Chemical Forums
Chemistry Forums for Students => Organic Chemistry Forum => Organic Chemistry Forum for Graduate Students and Professionals => Topic started by: cisdiols on July 04, 2008, 09:16:59 AM
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I have prepared at an 188 MW unit very non polar (acetonide, 5 carbon alkene); I seem to have trouble isolating the product after a Grignard addition.
I have tried distillation, and for some reason the product distils off with n-pentane and ether. I was thinking of a Vigruex column but i only have 30 mgs of product (for total synth) so not viable.
Any ideas?
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You could still get a Mass Spec of it when it is dissolved in solvent, right?
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i basically submit the crude sample for MS analysis.
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Not a ton of experience with volatiles but I have done some of these techniques with success:
-In one case I extract my compound with a higher boiling solvent than ether (say ethyl acetate or even benzene or toluene), which doesn't co-distill with my product... make sure you extract with as little a volume as possible or the yield will be poor.
-Depending on your next step you might be able to use the crude solution after filtering it through a frit or celite. Might be tougher in your case though with some soluble organometallic byproducts affecting the next reaction. It would depend on your next step.
-I've also used relatively "heavy" protecting groups to give the compound some weight and increase the bp. Maybe you can change the acetonide to a bigger derivative.
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Would it seperate though a prep-HPLC??
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Just curious, but if it is volatile how can you submit a crude?
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I submit my product in ether an solution, so it's not pure but I should see a molecular ion.
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I really think the only way you will be able to do this, is somehow extract it into a very different boiling point solvent and then distill, as mentioned above.
Out of intellectual curiosity can you tell us what the structure is?
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I can't for now, because I am filling for a support grant and I am not sure whether I am allowed under the agreement or not.
But a clue is, if you look up Martin G Banwell it's to do with his cis-diols work
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As a carbon-14 chemist I have a lot of experience of working with the isolation of volatile components.
We often use tetradecane to extract the volatile into and simply distill this mixture into a -196 manifold trap just by pumping at room temperature.
You could also use pump your material from a pentane solution through a -40 oC spiral trap into a -196 manifold trap. Your material should stay in the spiral trap and the pentane or ether will zip off at -40 oC (decent vacuum manifold) to leave your pure product.
However good thing about using C-14 is you can easily see where it has gone. No doubt will be a pain in the ass having such a tiny ammount of inactive.
Regards
R.
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kinda worked out a way to concentrate my reaction...i basically vac. it down under pressure in a -10 bath and ether comes off drop wise. However, one problem is that i get water into the reaction. Now I have to think of a way to prevent water getting in.
Any suggestions?
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However, one problem is that i get water into the reaction. Now I have to think of a way to prevent water getting in.
Any suggestions?
How does it 'get in'? Dry all of your solvents before use (distill, add drying agents etc.). Degass everything by bubbling through with something inert like Argon. Flush your vac line with Argon a few times as well so no air gets in (moisture in air). If you do the reaction in something like a shlenk tube - then keep your product in this tightly sealed tube under argon as well for storage. Although you will be doing this already if you are not wanting water or oxygen in your reaction so I'm sure how you are having a problem with water 'getting in' to your reaction. What do you mean?
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However, one problem is that i get water into the reaction. Now I have to think of a way to prevent water getting in.
Any suggestions?
How does it 'get in'? Dry all of your solvents before use (distill, add drying agents etc.). Degass everything by bubbling through with something inert like Argon. Flush your vac line with Argon a few times as well so no air gets in (moisture in air). If you do the reaction in something like a shlenk tube - then keep your product in this tightly sealed tube under argon as well for storage. Although you will be doing this already if you are not wanting water or oxygen in your reaction so I'm sure how you are having a problem with water 'getting in' to your reaction. What do you mean?
I think he meant at the distillation step.
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I think he meant at the distillation step.
Then I'm still confused. If he is doing the distillation under vacuum, then where is this moisture coming from?