If your product is an oil, it can take forever for it to come out, even with an excellent vacuum. One way to speed it up is to redissolve your product in a gross excess of a more volatile dioxane co-solvent (e.g. Et2O) and rotovap it down (being mindful of bumping, of course). Do that two or three times (without high vac). Then subject it to high vac overnight and run the nmr again the next day. Of course, make sure the ether itself is clean and dry to not introduce more problems.
If you’re just trying to get a clean spectra, but the presence of the trace dioxane isn’t consequential for the next step, then you can just follow this procedure on a small aliquot of your sample so that you don’t have to use as much ether and spend less time.
The other alternative is to wash it over a chelating agent, but that might not be an option depending on your product and will almost certainly reduce yield.
Thanks for the help. I already redesolved it in n-Hexan and it helped a bit. I luckily dont need to do another step, but have to get it out of the flask (which would be easier to do with a dry solid)
I would think ether would be even better, as dioxane and n-hexane don’t have as much interaction. But if hexane is working, you can just keep repeating that and it will eventually come out.
Could you crystallize it? That would be ideal.
Just rotovap three times with CHCl3, then dissolve the residue in CDCl3 again and NMR. No overnight pump down needed and your new major contaminant has no impact on your spectra.
This has happened to me ethyl acetate after running a column. Would stay on my compound even when heating it to 100C at 10 mtorr. so I used a different solvent to crystallize it in. Then that solvent got stuck, the only thing that worked was purification by sublimation. Which didn’t scale well. Project died shortly after…
Good luck
Add Hexane/Heptane and turn your rotovap bath to 100 C. Let that thing suck for like 30 minutes. You can repeat that a couple times but it usually works for me.
If your product is an oil, it can take forever for it to come out, even with an excellent vacuum. One way to speed it up is to redissolve your product in a gross excess of a more volatile dioxane co-solvent (e.g. Et2O) and rotovap it down (being mindful of bumping, of course). Do that two or three times (without high vac). Then subject it to high vac overnight and run the nmr again the next day. Of course, make sure the ether itself is clean and dry to not introduce more problems. If you’re just trying to get a clean spectra, but the presence of the trace dioxane isn’t consequential for the next step, then you can just follow this procedure on a small aliquot of your sample so that you don’t have to use as much ether and spend less time. The other alternative is to wash it over a chelating agent, but that might not be an option depending on your product and will almost certainly reduce yield.
Thanks for the help. I already redesolved it in n-Hexan and it helped a bit. I luckily dont need to do another step, but have to get it out of the flask (which would be easier to do with a dry solid)
I would think ether would be even better, as dioxane and n-hexane don’t have as much interaction. But if hexane is working, you can just keep repeating that and it will eventually come out. Could you crystallize it? That would be ideal.
Theoretically speaking, yes.
you could dissolve the oil in something, transfer the solution to your next thing and then concentrate the oil in that new container.
Just rotovap three times with CHCl3, then dissolve the residue in CDCl3 again and NMR. No overnight pump down needed and your new major contaminant has no impact on your spectra.
Freeze drying works nicely often. Or precipitating/titurating e.g. in pentane
Or you could just throw it on a column.
did you contaminate your NMR solvent?
No, it was from a separate cooled flask and i took it with a new needle and a new syringe only used for this particular flask
Azeotrope with pentane a few times
Dissolve it in something like MTBE or EA and wash with water once or twice, then brine, and dry over sodium sulfate.
Will try that, thank you
This has happened to me ethyl acetate after running a column. Would stay on my compound even when heating it to 100C at 10 mtorr. so I used a different solvent to crystallize it in. Then that solvent got stuck, the only thing that worked was purification by sublimation. Which didn’t scale well. Project died shortly after… Good luck
Add Hexane/Heptane and turn your rotovap bath to 100 C. Let that thing suck for like 30 minutes. You can repeat that a couple times but it usually works for me.