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Topic: TLC - What to do when your product won't move at all  (Read 9833 times)

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Offline DoctorDomo

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TLC - What to do when your product won't move at all
« on: March 12, 2014, 09:31:42 AM »
I ran a few reactions in similar conditions, each product is similar and fairly similar to the starting material (starting material and products are all fluorescent and easy to visualize), and one issue I'm having is TLCs. I tried a range of different mobile phases, and they all move the purified starting material but when I spot my reaction mixture on the plate, it won't move at all. I end up with a fluorescent blotch in the exact same place I spotted it, and my starting material which moves up the TLC plate (im using silica plates).

Firstly, my solvent is DMF, I hear it sticks to the silica and causes the compound to smear, but even after my best attempt to get rid of the DMF, I'm still getting stationary spots. Also, I've only seen it smear once or twice, most of the time I end up with a blotch that didn't move at all. My friend whos working the same kinda reaction (in DMF) has the same issue. I'm familiar with TLC but this is the first time I've done it in a real life application so I'm struggling. What does it mean when you spot doesn't move at all, and how do you go about solving it?

BTW the mobile phases I've tried are ethyl acetate/hexane, ethyl acetate/chloroform and ethyl acetate/petroleum ether, all in varying ratios, if you could recommend some other good solvent systems, I'd greatly appreciate it.

Offline 408

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Re: TLC - What to do when your product won't move at all
« Reply #1 on: March 12, 2014, 09:57:27 AM »
What does your compound look like?  Toss some way more polar solvent in.  Like acetonitrile or something. 

Offline Corribus

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Re: TLC - What to do when your product won't move at all
« Reply #2 on: March 12, 2014, 10:55:38 AM »
Some things just don't purify well on silica. You could try a different stationary phase, or a different type of chromatography (e.g., size exclusion).

You can also try adding a little methanol to the eluent (few percent).
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Offline Dan

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Re: TLC - What to do when your product won't move at all
« Reply #3 on: March 13, 2014, 09:25:37 AM »
Just to clarify DoctorDomo, are you saying that the purified product runs fine, but the crude doesn't move?

For the TLC where the spot doesn't move, are you analysing the reaction directly, or are you working up an aliquot?

For reactions in DMF I always work up an aliquot. I partition a drop or two of the crude between a small quantity of brine and ethyl acetate (0.05-0.10 mL) in a small vial and TLC the organic layer (most of the DMF stays in the aqueous).

It would be helpful to know what the reaction is. E.g. it could be that the reaction forms a salt of your product that is not mobile on silica - only after workup, where you generate a neutral form, will it move. Provide as much information as you can, at the moment there's not much to go on.
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Offline DoctorDomo

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Re: TLC - What to do when your product won't move at all
« Reply #4 on: March 13, 2014, 02:20:50 PM »
UPDATE: I used TLC to monitor the progress of my reaction, and the reaction wasn't complete so there was starting material in there, and it had no trouble getting away from the DMF, it traveled the exact same distance as the pure starting material on the left lane.

What does your compound look like?  Toss some way more polar solvent in.  Like acetonitrile or something.

Heres my starting compound:

I would have expected that compound to be fairly lipophilic, but it isn't really. Polar mobile phases make it race up the plate. It exhibits purple fluorescence which surprises, the same colour that my products fluoresce at. I wanna try acetonitrile next, along with the 90:10 methanol phase.  .

 

What does your compound look like?  Toss some way more polar solvent in.  Like acetonitrile or something.


Some things just don't purify well on silica. You could try a different stationary phase, or a different type of chromatography (e.g., size exclusion).

You can also try adding a little methanol to the eluent (few percent).

I'm gonna try this tomorrow. A friend is running a reaction (completely different compounds though) in DMF and he told me that hes had no trouble with sticking or streaking, hes been using 90:10 ethyl acetate:methanol. I ran some solubulity tests on my compound and couldn't find anything that its easily soluble in. Diethyl ether, n-hexane, acetone, water, ethanol, none of them dissolve it well, and oddly enough it seems least soluble in ether and hexane. I added some DMF soaked precipitate from my flask to a mL of diethyl ether and white flakes dropped to the bottom of the vial. It cleaned the yellow tint out of my product. Does this mean I might be able to use ether to crash the product out of solution?

What does your compound look like?  Toss some way more polar solvent in.  Like acetonitrile or something.
No, I'm saying that the starting material (the structure I posted above) moves fine, but my product won't move at all. Also when the reaction isn't complete and theres starting material left over, I can easily separate it with TLC because the starting material (the fluorinated phthalonitrile) has no trouble separating from the crude product either. I tried both analysing directly and diluting it in DCM. It doesn't dissolve so well in DCM.

