Wednesday, November 17, 2010

Process Wednesday: aqueous extractions with water-miscible solvents

Another handy tip from Anderson's Practical Process Research and Development:
To rapidly develop extractions of reactions run in water-soluble solvents, e.g. THF and acetonitrile, first try washing reaction aliquots on a test-tube scale. Note any phase separation, the presence of any emulsions that take a significant amount of time to separate, and any losses to the aqueous phase. If problems arise, inorganic salts may be added to the mixture in order to "salt out" the organic product into the organic phase. Effective phase splits may occur if there is less of the organic solvent present, i.e. if the reaction were run more concentrated. A cosolvent such as ethyl acetate or toluene may be added to improve the partition coefficients of the product in the organic and aqueous phases, thus elimination emulsions. As a last resort, replace the reaction solvent with a higher-boiling, more water-immiscible solvent, and reexamine extractions.
This is a tip that has a little bit of relevance to the small-scale aqueous extraction that is being annoying and forming an emulsion as well. Remember, as the chemist, you control the vertical and the horizontal.

1 comment:

  1. Great to see the reference to Neal Anderson's book. I usually stay away from those water-miscibles if I can, but I have worked on processes that required it. What about the emulsions that you can't help avoid if you use certain reagents. Like the dreaded tin chloride reduction. You get a milky white emulsion from all the tin salts.

    In my last job, it wasn't a chemical but biological system using enzymes. I was still wet behind the ears in this area. Someone on my team advised me to just throw in some ethyl acetate. After I stirred for 20 minutes, the subsequent emulsion lasted hours if not days. I soon realized a centrifuge was my best friend.

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