The Art and Science of Recrystallization

Recrystallization as a means of purifying organic compounds is a time honored method. Long before Ultra High Performance Liquid Chromatography and Supercritical Fluid Chromatography there was recrystallization. Of course, recrystallization is directly related to solubilities. The science of recrystallization is that a compound may be appreciably more soluble in a hot solvent than a cold one. “Dissolve hot and filter cold,” is the recrystallizer’s motto. A variation of this may be to dissolve the compound in one solvent and then change the solubility properties of the solution by adding a second solvent that causes the desired compound to precipitate. Recrystallization has been, and is now, a very empirical process. Small molecule recrystallization databases are still in their infancy. An example of online databases are the NIST solubility database and the Open Notebook Science solubility challenge project. Even the venerable CRC Handbook has little information on solubility and recrystallization. Solubility data of organic compounds in any other solvent but water is sketchy. The bottom line is that solubility characteristics of organic compounds are very difficult to predict beyond the “like dissolves like” aphorism. Casual browsing led me to a nice little document on entitled “Recrystallization and Melting Point Determination” by Bunnelle, Meyer, & Glaser out of the University of Missouri  which gives a lot of very practical “how-to” advice on performing a recrystallization. Another aspect of recrystallization is the beauty of the new crystals produced. Crystals evidently can be photographed quite nicely with a microscope. Should the microscopic examination and photography of crystal structures be included in Organic Chemistry laboratory? (image source)

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One Comment

  1. Posted October 27, 2010 at 7:02 am | Permalink

    Brent – we now have some web services for predicting solubility – it would be interesting to see if these could help with selecting a solvent for crystallization in your lab. What kind of compounds are you looking at?
    http://onswebservices.wikispaces.com/solubility


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