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Topic: Removal of water as it is formed as a product  (Read 6505 times)

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Offline Cesium-137

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Removal of water as it is formed as a product
« on: October 17, 2010, 07:54:10 PM »
Does anyone know an inert, efficient chemical method for removing water from reactions as it is formed? Activated molecular sieves? Mg/NaSO4? CaCl2? Do I run the risk of the latter two oxidizing/halogenating products?
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Offline g-bones

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Re: Removal of water as it is formed as a product
« Reply #1 on: October 18, 2010, 03:04:19 AM »
What kind of reaction are you looking to remove water from?  On small scale activated molecular sieves work fine.  On larger scale, Dean-Stark is king (http://en.wikipedia.org/wiki/Dean-Stark_apparatus).  I have also seen people stirring up reactions with things like phosophorous pentoxide, magnesium sulfate or martin sulfurane.  These depend on why you are removing water (dehydration of an alcohol to an olefin, esterification of of an acid to an ester etc)  hope this helps!
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Offline discodermolide

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Re: Removal of water as it is formed as a product
« Reply #2 on: October 18, 2010, 04:04:14 AM »
Quote from: Cesium-137 on October 17, 2010, 07:54:10 PM
Does anyone know an inert, efficient chemical method for removing water from reactions as it is formed? Activated molecular sieves? Mg/NaSO4? CaCl2? Do I run the risk of the latter two oxidizing/halogenating products?
The absolutely best thing is a Dean-Stark apparatus using a solvent such as toluene.
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Offline Cesium-137

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Re: Removal of water as it is formed as a product
« Reply #3 on: October 18, 2010, 09:37:46 PM »
Dean Stark is great, but requires refluxing and high heat to vaporize water/azeotrope. Im attempting a dehydration to form a ketone, but the molecule is sensitive to heat (rxns run at -78C to RT, under Ar, in THF generally). Perhaps at these temperatures a dehydration is impossible?
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Offline MissPhosgene

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Re: Removal of water as it is formed as a product
« Reply #4 on: October 18, 2010, 10:07:14 PM »
Can you be more specific about the reaction you are doing? There are many methods to dehydrate a reaction mixture, but they have certain requirements.
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Offline Cesium-137

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Re: Removal of water as it is formed as a product
« Reply #5 on: October 19, 2010, 04:13:07 AM »
I just realized I said dehydration to form a ketone... What I meant was Dehydration to an alkene. alcohol + methanolic acid (probably HCl) to form a haloalkane, then treated with NaH to eliminate to the alkene. I may be able to substitute the acid for SOCl2...
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Offline Doc Oc

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Re: Removal of water as it is formed as a product
« Reply #6 on: October 19, 2010, 08:18:38 AM »
The more common method I've seen used to generate alkenes is to mesylate the alcohol and use triethylamine or similar base.
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Offline MissPhosgene

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Re: Removal of water as it is formed as a product
« Reply #7 on: October 19, 2010, 01:33:13 PM »
J-Bone's method is very effective. What stereochemistry do you want at the alkene?
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Offline Cesium-137

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Re: Removal of water as it is formed as a product
« Reply #8 on: October 19, 2010, 11:10:10 PM »
Ill give the mesylate a shot! Its part of a ring so stereochem is not an issue. So Z/cis  :P
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