Recrystallization.
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Recrystallization.

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Chem 347 Fall 2010 Prof. Rob Ronald. In order to produce a super-saturated solution the solute is dissolved in the boiling solvent, which is then filtered to remove insoluble impurities, and then allowed to cool. As the temperature drops the solution becomes super-saturated in the so
Recrystallization.

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1. Chem 347 Fall 2010 Prof. Rob Ronald Recrystallization. Recrystallization is a technique that is used for solid, crystalline samples. It is widely applicable, and produces samples of the highest purity ? generally much higher than that obtained by any other means. The procedure is simple: the sample is dissolved in a suitable solvent, filtered to remove insoluble impurities, and then the solution is allowed to become super-saturated in the solute, which forces it out of solution. Because in the crystalline phase the molecules are arranged in an orderly array, the crystalline lattice, impurities are usually expelled into the solution, thus producing solid crystalline material of high purity. The highest purity crystals are usually obtained by recrystallizing twice: once from a polar solvent to preferentially dissolve polar impurities; and once from a non-polar solvent to remove non-polar impurities.

2. Chem 347 Fall 2010 Prof. Rob Ronald In order to produce a super-saturated solution the solute is dissolved in the boiling solvent, which is then filtered to remove insoluble impurities, and then allowed to cool. As the temperature drops the solution becomes super-saturated in the solute, and crystals begin to grow in the solution. Usually the solution is chilled after crystal growth at room temperature is complete to maximize the amount of crystalline product obtained. The crystalline material is removed from the liquid (mother liquor) by filtration. The filter-cake (the crystalline product) is then rinsed with some chilled, clean solvent to removed traces of the mother liquor (the solution from which the crystals grew). Sometimes the mother liquor can be concentrated and a second crop, usually less pure, can be obtained.

3. Chem 347 Fall 2010 Prof. Rob Ronald Another way to super-saturate a solution is to slowly add a second solvent, one in which the crystalline material is insoluble, to the solution. This is much easier to do on a large scale, and is often employed in the pharmaceutical industry in reactors in which hundreds of kilograms of product in thousands of liters of solvent are being used. On a small scale this can sometimes be achieved by placing the solution contained in a small flask in a jar containing a volatile solvent and allowing the concentration of the volatile solvent to slowly increase by diffusion.

4. Chem 347 Fall 2010 Prof. Rob Ronald Important point. When dealing with an unknown substance if you get crystals by any means some of these should be reserved in a small vial to be used as seed crystals. While crystallization is a simple procedure, a lot of ?art? is involved in the process, and obtaining good crystals requires care and patience. The first task in recrystallization is to find a suitable solvent or solvent mixture. This is a trial and error process, and is one of the reasons that you will be asked to determine the solubility of your unknowns in organic solvents (both cold and hot solvents). Sometimes a solvent mixture is better than a pure solvent, but when using solvent mixtures it is always best to have the more powerful solvent (the one that dissolves your material the best) have a lower boiling point than the main solvent so as the solvents evaporate the crystals become less, rather than more soluble.

5. Chem 347 Fall 2010 Prof. Rob Ronald Dichloromethane, also called methylene chloride (CH2Cl2, bp 42 ?C) is often an ideal co-solvent for recrystallizations. The crude crystalline material can be dissolved in a small amount of boiling dichloromethane, And then at the boiling point of the solution the main solvent can be added drop wise until the cloud-point is reached. If seed crystals are available the solution can be seeded by adding a few crystals. Sometimes the bottom of the flask can be scratched with a glass rod, or a metal spatula to induce crystallization. If the product oils out no purification will be achieved. To avoid oiling a few drops of the CH2Cl2 can be added to dissolve the oil and keep the solution clear until crystals begin to form.