I'd dont wannaoo much about the products since they're new compounds, since I haven't even started characterising them so which compounds and isomers will be formed is uncertain. Basically I'm doing nucleophilic aromatic substitutions onto the ring in order to replace some of the electron withdrawing groups with electron donating ones. Each reaction I've done so far as at least 2 isomers, but I've yet to see anything move, let alone separate on the TLC plate. I'm spotting directly from my reaction mixture, but I tried dissolving a drop of the rxn mixture in some DCM to dilute the DMF but had the exact same result, I also tried just drying the DMF off with the nitrogen pump but no luck. There usually isn't even any smearing like you would expect with DFM, however I tried a 50:50 ethyl acetate/toluene moebile phase and it smeared my product up the plate about a cm. Thats a start I suppose.

Thanks a lot for the tip! Whats the salt for, just for dissolving emulsions or will it help get the DMF into the water layer too? Either way, I'd be inclined to spot both layers to see how much product is still present in the water.

I've tried multiple reaction conditions, for example, in one of my reactions involved attaching alkoxy groups to the ring. I tested two routes, first one I just added lithium metal to the ethylene glycol, and used the glycol as my solvent. Then I added the phthalonitrile and let the alkoxide ions do their thing. Second time around I used potassium carbonate to the propylene glycol then added the phthalonitrile and let it react over night. Didn't have any effect on the TLCs. Surely an excess of lithium metal would deprotonate both -OH groups of the glycol. Then I did the rxn in less harsh conditions by simply adding the precursor, along with some K2CO3 to the propylene glycol and adding a few mL of DMF. Intrestingly, the addition of DMF instantly turned the solution from clear to light yellow. Could that be a solvation effect?
« Last Edit: March 13, 2014, 03:19:37 PM by DoctorDomo »

Offline kriggy

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Re: TLC - What to do when your product won't move at all
« Reply #5 on: March 14, 2014, 02:14:29 AM »
Why exactly is a problem that your product doesnt move? If you use it for monitoring the progress of the reaction, you can just do the TLCs until the starting materialĀ“s spot disappears.

Offline DoctorDomo

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Re: TLC - What to do when your product won't move at all
« Reply #6 on: March 14, 2014, 10:00:10 AM »
My product is a mixture of isomers, I need to be able to do a proper TLC analysis when the reaction is complete. I'm characterizing new compounds, so knowing the rate of the reaction is just one detail, I'll need to find a proper mobile phase and record the Rf values of every compound. Some of the isomers might not even separate but you don't know until you test it.

One of my latest compounds happens to be green, I added some precipitate to diethyl ether and watched white flakes float to the bottom and the ether turn yellow. After evaporation I was left with two substances, one green and one white. Interesting observation but why the hell are both of these products sticking while the starting material runs up the plate. From melting point and TLC tests its clear theres no starting materials left, so I'm gonna figure out what the hell these water soluble white flakes are.

Offline DoctorDomo

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Re: TLC - What to do when your product won't move at all
« Reply #7 on: March 14, 2014, 11:54:48 AM »
PROBLEM SOLVED: A 90:10 chloroform/methanol mobile phase moves the product easily. A bit too well, so I'm gonna take it down to 95:5. Good to know, this mobile phase counteracts the DMF issue.

Offline Corribus

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Re: TLC - What to do when your product won't move at all
« Reply #8 on: March 14, 2014, 12:01:35 PM »
FYI, I have always been told not to add more than 5-10% methanol on a silica column, due to partial dissolution of the silica. I haven't every experimentally verified this, but it seems to be a pervasive nugget of wisdom among chromatographers, so it must have some truth to it. When I was doing lots of column purification, I never put more than 5% methanol in an eluent.
What men are poets who can speak of Jupiter if he were like a man, but if he is an immense spinning sphere of methane and ammonia must be silent?  - Richard P. Feynman

Offline DoctorDomo

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Re: TLC - What to do when your product won't move at all
« Reply #9 on: March 14, 2014, 05:14:03 PM »
I heard the same thing, but after testing 10% methanol, I'm gonna have to say its a myth. A higher proportional of methanol probably will dissolve the silica, but this ratio seems fine. I've been using disposable silica backed plates though, I suppose this would be a bigger issue if you reuse your TLC plates. Thats something I've been wondering. I see people throwing out TLC plates after 1 run, I don't see why you can't reuse TLC plates if you're using a non destructive staining method. I've been using my plates 2 or 3 times and it seems to work fine, but I don't clean them, I just make note of where the old spots are, and add new spots in places where they won't collide with the old ones. I've been wondering why don't I just dip the plate into a non polar solvent to clean away the spots, then reuse them. I don't see the phosphor at the bottom of the plate disappear after leaving it sit in an NP solvent, so I don't think loss of that would be an issue.

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