Chemical Forums
Chemistry Forums for Students => Organic Chemistry Forum => Topic started by: taurean on January 24, 2011, 01:47:20 PM
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How to make anhydrous acetone for a moisture sensitive reaction?
Thank you,
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It is difficult to dry acetone, because it is in slow equilibrium with diacetone alcohol and water (aldol reaction of acetone with itself), so many drying agents drive the equilibrium towards diacetone alcohol (4-hydroxy-4-methyl-2-pentanone) by removing the water that forms during the reaction. You would be best served to obtain the purest acetone that you can first, then dry it quickly with sodium sulfate and filter to use.
That is why no one sells "anhydrous" acetone. The best you can get is about 0.5% water, maybe 0.1%, but that is tough to do. Fortunately, in many reactions, a trace of water is acceptable.
Bob
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Distillation from boric anhydride can yield acetone of about 18 ppm according to J. Org. Chem, 43, 1978, 3966.
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Actually it is pretty easy to dry over CaSO4 (commercially sold as Drierite). That is one of the recommended procedures from this book (http://www.amazon.com/Purification-Laboratory-Chemicals-Fifth-Christina/dp/0750675713).
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99,8% acetone is commercially available and this should be fine for most of the reactions. If not you can dry it by keeping it over phosphorous pentoxide overnight followed by distillation. Then store it under inert atmosphere over 3A molecular sieves.
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Actually it is pretty easy to dry over CaSO4 (commercially sold as Drierite). That is one of the recommended procedures from this book (http://www.amazon.com/Purification-Laboratory-Chemicals-Fifth-Christina/dp/0750675713).
This. I've done this distillation before, it's incredibly easy and worked well for my purposes.
There's a recent paper in JOC (2010, 75, 8351) that demonstrates the incredible drying power of simple molecular sieves. They just let the solvents sit for a day or two and the water in some of them got into single digit ppm (even for notoriously hygroscopic solvents like ethanol or THF). It might be worth trying with your acetone.
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Thank you very much for all your replies.
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I have been reading the Burfield/Smithers paper. If one is stirring with boric anhydride, is there any advantage in decanting the acetone away from the reagent before distilling?