I have a question about solvent evaporation. If I had an organic layer and an aqueous layer in an unknown mixture of liquids and wanted to obtain the solid of a compound(s) dissolved in the layers, is it reasonable to assume that I can run the compound in the IR spectrometer (with KBr pellet) with no problems from interfering solvent residue spectra?

Question

If I will have solvent spectrum interfering with the compound spectrum, how would I solve this problem?

I was planning to determine how many compounds are in the unknown mixture by running a TLC plate, but if I don't know the solvents that the unknown is dissolved in specifically, does it matter, since the solvent that the compound is dissolved in evaporates off after you apply it to the plate anyway? or does it matter during the TLC plate development?

I really need help on this,
Thanks

Answer 1

I assume you mean to make a KBr pellet from the solid obtained from one of the layers. Most organic solvents will evaporate and I don't think there will be interference to worry about. If it is water, the water may not evaporate so the IR spectrum could be that of the compound plus the trace of water. Water, however, has a fairly simple spectrum. You can look up the spectrum of water and get an idea of how much it might interfere. The O-H stretching frequencies are in the 2.75 (near the C-H stretching) micron region and the O-H bending frequencies are in the 6 micron region (near the C=O and anhydride region). However, if you are running this on a FT-IR, it's quick enough to do and it doesn't waste a lot of time if it doesn't pan out. Often, however, when running a MIXTURE of compounds on the IR, you can identify functional groups but I don't think it is reasonable to assume you can identify any specific compounds.
Regarding the TLC, I would suggest you evaporate the solvent to get the solid and dissolve the solid in a known solvent and go from there.

Answer 2

I assume you mean to make a KBr pellet from the solid obtained from one of the layers. Most organic solvents will evaporate and I don't think there will be interference to worry about.
Yes, I mean to add the dry solid to KBr to form a pellet. The chance of interference was what was really worrying me so that's good to know.

However, if you are running this on a FT-IR, it's quick enough to do and it doesn't waste a lot of time if it doesn't pan out.
We're using FT-IR so I can see the background spectrum and compare it with what I get to see if there's a problem.

Often, however, when running a MIXTURE of compounds on the IR, you can identify functional groups but I don't think it is reasonable to assume you can identify any specific compounds.
True, and this is why I have to perform an extraction if I see that there is more than one unknown compound present in the mixture (TLC) but I would still have to evaporate off solvent in the end.
Regarding the TLC, I would suggest you evaporate the solvent to get the solid and dissolve the solid in a known solvent and go from there.
Hm...but if I can use GC to find out the solvent(s) that the unknown compound(s) is dissolved in, would it be fine to use it directly? (I'm apprehensive to drying off the solvent if I don't know if there's more than one compound dissolved in the solvent(s))

If it is still better to evaporate off the solvent and just use my own solvent that I choose, how would I choose a solvent to run the TLC plate?

Can you use the same solvent that you use to re-dissolve the compound(s)in, to develop the TLC plate being used, or does it have to be a different solvent?

Thanks alot,
Dr.Bob

Answer 3

Using a GC to determine the solvent is good; however, whether to use that solvent or not is not the question. That solvent may not be suitable for separating the compounds with TLC. As for which solvent to choose, since you know nothing to little about the compounds, it's a guessing game. Also, one solvent may show that you have one compound, another solvent may show you have two or more when using TLC. Again, it's a guessing game. With unknowns in which you know almost nothing (just like crime scenes) you must search around as if looking for a needle in a haystack until you find some clues to go on. Each bit of evidence tells you something and that leads to more work. Each piece begins to fit together like a puzzle and soon you know what the compound(s)is(are).

Answer 4

Hm..very true. I now recall that different solvents will give you different results, based on their interactions with the compound in question. Just like a crime scene.
My plan of attack is to evaporate and just try out different solvents.

Thanks for all your help,
Dr.Bob :D