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Author: Subject: Good solvent system for separating aromatic amines and/or oximes on alumina TLC plates?
Melgar
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[*] posted on 6-10-2017 at 17:54
Good solvent system for separating aromatic amines and/or oximes on alumina TLC plates?


I have DCM, chloroform, hexane (mixed isomers), n-heptane, diethyl ether, toluene, acetone, methanol, ethanol, isopropanol, n-butanol, ethyl acetate, acetonitrile, MEK, DMF, triethylamine, GAA, and probably a few others that it wouldn't make much sense to use. Ethyl acetate + hexane rarely if ever moves amines from their starting position, and a mixture of DCM and methanol just made a mess.

I've had a hard time finding references for mobile phase combinations that work for alumina, since they always seem to be for silica. Though, I have found references indicating that alumina is supposed to work better for amines due to its slightly basic characteristics, but they don't specify what solvents to use.




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[*] posted on 6-10-2017 at 18:24


Ethyl acetate and methanol 2:1 with a few drops of ammonia solution worked well for me to separate tryptamine from byproducts on silica gel. 0.5 to 1% triethylamine would have been better probably, but I didn't have any and ammonia worked fine. Maybe with basic alumina the basification of mobile phase wouldn't be necessary.
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[*] posted on 6-10-2017 at 18:38


Melgar,

I have chromatographed fairly strongly basic amines on Woelm Activity 3 alumina using ether-benzene mixtures. I did this years ago when benzene was considered innocuous. The separations were clean and sharp.

You could use silica gel with butanol-acetic acid -water (40:10:50 - use upper layer). Ethyl acetate is sometimes included in the mix. Benzene-methanol (95:5), ethyl acetate-methanol-formic acid (80:10:10) and dibutyl ether-ethyl acetate-acetic acid (50:50:5) have been used for aniline type bases on silica gel. The benzene-methanol solvent tends to work best with simpler aromatic amines.

Oximes of benzaldehyde derivatives separate on silica gel using benzene-ethyl acetate (83:17). The syn and anti isomers can be separated.

This information comes from Stahl's book "Thin-Layer Chromatography"

AvB
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Melgar
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[*] posted on 6-10-2017 at 19:14


Quote: Originally posted by AvBaeyer  
Melgar,

I have chromatographed fairly strongly basic amines on Woelm Activity 3 alumina using ether-benzene mixtures. I did this years ago when benzene was considered innocuous. The separations were clean and sharp.

I don't have any silica TLC plates, but I do have ether and toluene. Any reason that mixture might not work? In my experience, mixtures that are indicated to work well for silica are kind of a crapshoot with alumina, and the ratios need to be adjusted. For example, hexane and ethyl acetate worked with alumina, but I needed to use ratios of more like 60:40 than the 90:10 that I seemed to keep finding in literature.




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AvBaeyer
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[*] posted on 7-10-2017 at 19:10


There are several variables which have large effects on the utility of alumina plates.

1. Are your plates acid, basic or neutral alumina? This is always stated as part of the labeling. The chemical state of the alumina has a huge effect on separation and the type of compounds that can be separated and what solvents can be used. In general, most plates are neutral.

2. Exposure to moisture has a much larger effect on alumina than silica plates. If your plates are old or have been stored by just lying around, you may want to try drying them gently in an oven, say at 100 C or so. Chances are your plates were originally Activity 2 or 3 since water would have been used in the manufacture of the coating. If your plates have absorbed significant moisture, you will need to use more polar solvent mixtures for polar compounds and less polar solvents for non-polar compounds.

Alumina hydration is denoted by the "Brock" numbers. Activity 1 is anhydrous, Activity 2 has a certain amount of water added to Activity 1 material and so on. I do not recall the exact amounts of water at each activity level. However, Activity 3 is usually the most often used.


3. Are your plates glass or plastic? Though some claim no difference between the two, I have generally observed otherwise. Plastic seems to require less powerful solvent mixtures ( ie less polar). This may be due to the very thin coatings on plastic and the tendency to overload the plate when spotting - just speculating here.

*Toluene and ether mixtures should work.
* There is no reason to expect that solvent systems for silica can be transferred to alumina. They are two very different chemical absorbents.

Hope this helps,

AvB

[Edited on 8-10-2017 by AvBaeyer]
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[*] posted on 8-10-2017 at 03:17


Just to clarify, are you taking about aromatic amines (i.e. aniline "derivatives") or aliphatic amines with a pendant aromatic ring?

I've columned aromatic amines (well, heteroaromatic amines) on silica with EtOAc-Hexane mixtures. Sometimes as little as 20% EtOAc (depending on other groups), but usually around 50%

If you're talking about an aliphatic amine, then I'd suggest something like EtOAc, or up to 20% MeOH-DCM or EtOAc (all on silica). You may need 0.1% triethylamine added if you've got something stubborn. If it still won't move then you're really looking at C18 (reverse phase) chromatography.

In my experience (granted, not as extensive as AvBaeyer), alumina only comes out to play if there is a stability issue with silica...
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