Chemical Forums
Chemistry Forums for Students => Organic Chemistry Forum => Organic Chemistry Forum for Graduate Students and Professionals => Topic started by: repetelww on May 10, 2017, 12:20:28 AM
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Hello everyone,
I have been conducting a series of reactions using alkenes as reaction solvents. Mostly I have been using dodecene and octadecene (mostly because we had it laying around lab). I believe that there might be an issue where trace water is causing problems with my synthesis. This got me to thinking about drying solvents, and I realized that I did not know the best way to dry these long-chain alkenes.
The nanoparticle literature says that maintaining octadecene under high-vacuum at elevated temperature (~100°C) for a few hours dries the solvent sufficiently. But I am wondering how to get them drier. I know that calcium hydride is good at drying hydrocarbons but would I need to worry about hydrogenation during distillation? Any thoughts on the subject would be fantastic.
Cheers
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Calcium hydride should work fine. As long as there's no traces of Pd on your flask or anything, the small amount of H2 produced by reaction with water shouldn't touch your alkenes, nor should the CaH itself.
Activated molecular sieves can also be highly effective: http://pubs.acs.org/doi/full/10.1021/jo101589h?src=recsys
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Molecular sieves 3-4 Å work well. But they must be preheated (activated) at 105-110oC for several hours, before being used. Furthermore, the solvent containing bottle must be dried in the oven, prior being filled and then, be sealed with Teflon tape.
Remember that molecular sieves 3-4 Å absorb water at about 25% of their weight.
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We used to use sodium wire in our solvents after distilling them... the sodium mops up any water pretty well.
(PS - I think this might be an old fashioned technique due to safety concerns with sodium metal... maybe Calcium Hydride is used more commonly now)
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Molecular sieves are the gold standard of drying something well. Inert, easy to use, gets many solvents drier than even distillation over a hydride source or elemental group 1 metal.
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It seems that it is not quite as hopeless as I thought. I was quite concerned about the hydrogenation but it seems like it might not be that big of a concern. Thank you all for taking your time.
I might go for the sieves since it at least appears to go a pretty good job at drying. Maybe I have a bit of an outdated mindset but it seems like you would get drier solvents using metal hydrides or sodium than sieves. Is this notion outdated?
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Maybe I have a bit of an outdated mindset but it seems like you would get drier solvents using metal hydrides or sodium than sieves. Is this notion outdated?
I feel exactly the same - it's totally against my chemical intuition that adsorption onto sieves would be more effective than reaction with a hydride, but it's true!
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It seems paradox that a physical method works better than a chemical one, but it is not a mystery. The thin layer of alkali/alkali earth hydroxide that is formed over the suspended alkali/alkali earth hydride during the reaction, inhibits any further reaction with water, at the end. This can happen even to finest divided suspension of alkali/alkali earth hydride. Contrary, this does not happen to molecular sieves, which absorb water through their surface cavities.
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The thin layer of alkali/alkali earth hydroxide that is formed over the suspended alkali/alkali earth hydride during the reaction, inhibits any further reaction with water, at the end. This can happen even to finest divided suspension of alkali/alkali earth hydride.
Yea - this is why the sodium is extruded directly into the bottle as a very long thin wire to increase it's surface area.
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It makes total sense from a surface area argument, it was just not my first thought, I should be more careful.