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Oiling out of solution

  1. May 5, 2009 #1

    ~christina~

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    I have a question about compounds that oil out of solvents after drying.

    How would I analyze this? Could I make a KBr pellet, or would I just be able to analyze it in UV spec?

    I assume that the solvents used to dissolve the compound, would be used as a blank, right?

    I tried to dry off the solvent used to dissolve the compound and it wouldn't completely dry off, and this is why I think it was oiling out of solution. I was originally planning on trying extraction after drying off the compound. I had no choice but to try to go and re-dissolve the compound in a solvent (CH2Cl2). It looked like it dissolved, but I'm not sure since the pasteur pipette looked as if it had an oily residue after I pipetted the solution over to another container....
     
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  3. May 5, 2009 #2
    You are doing something wrong. DCM should EASILY evaporate off. Are you sure your compound is fully dissolving in DCM? DCM dissolves a loooooooooot of organic molecules, if your stuff isn't dissolving in it, there is is high probability you need to try something more polar.


    Do you have something like DMF in there?
     
  4. May 5, 2009 #3

    ~christina~

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    It is an Unknown solvent. I was trying to evaporate off that solvent (mixture) and then use one solvent (DCM), and then redissolve compound in it.

    TLC plate showed 2 spots: 1 strong 1 really weak spot (using ethyl acetate and methanol)
    -I wanted to find out if it was interaction with solvent that was causing second spot, as my prof. told me it just looked like one compound.

    I just noticed that the pasteur pipette had an oily residue stuck on the inside after pipetting the redissolved compound (using DCM) into another container.

    Why I am asking this question is, someone told me you can't use IR (KBr) if you have a compound that oils after evaporating off solvent.

    I should also mention that I'm really desperate for a solution to this problem as I still have to find out what this compound is after I analyze it so ANY help is appreciated.
     
    Last edited: May 5, 2009
  5. May 6, 2009 #4

    chemisttree

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    Don't worry Christina, you can perform an IR experiment by adding an oil to KBr. Don't add much and mix it together really well. Usually oils or liquids are analyzed between AgCl plates but if you don't have them, you can make a pellet.
    I would add a small amount of the sample to DCM (which may have methanol in it, so be aware) and add the KBr you intend to use to that. Slurry it together and dry it using DRY air or DRY nitrogen, argon, etc... Don't let it air dry in the lab. When you are finished, you will have a very thin coat of your product on the solid KBr powder.

    Alternatively, you could add the oil directly to the KBr and mix it using an agate mortar and pestle.

    Whoever told you that you can't use a KBr pellet for oils is.... limited.
     
  6. May 6, 2009 #5
    Can't you just take a NMR? IR is a waste of time here if you are desperate. NMR will tell you right away. Take a sample, dissolve it in DMSO, and run a simple H NMR. Next, go to your library and download this article from The Journal of Organic Chemistry, which is always one of the most read articles out of all the ACS journals:



    Find out what solvent it is that is giving you your "oily residue".

    If everything doesn't dissolve in DMSO, then you have MAJOR issues.


    Do you have an experimental procedure for the compound you are working on? This could VASTLY help to determine what is going on. Do you have some sort of organic salt? It could be extremely hydroscopic.
     
    Last edited: May 6, 2009
  7. May 7, 2009 #6

    ~christina~

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    I did that before I saw your answer. I also used NaCl disks to analyze the oil in DCM. The mixing of the oil and KBr and subsequent drying in the oven and final pressing of the pellet, created a cloudy pellet. The spectrum was usable though. It looked like the NaCl spectrum, so that's good.

    The problem is that my compound seems to be heat sensitive. I placed the watchglass holding the unknown solvent + compound, to dry off next to the oven because after applying direct heat the first time, it turned yellow. It turned yellow again and I decided to use it anyway. Would this affect the spectrum?
    lol

    Thanks Chemisttree
     
  8. May 7, 2009 #7

    ~christina~

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    We're not allowed to use anything else other than IR, UV, and GC to find our unknown.
    We might have that chemical but I've suceeded in making a KBr pellet with the oil.

    Yes, but only for the extraction procedure. Other than that, there is no procedure for, what to do if your compound oils. It is most likely a salt.
    How would I determine if it was hydroscopic?

    Thanks gravenewworld
     
  9. May 7, 2009 #8

    chemisttree

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    Yes, yellow is a problem. You need to dry it under an inert atmosphere and put Al foil around the sample to keep the light out. Your original sample wasn't yellow and handling it produced a new chromaphore. It is decomposing which should complicate the analysis.

    I would recommend against applying the material in solution to the KBr powder. Use the neat oil instead and avoid the drying step altogether.
     
  10. May 7, 2009 #9

    chemisttree

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    By the way, just how much of your unkown are you given to analyze? I'm thinking that you might have free-based an amine perhaps? Free based amines often produce oils. If you have enough of your material, you might try to form the HCl salt and better purify it.
     
  11. May 7, 2009 #10
    I will never understand why college professors limit you to certain forms of instrumentation when absolutely no chemist in the real world would turn to IR first to try to identify an unknown organic molecule.

    Anyway, what is your extraction procedure? List it step by step with as much detail as possible. It is quite cryptic what you are describing and what the problem really is. How did you get your compound? How did you synthesize it?

