Chemistry 30A3 Fall 2011 Labs Dr. Troendle 23

Chemistry 30A3 Fall 2011 Labs Dr. Troendle 23 -...

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Chemical Biology 3OA3, September 2009 Page 23 Recrystallization . [NOTE: review the process of recrystallization here: it is NOT a question of dissolving the compound in solvent then letting the solvent evaporate: this simply returns all the impurities to the sample!] Transfer about one third of the crude caffeine to a clean 50 mL beaker, add 5 mL of toluene, and heat on a steam or water bath (HOOD!) to dissolve the caffeine. Remove the beaker from the heat source, add 10 mL of petroleum ether (boiling range 60-90 o ), COVER with a watch glass, and allow the caffeine to crystallize. If the solution is appreciably coloured, add a small amount of charcoal ("norit")-sufficient to give a dark suspension. This should remove the coloured impurities, and after filtration, colourless crystals should be obtained. Collect the product by vacuum filtration (Hirsch funnel), wash it with 1 mL of petroleum ether, allow it to air dry, and determine its melting point.
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