column Chromatography
« Reply #5 on: April 08, 2022, 10:57:50 AM »
Sometimes you see this if one of the spots is unstable on SiO2 and decomposes to the other. Check by running a 2D TLC: use a square plate, run like normal along one direction, wait 10 mins, turn 90 degrees and run again. Any off-diagonal spots indicate decomp. on silica.

Also, use Celite rather than silica for dry-loading, since it doesnt retain your compound.

Good luck

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