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Topic: DMF contaminant in TLC samples used to follow a reaction  (Read 728 times)

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Offline Babcock_Hall

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DMF contaminant in TLC samples used to follow a reaction
« on: March 03, 2022, 12:58:55 PM »
Suppose that one is following a reaction in DMF solvent by TLC.  Unless one removes it, the DMF would be expected to increase the Rf values of reactants and products, at least when the running solvent is less polar than DMF.  Is there a convenient way to remove the DMF in such an instance?  Thinking out loud, one might try a micro-extraction.  One could heat the TLC plate to drive off the DMF so long as the product was not heat-sensitive.

My reason for asking is that we were reacting a UV-active thiol with a diethyl phosphonate bearing a tosylate group, produce a UV-active thiother.  We ran the reaction for longer than the protocol indicated, and the TLC suggested that the starting material disappeared and that the product had a higher Rf value.  I think that this was a misleading result.*  We are working the reaction up, and two lines of evidence suggest that the reaction proceeded less than 50% to completion: mass of the crude product and TLC of the post-extraction material shows a significant amount of starting material.  If we run the reaction again, we can use the DTNB test (Ellman's reagent) to look for loss of thiol (this can be done quantitatively).  We can also use P-31 NMR to monitor the reaction (I wish we had done so for the reaction just completed).  However, other possible reactions will not have these alternatives.

*I think that the DMF pushed the starting material and product together and we did not differentiate between them.
« Last Edit: March 03, 2022, 01:12:02 PM by Babcock_Hall »

Offline rolnor

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Re: DMF contaminant in TLC samples used to follow a reaction
« Reply #1 on: March 03, 2022, 01:15:59 PM »
Just use a hair-dryer or heating-gun to remove DMF. Vacuum can also be used if you can fit the plate in some vacuum container/flask. You dont need much heat, DMF is rather volatile

Offline wildfyr

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Re: DMF contaminant in TLC samples used to follow a reaction
« Reply #2 on: March 03, 2022, 01:21:31 PM »
Nothing wrong with dissolving the aliquot in EtOAc and adding a little water and giving it a good shake, then spotting the EtOAc layer.

I like that a bit better than RT hair dryer cooking DMF, but if its stable to ~150°C youre probably fine.

Offline Babcock_Hall

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Re: DMF contaminant in TLC samples used to follow a reaction
« Reply #3 on: March 03, 2022, 01:56:36 PM »
A vacuum desiccator might be one way to go (you might have mentioned this to me before, because it seems familiar).  A micro extraction using ethyl acetate vs. water is also something that looks attractive.  Thank you.

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