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Unformatted text preview: Recrystallization I 15 Chem 355 Jasperse RECRYSTALLIZATION Background: Impurities often contaminate organic compounds, whether they have been synthesized in the laboratory or isolated from natural sources. Recrystallization is the most important method for removing impurities from solid organic compounds. It is suitable for both small scale (<0.5 g) and large scale (>100g) work. The basic recrystallization plan is to dissolve an impure solid in a hot solvent, then cool the solution so that the desired molecules recrystallize while the impurities remain in solution . Subsequent filtration separates the solid crystal from the liquid solvent. The dependence of solubility on temperature is key . Solubility of sample in solvent will always be higher at high temperature, but will decrease at low temperature. A solvent that can fully dissolve a solid while hot may thus become saturated as the temperature is reduced, resulting in crystal formation. Soluble impurities stay in solution because they are not concentrated enough to saturate the solution. However, even when the solvent is cold, at least some (if not all) of the desired compound will remain dissolved and will be lost during filtration. The choice of solvent for a recrystallization is crucial. For a successful recrystallization the dissolving power of the solvent must be mediocre, neither too good nor too bad . Why? If the solvent is too good , then even when the solvent is cold the sample will remain dissolved and you wont be able to harvest any crystals. If the solvent is too bad , then even when the solvent is hot it still wont be able to dissolve the sample, and the impurities wont be freed from the original sample. An effective solvent must be mediocre , good enough to dissolve the sample at high temperature (so that the impurities are freed), but weak enough so that at least some of your sample crystallizes out after cooling (so that you get at least some yield harvested.) Necessary sources of mass loss : The yield for a recrystallization can never be 100%. Why not? Because while the chilled solvent is saturated and should release some crystals, at least some of your desired material will remain dissolved in the cold solvent and will be lost when the crystals and solvent are separated. The primary necessary source of mass loss is to the solvent. Obviously additional mass will be lost to physical handling, and some of the lost mass is simply the impurities that you wanted to lose. (But normally the mass of impurities is only a few percent or less.) Unnecessary sources of mass loss : While losing mass to the solvent is inevitable, unnecessary losses to solvent are common and should be avoided. Some unnecessary losses to solvent result from the following: Using too much solvent. The more solvent that you use, the more sample will remain in the solvent even after cooling....
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This document was uploaded on 02/26/2011.
- Spring '11