Experiment I. The Recrystallization And Identification Of An Unknown. Reading assignment: Techniques in Organic Chemistry 2nd ed pages 38-42, 47, 100-116. 3rd ed pages 38-42, 49-50, 183-197. Topics and Techniques i) macroscale and microscale recrystallization ii) decolorization of hot solutions iii) hot gravity filtrations and vacuum filtrations iv) use of a Craig tube and a centrifuge v) inducing crystal formation Introduction Recrystallization is the process of purification that involves dissolution of a solid in a hot solvent, filtration of the heated solution or mixture, crystal formation and the isolation of the crystalline compound. The technique requires you to have an understanding of the factors that dictate solubility and crystal formation of a solid as well as an adept ability to manipulate hot solutions. You are strongly encouraged to understand the principals and perfect this technique. Part A: Macroscale recrystallization. Heat 150 mL of de-ionized water (DI H2O) on a hot plate for use throughout this experiment. Weigh 0.5g of your impure unknown in a 50 mL Erlenmeyer flask. Add hot water in small amounts, roughly 5 mL at a time. Continue adding hot H2O, swirling and heating the contents of the flask gently on a hot plate until essentially all of the solid has been dissolved. The total amount of H2O used to dissolve your unknown solid should not be more than 25 mL. Keep in mind the following two statements: i) you should try to dissolve the compound in as little hot solvent as possible; ii) there are likely to be insoluble impurities present in your unknown that will not dissolve in hot solvent and as well can have a negative influence on the recrystallization process. The insoluble impurities are readily removed by hot gravity filtration. Soluble color impurities, on the other hand, are often removed by adding charcoal and subsequent hot gravity filtration. Cool the solution obtained from above slightly and add about six charcoal pellets and boiling chips. Never add the charcoal to a boiling solution, as this is likely to cause the solution to boil-over. A good rule to keep in mind is: Never add a solid substance to a solution at or near the boiling point. Heat the solution to boiling and filter the solution through a pre-warmed filter funnel fitted with a 11 cm Whatman #4 filter paper. The purpose of using a pre-warmed funnel during the hot gravity filtration is to avoid formation of crystals that often occur from a warming of the glassware and subsequent cooling of the solution. If a significant amount of crystals are present on the filter, dissolve the crystals by washing them with a small amount of hot water and collect the resultant filtrate. The filtrate from the hot gravity filtration should be clear and colorless. If fine grey or black particulates are observed in the filtrate, then charcoal has somehow passed into the filtrate and a second hot gravity filtration must be performed. After the filtrate has cooled, crystals should form. Crystal formation is dependent on the concentration of the solute and temperature of the solvent. Crystal formation is also dependent on what solvent(s) is used. A solute or compound that is extremely easy to solubilize in a solvent is less likely to form crystals. Occasionally, crystal growth may be induced by the addition of a seed crystal or scratching the surface of the glass flask with a spatula. This rather unusual manipulation creates a surface for nucleation and the formation of crystals. You may also try to cool the solution by placing the flask on an ice-bath (0 °C), or reheating the filtrate to boiling to evaporate and concentrate the2 filtrate. Try to avoid a spontaneous or rapid formation crystals which usually arises when from cooling the filtrate too rapidly. Time permitting, allow the solution to cool to room temperature with out an ice bath so that the crystals will form more slowly. A general rule to keep in mind is that the more slowly the crystals grow the more pure they are likely to be. After the solution has been cooled in an ice bath for about 10 minutes, collect the crystals by suction filtration using a Buchner funnel fitted with appropriately Whatman #50 filter paper. Wash the crystals with a few portions of cold water and remove excess water by pressing the pure crystals down by using a spatula. Transfer the crystalline unknown to dry piece of filter paper and place in a dessicator for over night at least. Weigh the crystals, determine the melting point, perform two mixed melting points and identify your unknown. Report the total weight obtained for the pure compound, the percent recovery, and the identity of your unknown. Place your product in a small plastic bag that is properly labeled and turn over the contents to your TA. Part B: Microscale recrystallization (Craig tube procedure). The Craig tube is frequently used in microscale recrystallizations, particularly when the amount of solid is in the range of 10-100 mg. Figure 1. Craig tube is used for microscale isolation of crystals Weigh about 75 mg of your unknown compound in a 10x75 mm test tube. Add to the test tube in a dropwise fashion, about 0.5-1.5 mL of hot water until essentially all of the solid is dissolved or a solution is obtained. Keep in mind, that there could be insolubles that simply will not dissolve in the hot solvent. Note: dissolution of your solid unknown will be greatly improved if the rod of a microspatula is rotated in the test tube during the addition of hot water and heating. Once your solid unknown is essentially dissolved, allow the contents to cool ten degrees or so then add to the warmed solution a charcoal pellet, a boiling chip and then reheat the mixture to a gentle boil. Perform a hot gravity filtration using a Hirsch funnel with a cotton plug, and collect the solution in a 2 mL Craig tube (see video). This step remove the boiling chip, insoluble impurities and the charcoal from the hot mixture. The filtrate should be clear and colorless. Avoid the accidental transfer or passage of residual charcoal into the Craig tube. If some charcoal is present in the filtrate, it will be necessary to perform