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Topic: TLC - Can you recommend some different solvent systems  (Read 10320 times)

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Offline DoctorDomo

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TLC - Can you recommend some different solvent systems
« on: April 24, 2014, 04:05:16 PM »
I've been honing my TLC skills lately, and my latest reaction produces two isomers which are a little less polar than the starting material. The starting material dissolves nicely in ethyl acetate, chloroform, DCM but won't dissolve in acetone (and anything more polar) or hexane (and anything less polar).

Heres the results I've got so far:

100% ethyl acetate: all compounds run together with the solvent front, so I take it these compounds are extremely soluble in ethyl acetate

50:50 ethyl acetate:hexane: - I got one spot in the product lane, with a retention factor of about 0.9, it runs slightly ahead of the spot in the starting material lane which has an Rf of about 0.8

20:80 EtAc/n-hexane: Now two spots are visible in the product lane, but are very close together, and are followed very closely by the starting material spot. Bad separation and high Rf values of 0.7-0.5

10:90 EtAc/n-hexane: Slightly better separation, and Rf values between 0.6-0.3 but still pretty bad separation.

100% hexane: Starting material sticks to the baseline. Products separate nicely, but extremely slowly

So to try get better separation, I experimented with other solvents.

100% petroleum ether (40-60): More or less the same result as 100% hexane

100% DCM - The plate looked pretty much like 10:90 ethyl acetate:hexane. The same Rf values, the same separation.

50:50 DCM/hexane - Got a strange result with this one. All spots moved slow with an Rf of about 0.3, and no separation at all, even the starting material was indistinguishable from the products.

100% toluene - The whole plate (up to the solvent front at least) turned blue under UV, I mistakenly assumed that the mobile phase was contaminated with a UV-active compound (in reality what likely happened is I didn't let the plate dry, so it was the toluene that turned the plate blue) and threw the plate out after a quick glance but I could see a smeared spot in the product lane which moved really slowly, about 2 cm in 24 hours (I left the plate in overnight).

Thats what I've tested so far. My experience with mobile phases is extremely limited so I'd appreciate any tips, suggestions or if possible, info on what I can learn from the results I obtained. Based on something I read, I want to try using the 10:90 ethyl acetate:hexane system, but switching the non polar to another non polar. So 10:90 EtAc/toluene or EtAc/pet. ether.

Offline Archer

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Re: TLC - Can you recommend some different solvent systems
« Reply #1 on: April 24, 2014, 04:37:37 PM »
.

100% toluene - The whole plate (up to the solvent front at least) turned blue under UV, I mistakenly assumed that the mobile phase was contaminated with a UV-active compound (in reality what likely happened is I didn't let the plate dry, so it was the toluene that turned the plate blue) and threw the plate out after a quick glance but I could see a smeared spot in the product lane which moved really slowly, about 2 cm in 24 hours (I left the plate in overnight).

Once the solvent front has reached the top of the plate your just passively diffusing your compounds into smears, the solvent is no longer moving so neither is your analyte.

Could you provide some information on your compounds? Structures would be very helpful but even just functional groups which are present. 

What stationary phase are you using?
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Offline Dan

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Re: TLC - Can you recommend some different solvent systems
« Reply #2 on: April 24, 2014, 05:51:54 PM »
My usual weapons are:

Weak solvents: Alkanes, toluene, DCM, chloroform (the latter for more polar substrates, unlikely to be applicable in this case as weak solvents)

Strong solvents: Acetone, EtOAc, diethyl ether, chloroform, DCM, toluene

Mix a strong solvent with a weak solvent and tweak the ratios to put your spot in the desired region.

For your particular system I would suggest trying:

10:90 acetone/hexane
10:90 diethyl ether/hexane
5:95 acetone/toluene
5:95 EtOAc/toluene
5:95 diethyl ether/toluene

Adjust the ratios as necessary.

Note that EtAc = butanone (methyl ethyl ketone, or MEK) - I assume you meant EtOAc?
My research: Google Scholar and Researchgate

Offline DoctorDomo

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Re: TLC - Can you recommend some different solvent systems
« Reply #3 on: April 24, 2014, 09:38:56 PM »
.

100% toluene - The whole plate (up to the solvent front at least) turned blue under UV, I mistakenly assumed that the mobile phase was contaminated with a UV-active compound (in reality what likely happened is I didn't let the plate dry, so it was the toluene that turned the plate blue) and threw the plate out after a quick glance but I could see a smeared spot in the product lane which moved really slowly, about 2 cm in 24 hours (I left the plate in overnight).

