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Unformatted text preview: revised 8/24/09 Recrystallization of Acetanilide 1 Techniques and topics: Recrystallization, Vacuum Filtration, Determination of Percent Recovery, and Melting point. Reading assignment: Fessenden, R.J., Fessenden, J.S., Feist, P. Organic Laboratory Techniques , 3 rd ed.; Brooks/Cole: Pacific Grove, 2001, pages 23-32, 39-48. Loudon, G.M. Organic Chemistry , 5 th ed.; Roberts & Company Publishers 2008, Section 8.4, p 339-46. Technique: Recrystallization Throughout the year in organic chemistry lab you will synthesize products that are solids at room temperature. These products are often contaminated with impurities such as unreacted starting material or reagents, by-products or solvent. The most convenient method of separation of impurities from a desired crystalline product is recrystallization. Learning the art of recrystallization is a rite of passage for every organic chemist. It takes a lot of practice, care, and patience. However, the rewards of doing a recrystallization correctly are great: beautiful pure crystals and minimum loss of product. Recrystallization involves dissolving a “crude” solid (usually containing a large amount of the desired solid along with some impurities) in a minimum amount of a hot solvent to produce a saturated solution. Recall that solubilities of most substances decrease at lower temperatures. As the solution cools it becomes supersaturated and the crystals precipitate out of solution (Figure 1) 2 . Vacuum filtration separates the pure crystals from the impurities, which are still dissolved in the liquid. Figure 1. An impure solid in: (A) cold solvent in which it has low solubility, (B) solvent that has been warmed up to dissolve all of the solid, and (C) solvent that has cooled down to the original temperature. The crystallized solid is now pure, while the impurities remain in solution. Ideally all of the impurities will remain in the solvent. However, if the solution is cooled too quickly , the impurities may become trapped in the crystal lattice as it forms. To prevent this from happening the solution is allowed to cool slowly by setting the solution in a cool water bath or on the benchtop (best). Choosing the correct solvent is imperative for a successful recrystallization. Try to find a solvent that meets the following requirements: revised 8/24/09 1. Dissolves the solid while hot. 2. Does NOT dissolve the solid while cold. 3. Impurities should remain dissolved in cold solvent. Finding this ideal solvent will require research and experimentation. First record the solubility data on your compound in your notebook (usually found in the CRC Handbook of Chemistry and Physics – online or hardcopy in library). If the handbook states that your compound is soluble in hot ethyl alcohol (alcohol: s.h. ) this implies that it is insoluble in cold ethyl alcohol and this is probably a good solvent for recrystallization. If the solubility data is not available in a handbook – then experiment....
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- Spring '10