    If you have an organic salt, you can easily tell if it is hydroscopic if it is nice solid/powder when it dries and starts to turn into a sticky oil if you leave it out.


    If this is the problem, and you do indeed have a salt, dissolve it in water. There will be a high probability that it will go into water. Next wash the water with a solution of saturated sodium bicarbonate until the pH is slightly basic. Extract thoroughly with DCM. Combine DCM layers and dry them with sodium or mag sulfate. Concentrate down. You should have your free based product. Re-salt it AND KEEP IT DRY.
     
  12. May 7, 2009 #11

    ~christina~

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    There was no synthesis, it was a compound(s) that was purchased and just dissolved by someone into a number of solvents and given to us, as is.

    My Extraction procedure: (note: by today I just got past the first extraction which seperates the acidic compound from the basic and neutral compounds. I observed a cloudyness in the aq layer upon changing pH of aq layer to ~10 so I know I have an acidic compound. Problem is: yellow colour stayed in the organic layer and thus it may be the other compound but is it light/heat sensitive? how to solve this issue.....)

    Using dried (yellow oil) compound re-dissolved in DCM after evaporating off original mix of unknown solvnets = Oranic layer with compound(s)
    1. Distilled water is added to Organic layer
    2. pH of water changed to ~pH 10 causing acidic compound to move to aq layer if it is present
    3. remove aq layer with acidic compound and then add CH2Cl2 to that and then change pH of aq layer to ~pH2 causing acidic compound to move to organic layer
    4. evaporate off organic solvent and obtain pure acidic compound
    5. Take original organic layer with the basic compound and neutral compound if present and add H2O. Change pH of water to ~pH2 which would cause the basic compound to move to the aq layer if present and leave the neutral compound in the organic solvent layer if it is present.
    6. dry off organic solvent with neutral compound and obtain that.
    7. Take aq layer with basic compound. Add CH2Cl2, change pH of aq layer to pH~10 which should cause basic compound if present to go into organic layer. Remove organic layer with basic compound and then dry to obtain it.
    Organic layer and compounds
    No, it dried to a oil directly.

    This oil wasn't dissolvable in water. I basically did something like you described. I tried this today and then just added CH2Cl2 when it was not miscible in water.

    I'm given about a little over 10ml of it. I used about 1/2 of that already including today. I used a bunch to dry off and try to extract today. (just in case I have 2 drugs, which I do think I have now based on the first extraction) I have no idea what I have or the characteristics of how a compound would act if it was what it is. If indeed it does oil, then maybe one is an oil. It would have to be either the basic or neutral compound because the yellow colour stayed in the original organic CH2Cl2 layer.(extraction procedure described above) I have no idea how to obtain it by extraction WITHOUT drying it off.....

    I used it....yellow. Probably not a good idea but it turned yellow even with no heat applied today. (now I'm thinking it's light sensitive) I can't analyze it as a yellow compound? How would that affect IR?
    I could dry it out without light but I'm not sure what factor caused it to turn yellow in the first place. I have a bottle of about 1ml it, and the bottle is clear. It has not turned yellow though. why?
    How would I use it without drying it? I don't know the solvent mixture used. (I sort of have an idea but am not 100% sure because one of the peaks may be a drug in GC) I can't do extraction if I don't know the mixture and I probably wouldn't want to try that if it wasn't using a solvent I knew. This is why I was drying it and using that to extract today.

    Thanks for all your help,
    gravenewworld and chemisttree
     
    Last edited: May 7, 2009
  13. May 8, 2009 #12

    chemisttree

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    OK, so you have three organic solutions. One has the compound that is soluble in pH 10 water, one has the compound that is soluble in pH 2 water and one has a compound that is not soluble in water at all, regardless of pH.

    Is there stuff in all three solutions? (Check by spotting each solution, drying and visualizing with UV - don't develop them in the TLC chamber) Remember that it is virtually impossible to cleanly seperate things and it is likely that all three solutions will have some of each of the compounds you were given. When you perform TLC tests on these 3 solutions, you might see 2 or more spots but one spot will be crazy dark and the other(s) will be lighter. You should see that the acid-soluble (in water, that is) spot(s) have one that is dark and one that is light. The intensity of the two spots will likely be reversed for the base-soluble solution. The last organic soluble solution (if you kept it) might have the same two spots, the intensity of which is determined by both concentration and UV absorbance (extinction coefficient) of the compounds.

    Something going from colorless to yellow might indicate that an oxidation has occurred. If you were given an aldehyde, for example, your IR analysis will show either some or a lot of a carboxylic acid. If a rearrangement has occurred the IR spectrum of the rearranged product would likely be different than that of the colorless material. It is impossible to tell if enough of the material has changed to affect your analysis, but be aware that it is possible (although unlikely). I can't imagine a worse compound to give a student than one that falls apart when handled a little rough. 'Rough' here means you didn't neutralize the extracted solution from either the acid or base extraction and some of that acid or base came along for the ride and now contaminates your solutions. pH 10 and pH 2 can do further chemistry on your product(s) especially when they are dispersed onto a surface and exposed to oxygen, heat, or light.
     
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