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Author Topic: TLC - Can you recommend some different solvent systems  (Read 95 times)

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DoctorDomo

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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.
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Archer

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Re: TLC - Can you recommend some different solvent systems
« Reply #1 on: Today at 10:37:37 AM »

.

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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Dan

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Re: TLC - Can you recommend some different solvent systems
« Reply #2 on: Today at 11:51:54 AM »

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?
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DoctorDomo

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Re: TLC - Can you recommend some different solvent systems
« Reply #3 on: Today at 03:38:56 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.
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