6. Chem 347 Fall 2010 Prof. Rob Ronald Some useful solvent mixtures for crystallization

7. Chem 347 Fall 2010 Prof. Rob Ronald CRYSTALLIZATION PROCEDURE. The best glassware for laboratory crystallization is the Erlenmeyer flask (or a suction flask) with a narrow neck. Avoid recrystallizing in round-bottomed flasks Pear shaped recovery flasks with round bottoms are okay for recystallizations For very small scale recrystallizations (10-20mg) a small test tube can be used. Small test tubes are especially good for testing various solvents for their suitability for recrystallization. When testing solvents start with the least polar and most volatile solvents. Do not use beakers or round-bottomed flasks for crystallizations.

8. Chem 347 Fall 2010 Prof. Rob Ronald This is a procedure for recrystallization from mixed solvents (1) Completely dissolve your separated unknown material (0.1-1.0g) in 1-2 parts (1-2mL) of the more powerful solvent in a 25-50mL Erlenmeyer flask. Heat to a gentle boil on a stirring hot plate; use a magnetic stirring bar to avoid bumping. Begin to add the less powerful, higher boiling solvent, at such a rate that the boiling never stops. Continue to add this solvent until you have a volume of 10-15mL If the solution clouds add small amounts of the more powerful solvent to keep the material in solution. If crystals form add more of the powerful solvent to keep the material in solution (2) While you are dissolving your crystals, heat 5-10mL of a clean solvent mixture of the approximate concentration you are using for the crystallization in a clean Erlenmeyer flask. You will use this hot solvent mixture to rinse your Celite filter. Chill what is left over to be used to rinse your filtercake.

9. Chem 347 Fall 2010 Prof. Rob Ronald (3) Also while your compound is dissolving prepare a filter to clarify your crystallization solution Fit a Hirsh or B?chner funnel to a small suction flask (25-50mL) using a rubber filtering adapter. Secure the flask with a clamp Connect it to the house vacuum. Place a paper filter circle in the funnel and open the vacuum just enough to suck the paper tight against the perforated surface; do not open the valve all the way. While the vacuum is maintained pack a layer of Celite 5-10mm deep onto the filter paper using a spoon-shaped spatula. Always keep the vacuum maintained on the Celite layer to keep it stable.

10. Chem 347 Fall 2010 Prof. Rob Ronald Crystallization apparatus

11. Chem 347 Fall 2010 Prof. Rob Ronald (4) If the solution of your crystals is colored due to impurities it can sometimes be decolorized with charcoal When the charcoal is added the solution may boil violently Remove the boiling solution from the hot plate before adding charcoal Let the solution cool briefly before adding charcoal Add the charcoal a small amount at a time. Make sure that the material does not begin to crystallize; if it does add more of the more powerful solvent and very carefully reheat. (5) Quickly pour the hot solution into the funnel and suck it into the suction flask. Maintain only enough vacuum to suck the solution through the Celite layer ? the filtration should be very rapid. Quickly rinse the Celite layer with a small amount of the hot solvent. (The solvent in step 2) If your compound crystallizes in the funnel you will have to start over, so pay attention to your solvent compositions. On a larger scale clarification 250mL ? 2500mL the vacuum can be shut off as soon as about 10% of the solution has been filtered as the temperature differential will keep the flask under vacuum.

12. Chem 347 Fall 2010 Prof. Rob Ronald (6) To crystallize your product Place the largest magnetic stirring bar that will fit in the suction flask and still permit SLOW stirring of the solution. Carefully boil the solution, with stirring to avoid bumping, until the solution just clouds or until crystals begin to form in the solution. The ideal volume for recrystallizations is between 5 and 20 parts If the material oils out then add more of the more powerful solvent, and then try seeding the solution. As soon as crystals start forming in the flask, either spontaneously, or from seeding, remove the flask to a cool stirring plate and stir very slowly until a large mass of crystals have formed. At this point the stirring bar can be removed from the solution and rinsed off with a few drops of the more powerful solvent if necessary. Allow the solution to become completely equilibrated at room temperature. You can chill the solution in an ice-bath, but sometimes this brings down unwanted impurities. Don?t be greedy, your goal is a pure product rather than a quantity of impure material.

