4 - experiment #2 - synthesis and recrystallization of...

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experiment #2 - synthesis and recrystallization of dibenzalacetone Introduction and Purpose Recrystallization is, by far, the most common way in which solid organic compounds are purified. Since experimental chemistry can be a challenge under the best of circumstances, most practitioners try to operate under those circumstances. This means, among other things, working with pure compounds. Consequently, solid products of reactions are routinely recrystallized before subsequent use. The main purpose of this experiment is to give you some understanding of the process of purification of solids by recrystallization and to give you some practical experience in carrying out this process. Theory The problem at hand is that you have a solid substance that contains relatively small, but significant, amounts of impurities, and you want to dispose of the impurities but keep as much of the compound as you can. [Recrystallization is less likely to be successful if there are really large amounts of impurity. See if you can figure out why, after you have read this.] The solution to this problem is to find a solvent that has the following properties: your compound is soluble in the hot solvent, your compound is insoluble, or only slightly soluble, when the solvent is cold, and the impurities are either completely soluble when the solvent is cold, or completely insoluble when the solvent is hot. This set of properties, combined with some work on your part, will enable you to clean up impure solids by separating out the impurities and throwing them away. The idea, then, is to place the material to be recrystallized in an Erlenmeyer flask, add a small amount of solvent that has the properties described above, and warm the mixture, bringing it to a boil. If all of the compound does not dissolve after a few minutes of boiling, you slowly add more solvent, keeping the mixture at a gentle boil, until all of the compound has dissolved. If any insoluble impurities – rust, dandruff, floor sweepings, etc . – are observed, use gravity filtration (conical funnel and folded filter paper, not Buchner funnel with vacuum and flat filter paper) to remove them and discard. After you have done this, or if there were no insoluble impurities, allow the filtrate to cool and collect the crystals of product that form, on a Buchner funnel. The soluble impurities are now in the filtrate, which is discarded. The solvent adhering to the crystals in the Buchner funnel is allowed to evaporate. The product is now pure. The Art of Recrystallization O.K., in practice, things get a little more complicated than suggested above. But we're definitely not talking about brain surgery here. Even a Hartwick student can perform a successful recrystallization (well. .., some of them).
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This note was uploaded on 02/02/2012 for the course CHEM 211 taught by Professor Santos,j during the Spring '11 term at Abu Dhabi University.

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4 - experiment #2 - synthesis and recrystallization of...

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