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### Topic: Working with volatile low molecular products  (Read 11402 times)

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#### cisdiols

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##### Working with volatile low molecular products
« on: July 04, 2008, 09:16:59 AM »
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?

#### Mitch

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##### Re: Working with volatile low molecular products
« Reply #1 on: July 04, 2008, 02:36:18 PM »
You could still get a Mass Spec of it when it is dissolved in solvent, right?
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#### cisdiols

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##### Re: Working with volatile low molecular products
« Reply #2 on: July 07, 2008, 06:27:34 AM »
i basically submit the crude sample for MS analysis.

#### reflux

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##### Re: Working with volatile low molecular products
« Reply #3 on: July 07, 2008, 08:26:16 AM »
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.

#### P

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##### Re: Working with volatile low molecular products
« Reply #4 on: July 07, 2008, 12:33:40 PM »
Would it seperate though a prep-HPLC??
Tonight I’m going to party like it’s on sale for $19.99! - Apu Nahasapeemapetilon #### Mitch • General Chemist • Administrator • Sr. Member • Posts: 5294 • Mole Snacks: +376/-2 • Gender: • "I bring you peace." -Mr. Burns ##### Re: Working with volatile low molecular products « Reply #5 on: July 07, 2008, 01:21:57 PM » Just curious, but if it is volatile how can you submit a crude? Most Common Suggestions I Make on the Forums. 1. Start by writing a balanced chemical equation. 2. Don't confuse thermodynamic stability with chemical reactivity. 3. Forum Supports LaTex #### cisdiols • Regular Member • Posts: 9 • Mole Snacks: +0/-0 ##### Re: Working with volatile low molecular products « Reply #6 on: July 07, 2008, 07:55:15 PM » I submit my product in ether an solution, so it's not pure but I should see a molecular ion. #### Mitch • General Chemist • Administrator • Sr. Member • Posts: 5294 • Mole Snacks: +376/-2 • Gender: • "I bring you peace." -Mr. Burns ##### Re: Working with volatile low molecular products « Reply #7 on: July 07, 2008, 08:03:01 PM » 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? Most Common Suggestions I Make on the Forums. 1. Start by writing a balanced chemical equation. 2. Don't confuse thermodynamic stability with chemical reactivity. 3. Forum Supports LaTex #### cisdiols • Regular Member • Posts: 9 • Mole Snacks: +0/-0 ##### Re: Working with volatile low molecular products « Reply #8 on: July 09, 2008, 08:16:46 PM » 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 #### russellm72 • Chemist • Full Member • Posts: 133 • Mole Snacks: +10/-7 • Gender: ##### Re: Working with volatile low molecular products « Reply #9 on: July 10, 2008, 07:56:42 AM » 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. #### cisdiols • Regular Member • Posts: 9 • Mole Snacks: +0/-0 ##### Re: Working with volatile low molecular products « Reply #10 on: July 24, 2008, 04:49:24 AM » 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? #### P • Full Member • Posts: 639 • Mole Snacks: +64/-15 • Gender: • I am what I am ##### Re: Working with volatile low molecular products « Reply #11 on: July 24, 2008, 06:42:13 AM » 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? Tonight I’m going to party like it’s on sale for$19.99!

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#### Mitch

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##### Re: Working with volatile low molecular products
« Reply #12 on: July 24, 2008, 01:15:58 PM »
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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#### P

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##### Re: Working with volatile low molecular products
« Reply #13 on: July 25, 2008, 04:14:24 AM »

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?
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