13. Chem 347 Fall 2010 Prof. Rob Ronald To collect your crystalline material (7) After crystallization is complete stir the crystals with a clean spatula, and use a spatula to loosen any that adhere to the sides of the flask. The stirring during the crystallizations tends to minimize caking of the crystals and also produces crystals of a very even size. (8) Take a clean, dry suction flask (25-50mL) and fit it with a clean, dry Hirsh or B?chner funnel and filtering adapter. Clamp it and connect it to the house vacuum and open the valve just enough to seal the paper on the perforated plate. Quickly transfer the contents of the crystallizing flask on to the filter using a spoon-shaped spatula to help scrape out the crystals. Add a small amount of the clean chilled solvent and swirl it around to help wash out all the crystals onto the filter paper. Rinse the filtercake with a small amount of the chilled solvent to remove the mother liquors adhering to the crystals.

14. Chem 347 Fall 2010 Prof. Rob Ronald (9) Disconnect the house vacuum and pour the mother liquor solution (filtrate) back into the original crystallizing flask. Replace the funnel on the suction flask and reconnect to the vacuum; place a larger filter circle on top of the funnel to avoid sucking dust onto your crystals. The mother liquor solution can be concentrated to obtain another crop by repeating steps 6-8. (10) Let the crystals dry completely, it is a good idea to let them air dry overnight before they are weighed and the melting point determined. If the melting point has a 2 ?C melting range (or less,) and the TLC indicates that the crystals are homogeneous, transfer the crystals to an appropriately labeled vial. If your melting point range after overnight drying is more than 2 ?C; or if the TLC indicates the presence of impurities the recrystallization should be repeated using a different solvent system. Typically, recrystallized materials that are completely dried and have melting ranges of = 2 ?C will give acceptable combustion analysis results, as well as 300MHz (Proton)/75MHz (carbon) NMR spectra with no visible signals from impurities.

15. Chem 347 Fall 2010 Prof. Rob Ronald Recrystallization in a Test Tube 1- place a 10-20mg sample in a test tube 2-dissolve in the appropriate solvent(s) 3-place a short pipette with a cotton plug in another clean tube 4-filter the solution into the new tube through the cotton filter 5-allow the crystals to grow 6-remove the solvent with a fine tipped pipette 7-remove the pipette containing the solvent 8-dry the crystals in the tube

16. Chem 347 Fall 2010 Prof. Rob Ronald Criteria of purity for organic compounds For solid compounds the melting point ranges should be =2 ?C Similarly liquid samples should have boiling point ranges =2 ?C. For known compounds there should be a literature citation with the literature values cited ? this may mean going to the library and looking up things in original journals! Unfortunately, not all the data of compounds is available on the Internet. The accepted way to report melting point data is the following: mp 79-80 ?C, Lit.5 80-81 ?C (the superscript 5 would refer to the source of this data, preferably a journal reference, but a citation from the Dictionary of Organic Compounds or from Beilstein?s Handbuch would be acceptable ? the Aldrich catalog is not an acceptable source of mp data!). These are the criteria we are going to use in Chem 347.

17. Chem 347 Fall 2010 Prof. Rob Ronald For new compounds the criteria are somewhat more involved. New compounds should be submitted for combustion analysis: this is done by a professional service and the results are returned as the %C, %H, %N, and % of each other elements. Oxygen is not listed as the compounds are burned in an atmosphere of pure oxygen. In journals the results of combustion analysis are reported in the following manner: Anal. Calcd for C12H19NO4: C, 59.73; H, 7.94; N, 5.80. Found: C, 59.37; H, 7.82; N, 5.81. Acceptable combustion analysis results should be within <0.4% of the calculated values; usually C close to 0.3 and H close to 0.1 are generally accepted. High Resolution Mass Spectroscopy (HRMS) can be substituted for combustion analysis for proof of identity, and is reported in the following manner: HRMS (EI) Calcd for C20H21NO: m/z 291.1623. Found: m/z 291.1618. HRMS data has to agree with the calculated value within 5ppm. NMR data at 300MHz (proton)/75MHz (carbon) is also accepted as proof of homogeneity if impurity peaks are =5% of the tallest peak at a signal to noise level adequate to observe this.


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