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Professor Kathleen V Kilway and Robert Clevenger Department of Chemistry University of Missouri Kansas City 2006 1 Crystallization J R Mohrig C N Hammond and P F Schatz Chapters 3 14 and 15 21 28 lab notebook and yield 183 197 recrystallization 174 183 melting points Vocabulary Solute Solvent Miscible Filtration Trituration Solubility Melting point Filtrate Aliquot Reflux A compound which is dissolved in another compound and present in a lesser amount The compound which is present in the excess It is capable of dissolving a solute but not reacting with it Two or more compounds which dissolve in one another to form a homogenous solution are said to be miscible A process by which one material is separated from another based on differences in particle size Repeated washing of a solid with a solvent The degree of which a compound dissolves The temperature range at which a compound melts from the observation of the initial melting of the first to the last crystal It is reported as a range The material that passes through the filter and remains in the receiving flask It is sometimes referred to as the mother liquor A small amount of a liquid which is either added to or taken from a solution Heating a solvent compound to its boiling point It is evident from the condensation of that liquid on the walls of the container Crystallization 1 This procedure and material has been adapted from the microscale procedure described in Macroscale and Microscale Organic Chemistry Experiments by Kenneth L Williamson and Experiments in Organic Chemistry by Louis F Fieser In a nutshell Recrystallization is a technique used to purify a solid We use it to purify starting materials as well as the products of a reaction Once a solid is isolated at the end of a reaction then we dissolve it with the smallest amount of solvent liquid possible In theory the impurities will stay in solution or be filtered while the pure material will slowly crystallize out leaving the impurities behind Background What is crystallization According to the American Heritage Dictionary of the English Language American Heritage Publishing Co and Houghton Mifflin Company Boston 1973 pg 319 a crystal is a three dimensional atomic ionic or molecular structure consisting of periodically repeated identically constituted congruent unit cells and crystallize is to cause to form crystals or assume a crystalline form There are examples of crystalline solids which we run across in our everyday lives Salt NaCl ionic solid and sugar are typical examples of crystalline solids As organic chemists we use recrystallization as a technique to purify either a desired product or a starting material If you start with a compound that is pure you have a greater chance for your reaction to succeed In crystallization it is necessary to consider the properties of the solvent The reason why is because organic chemists usually say that like dissolves like Therefore chemists look to select solvents for recrystallization that have structures similar to the solute In organic lab you will be using solvents like water toluene and hexanes Water contains networks of hydrogen bonds It easily dissolves ionic solids such as NaCl and polar compounds such as alcohols R OH Toluene is an aromatic solvent which contains a benzene ring so other aromatic compounds should also be soluble in it Hexane is an aliphatic hydrocarbon so it would be expected to dissolve hydrophobic compounds 2 Professor Kathleen V Kilway and Robert Clevenger Department of Chemistry University of Missouri Kansas City 2006 In general there are seven possible steps for the recrystallization of a compound In some cases you would use them all In order they are Choose a solvent Dissolve the solute your compound Decolorize to remove any colored impurities Filter to remove any insoluble impurities Crystallize usually by cooling the solution Wash the product rinse the desired crystals Dry to remove the solvent The first step finding a suitable solvent is sometimes the most difficult one Why Well you want to choose a solvent that will dissolve the solute at temperatures higher than room temperature If the material goes into your solvent at room temperature then you will never be able to recover your compound That is not a good thing if you are being graded on recovery or yield Ideally the solubility of the solute should be at its maximum in the hot solvent and at its minimum in the cold solvent A good solvent will have that property and will dissolve 18 g 100 mL of solvent at its boiling point but 0 5 g 100 mL at ice water temperature 0 oC In some cases you will not be able to find a great solvent for recrystallization so you will have to use a two solvent or mixed solvent pair This technique will be discussed in your procedure but essentially you need to use two solvents that are miscible in one another i e they dissolve in one another Additionally the compound to be recrystallized must be soluble in the first solvent but insoluble in the second solvent The solid to be crystallized is first dissolved in a a mimimal amount of hot soluble solvent then cold insoluble solvent os added dropwise until the solution becomes cloudy One such mixed solvent system is ethanol ethyl alcohol and water The general rule is like dissolves like Once you have chosen a solvent or solvent system then you dissolve your solid in your chosen solvent system Never add more than 10 mL solvent per gram of solute Also whenever you heat a liquid you must use either a boiling chip or boiling stick Otherwise the solution may superheat and bump or boil out all at once The third step is important if you know that your compound should be colorless white but isn t Unfortunately most organic compounds are colorless If necessary we use a decolorizing agent which is usually charcoal Colored compounds impurities are large aromatic compounds which then stick to the charcoal The fourth step is to filter away the impurities which may include the charcoal It is necessary that the solution is hot for this step so that the crystallizing solid does not hinder the filtration Crystallization 3 Then the solution is cooled to room temperature and sometimes even colder with the aid of an ice water bath The best crystals come from letting the solution cool slowly so that the impurities are not trapped again in the solid Once the crystallization step is complete no more crystals are formed the crystals are filtered and washed with a small amount of the solvent It


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UMKC CHEM 321L - Crystallization

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