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Topic: Removing residual hexanes  (Read 8822 times)

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Offline nox

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Removing residual hexanes
« on: March 11, 2011, 09:39:02 PM »
I ran a column on my product in a hexanes/EtOAc mix. I rotavapped it down a couple of times and put it on the highvac line for an hour. It failed to solidify and I got a gummy paste instead. I know based on my past experience that these types of compounds are nice solids, so I figured something was wrong.

NMR came back and it tells me I have ~20% hexanes still stuck in there. I have absolutely no idea how this could happen, seeing as all the EtOAc is gone. Anyway, anyone have good ideas for removing that annoying residual hexanes?
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Offline MissPhosgene

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Re: Removing residual hexanes
« Reply #1 on: March 11, 2011, 10:32:38 PM »
It's probably not from the flask. Check the NMR solvent and tube. Maybe the isolated product isn't the desired product.
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Offline nox

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Re: Removing residual hexanes
« Reply #2 on: March 12, 2011, 02:14:59 AM »
Actually I just rechecked and it seems that the impurity is not hexanes, but grease.

I've no idea how it got in there, but I have a sneaking suspicion somebody greased up the highvac line a bit too much than is healthy.

Also the guy before me left me just enough material for one reaction, so I'll need to make more substrate. This'll set me back a week, FML.
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Offline Dan

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Re: Removing residual hexanes
« Reply #3 on: March 12, 2011, 05:56:52 AM »
A short column will easily take the grease out in less than a week.
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Offline Doc Oc

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Re: Removing residual hexanes
« Reply #4 on: March 12, 2011, 11:47:45 AM »
Rotovap joints are also notorious for allowing grease into your reaction flask if you slather too much grease on the *Ignore me, I am impatient* trap.

You can try re-dissolving the gum in something like DCM and rotovap it to see if that'll help pull the grease out.  Otherwise, I agree with Dan, you can run a very short silica plug.  Use pure hexane at first and the grease will fly out with that.  Then flush with EtOAc to get your compound back, it should be relatively pure.
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Offline nox

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Re: Removing residual hexanes
« Reply #5 on: March 12, 2011, 01:13:57 PM »
Tried the DCM rotavap idea already, that's what I normally do to pull out any last traces of EtOAc anyway.

Here's the thing: my compound is massively greasy, so it might be a bit tricky not to get the stuff to co-elute with hexanes. But yeah I'll give it a shot.

Also the other problem is I'm finishing up a scope expansion project, and this is the single most important example, so I want to be fair to myself and not cheat myself out of like say 10% yield or something. So depending on how things go, it might be worthwhile to redo the reaction anyway.
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