Once the solvent front has reached the top of the plate your just passively diffusing your compounds into smears, the solvent is no longer moving so neither is your analyte.

Could you provide some information on your compounds? Structures would be very helpful but even just functional groups which are present. 

What stationary phase are you using?

Ah right, thanks for informing me about what happens when the solvent front runs off the plate. The compounds are electron deficient benzene rings, with 1 or more long chain (6 carbon) alkoxy groups attached (via a nucleophilic substitution). The alkoxy groups are what make it less polar than the starting material. The stationary phase is silica but I added a drop of triethylamine to the mobile phases to prevent the silica protonating the slow moving or stationary compounds.

My usual weapons are:

Weak solvents: Alkanes, toluene, DCM, chloroform (the latter for more polar substrates, unlikely to be applicable in this case as weak solvents)

Strong solvents: Acetone, EtOAc, diethyl ether, chloroform, DCM, toluene

Mix a strong solvent with a weak solvent and tweak the ratios to put your spot in the desired region.

For your particular system I would suggest trying:

10:90 acetone/hexane
10:90 diethyl ether/hexane
5:95 acetone/toluene
5:95 EtOAc/toluene
5:95 diethyl ether/toluene

Adjust the ratios as necessary.

Note that EtAc = butanone (methyl ethyl ketone, or MEK) - I assume you meant EtOAc?

Thanks a lot! Yeah I meant EtOAc, sorry. I'm not familiar with the concept of weak and strong solvents. Besides acetone/hexane (although in different ratio), I haven't tried any of those solvent systems. I'll give some of those a try. I'll give acetone/hexane a try first, maybe I need a more polar solvent with the hexane to give decent separations considering ethyl acetate fares out better than DCM.

Offline Archer

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Re: TLC - Can you recommend some different solvent systems
« Reply #4 on: April 24, 2014, 10:41:25 PM »
Triethyamine is not always your best friend in TLC, it is helpful but sometimes modifiers migrate up the plate more slowly than the solvent front (depending on the solvent system you are using.

I occasionally find with silica it can help (not always) to saturate the plate with the modifier in the vapour phase in the tank. don't put the plate in the solvent, just sit it on anything that sits it above the bottom of the tank. let it bind to the active sites as a gas then remove it to dry a little just while you do the next bit, then add the rest of the solvent and equilibriate the tank as normal so the vapour is saturated. Spot your compound as normal and run the TLC. This gives a better model for a column later where you run a couple of volumes of the solvent before you start your separation. If this sounds like too much work, you do a couple of blank runs with the plate in your solvent mixture with the modifier added, letting it dry between each run this can help stop your compound adsorbing onto the silica too strongly.

You can also condition the plates first by drying in a hot oven.

Or try using alumina plates. .

People often forget about acetonitrile as a solvent but it is useful at times, not just for HPLC but not cheap either. 

You can also consider 2D TLC or gradient elution.

I was able to separate two disastereoisomers (THP protection products of a chiral alcohol) using pseudo gradient elution by running the plate first, let it dry and swap to another ratio or add a third solvent and run the same plate again. This is a crude model of gradient elution on column but it can give you the separation needed.

In your case, if i followed it right, start with hexane and then when it is complete run the same plate in 20:80 EA : Hex and see if your separation improves.

When it comes to the column you can just keep recycling the hexane for a few volumes then slowly increase the EA ( I always preferred ether / hexane myself as it is easier to remove from the product. ) there is a lot of tweaking with TLC which makes it a pain. Oh how I love short path distillation when it's appropriate to use.

“ I love him. He's hops. He's barley. He's protein. He's a meal. ”

Denis Leary.

Offline Dan

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Re: TLC - Can you recommend some different solvent systems
« Reply #5 on: April 26, 2014, 03:26:30 PM »
I'm not familiar with the concept of weak and strong solvents.

Weak and strong just mean the less polar and more polar component of the eluent, respectively.

I've had very good results separating isomeric aromatic compounds using toluene based systems before - they often give much better separation than alkane based systems (but not always).
My research: Google Scholar and Researchgate

Offline 408

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Re: TLC - Can you recommend some different solvent systems
« Reply #6 on: April 27, 2014, 03:52:25 AM »
I have had insane luck with benzene.  Whenever I have a tricky separation, I add benzene in 10-30%, and the majority of the time that solves the problem after ratio adjustment. 

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