Wednesday, February 15, 2012

Process Wednesday: Reslurrying

Reslurrying is a technique that isn't much used in academic organic chemistry. I suspect that it has much more to do with scale than anything else -- why think about a quicker technique than recrystallization when you're not worried about saving much time? Nevertheless, if you can find a solvent that your product is not very soluble in, and which the impurities are soluble, it might be a good reslurrying solvent (as well as a pretty decent recrystallization solvent). Simply toss your crude product in, stir for a while and filter. 

Nevertheless, it's useful. Let's hear what our mentor-by-literature, Neal Anderson (and his book), has to say on the matter:
Reslurrying a solid can sometimes be an attractive alternative to recrystallization. Reslurrying is analogous to trituration. Since attention does not need to be paid to dissolving and recrystallizing, reslurrying is less labor-intensive than recrystallization. Reslurrying can remove impurities adsorbed to the surface of crystals, and after suitable contact with solvent the product is isolated by filtration. 
 Typical of Anderson, he also suggests ways to avoid reslurrying altogether:
TIPS: 
  • Reslurrying can sometimes be avoided by simply changing the work-up or crystallization procedures. If reslurrying removed water-soluble impurities, including an aqueous wash during the work-up may eliminate the need to reslurry the product. Alternatively, the product may be crystallized from solvents that dissolve impurities that would otherwise be removed by reslurrying.  
  •  Recrystallization from biphasic solvent combinations may be effective. 
I'll have to try biphasic recrystallization sometime -- it sounds intriguing. 

4 comments:

  1. The term I've always used for this technique was trituration. If I'm scaling up chemistry, I typically give this a try first. I've had a fair amount of success this way, even on say 100mg scale of final product.

    It also typically means your compound has some pretty poor solubility, in general...

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    1. Poor solubility? Well, that's not a big deal, now is it?

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  2. I have used this technique before, and I suggest using a sonicator whenever possible. I it works much better than just stirring. Although I suppose upon scale up this may not be feasible.

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  3. I did some fairly nasty Friedel-Crafts reaction of substituted phenylacetic chlorides and oxindole, in CS2, with AlCl3. Adding water and hexane into this brew actually produces biphasic mix from which a pure oxindole-benzylketone product precipitates over time. One thing though about precipitation from biphasic mxtures - youwant to give the filter cake a throurough rinse, by alternating the two immiscible solvents, and perhaps re-slurry on Buchner because the particles of the precipitated material may get coated with one of the (mutually immiscible) solvents and refuse to be wetted by the other one

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