Recrystallization handout

Recrystallization handout - IDENTIFICATION OF A...

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IDENTIFICATION OF A RECRYSTALLIZED SOLID BY MELTING POINT AND INFRARED SPECTRUM I. RECRYSTALLIZATION A. Introduction One of the most useful and necessary techniques to be mastered by an organic chemist is recrystallization. Many organic compounds are solids and the technique of choice in most instances for purification is recrystallization. Recrystallization is a procedure in which a crystalline material is dissolved in a solvent and then made into a crystalline solid again by precipitation. Thus, the solid is said to be recrystallized. Since the total concentration of impurities in a specific sample of a solid is usually significantly lower than the concentration of the substance being purified, on cooling, the low concentrations of impurities will remain in solution, where as the highly concentrated product precipitates. In recrystallization, a solvent is generally chosen such that the impurities are more soluble than the substance being purified. Crystals grown from such a solution will be of much greater purity than the original solid if the impurities are present only to the extent of a few percent. For a larger concentration of impurities, two or more recrystallizations may be necessary. B. Outline of the technique 1. Select an appropriate solvent. 2. Dissolve the solid to be purified in the solvent at or near the boiling point. 3. Decolorize the hot solution with activated charcoal if the original solid was colored. 4. Filter the hot solution to remove insoluble impurities and activated charcoal (if used). 5. Crystallize the solid from solution by cooling. 6. Separate the crystals from the supernatant solution. 7. Wash the crystals to remove adhering solution. 8. Dry the crystals. C. Selecting a solvent Crystal formation of a solute from a solvent is a selective process. When a solid crystallizes under appropriately controlled conditions, an almost perfectly pure crystalline material can result. This is because only molecules of the right shape fit into the crystal lattice. In recrystallization, dissolving the impure solid in a suitable hot solvent destroys the impure crystal lattice; crystallization from the cold solvent selectively produces a new crystal lattice with fewer impurities. Choice of solvent is the most crucial step in the recrystallization procedure. The solute should have a maximum solubility in the hot solvent and a minimum solubility in the cold solvent. Though choice of solvent is a trial and error process, there is some relationship between the solvent’s molecular structure and the solubility of the solute--i.e., like dissolves like. For instance, hydrocarbons and the alkyl halides (nonpolar compounds) are recrystallized from nonpolar solvents, while carboxylic acids (and other polar compounds) are often recrystallized from water. A useful recrystallization solvent will meet the following criteria:
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This note was uploaded on 02/26/2012 for the course CHEMISTRY 231 taught by Professor Uc during the Spring '12 term at Kentucky.

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Recrystallization handout - IDENTIFICATION OF A...

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