Chemical Forums
Chemistry Forums for Students => Organic Chemistry Forum => Organic Chemistry Forum for Graduate Students and Professionals => Topic started by: zuriel on March 14, 2014, 11:54:44 AM
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Hi folks,
I carried out a column on a crude oil recently and since the product is an aryl acetic acid, I used 1% acetic acid in the eluent to improve separation.
The NMR looks perfect but I need to remove the residual AcOH. I tried leaving it on the Ro-Vac for an hour at 70 degC but to no avail. Same with the high vac. line. Someone else suggested azeotroping it off with Toluene.
Does anyone else have any other practical tricks? I can't wash with a base since my product is also an acid.
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The toluene trick usually does it for me.
If the material really seems to love a volatile you will have to do several cycles of dissolving/evaporating for it to work. I'm assuming you are running the NMR in CDCl3, so after the runs with toluene do a few runs with regular chloroform. I've also heard that dioxane is able to azeotrope acetic acid more efficiently than toluene, so you might give that a shot as well.
Is your product an oil by chance? If so, after doing the above runs, drop a stir bar in your RBF before you leave it on high-vac overnight.
It also might be good to double check that your high-vac is pulling as well as it should.
If these tricks don't work, double check that your CDCl3 isn't contaminated.
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Reading the title might have answered my question. Do the stir bar trick for sure.
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Thanks TheUnassuming I'll give both a shot :)
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Wash it out with aq. NaHCO3? Or dilute NaOH is a bit safer if you worry about co2 evolution...
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I should have also mentioned that using an oversized flask helps with pulling volatiles out of oils.
Since your product is an acid, I wouldn't suggest using sodium bicarb wash or NaOH wash. One thing you can do, though it will cost you a bit of your product is to look at the most recent NMR and calculate how much AcOH is present in your sample. You should then be able to calculate how much is actually present in your flask. Since your product in theory has a lower pKa than acetic acid, you might be able to get the AcOH deprotonated and force it into the water layer during an extraction by using an exact amount of base. I've used this trick on other acid systems before, just not with AcOH.
Of course the other solution that will cost you a bit of your product will be to just re-run the column but without AcOH. Since your product is pure aside from the AcOH it shouldn't matter that it streaks aside from the slight loss of product on the tail end.
Neither of the two options above is close to optimal and should be "last resort" material.
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I should have also mentioned that using an oversized flask helps with pulling volatiles out of oils.
^This and also throw in a stirrer bar - it provides nucleation sites for acetic acid. I'd try the high vac with heat and stirring.
Base wash is definitely a bad idea for the reason already pointed out.
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Good suggestions guys, thanks. I've concentrated the sample from Toluene a few times now and I'm going to put it on the high vac in a while.
I'll try the stir-bar trick too - thanks for that suggestion! :)
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As a carbohydrate chemist, I have tons of oils. I also used tons of acetic acid. Co evaporation with toluene 3 times is usually enough for trace amounts. If you have a lot, I suggest a few cycles with ethanol first and then toluene.
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So just to conclude this, before I put the sample on the high vac I azeotroped from Toluene twice and left the oil on the ro-vac for a while at about 60 degrees. The NMR spectrum then showed no acetic acid - so the Toluene azeotrope worked without any need for the high-vac. Thanks everyone for your suggestions!
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There is one multistep synthesis I do which produces (TMS)2NH in one step and TMS-F in another step. The protocol I use calls for a strip using toluene after an initial rotary evaporation at both steps. Is it possible (likely?) that toluene forms azeotropes with these substances as well? Are there solvents that more commonly form azeotropes? It would be nice to be able to have a rational basis for designing a protocol for the removal of unwanted solvents.
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I'm somewhat aware that multiple evaporations from chloroform are also good at getting rid of any residual ethyl acetate but apart from that I'm not really sure about many others.
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If I need to remove a liquid from anything.. I co evap with toluene. Worked for me for acetic acid, methanol, ethanol, pyridine, and water. Toluene is magical. Azeotrope or just high bp enough that the other stuff comes off too, whateves. It works.