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Organic Chemistry Laboratory
Recrystallization and Drying Agents

Recrystallization
The principle of recrystalllization is to dissolve the compound to be purified in a warm solvent while the impurities either do not dissolve, or the impurities remain dissolved after the pure compound crystallizes from the solution when the solution is cooled back to room temperature or put in an ice bath. A proper solvent must be selected.  The properties of a good recystallization solvent is one that the compound dissolves easily when the solvent is warm, but that is less soluble at room temperature or when cooled in an ice bath.  The solvent should relatively low boiling  (typically below 100C).  For most organic compounds, water is not a good recrystallization solvent.
Recrystallization requires significant patience so be prepared to be patient. 

Procedure
Identify an appropriate solvent to do the recrystallization.  Use the data generated from your solubility experiments to select a proper recrystallization solvent for your unknown.  Do not use water or any of the other acidic or basic aqueous solutions for recrystallization.  For approximately 2 grams of a solid, use approximately 30-40 ml of recrystallization solvent.  Dispense the solvent into a 100ml  beaker and gently warm the solvent in a water bath to just below its boiling point.  Add the unknown to the warm solvent and swirl or stir the solution. (If the compound does not completely dissolve, try adding ~5-10 ml more of solvent)
                               Figure 1:   Gravity Filtration   
If you observe particles in the solution still, use a gravity filter remove them (hot gravity filtration).  Keep the solution warm and set up a clean Erlenmeyer flask (125ml) with a glass funnel and piece of filter paper (Figure 1 at left)  and quickly pour the warm solution through the filter.  Set the the Erlenmeyer flask down and let is sit undisturbed for 15-30 minutes, or until crystals or a precipitate forms. If no precipitate or crytallization results after 30 minutes, immerse the beakier in an ice bath and allow it to cool down, undisturbed for 15-20 minutes.

Crystals (i.e., the pure compound) should form in the flask and leave the soluble impurities behind in the solution.  The pure compound should be isolated using vaccum filtration (Figure 2 at right).  (DO NOT ATTEMPT TO FILTER IF THERE DOES NOT APPER TO BE ABOUT 1.5-1.8 g of crystals formed).  Collect the pure compound from the filter paper and allow the material to dry for at least 24 hours before attempting to take a melting point.

Figure 2: Vacuum Filtration

Drying Organic Solutions
The process of isolation of pure organic compounds often requires removing trace amounts of water.  For example, in aqueous extractions trace amounts of water will be transferred into the organic layer because of the partial solubility of water in the organic phase. This water must be removed before the organic compound can be properly characterized.  Two general methods of drying organic solutions are commonly used; saturated aqueous sodium chloride and solid drying agents.

Saturated Aqueous Sodium Chloride
The bulk of the water can often be removed by shaking or "washing" the organic layer with saturated aqueous sodium chloride. The salt water (brine) works to pull the water from the organic layer to the water layer. The concentrated salt solution tendency is to become more dilute which promotes the migration of water from the organic layer into the water layer. 

Procedure
Place the organic solution in a separatory funnel. The organic solvent might be methylene chloride, diethyl ether, hexanes, etc., as long as it is not, of course, water.  Add an amount of saturated aqueous sodium chloride, less than or equal to the amount of organic solution you have.Stopper the funnel and shake as in an extraction.  Allow the layers to separate. The rules as to which layer is on top are the same as for extraction.  Drain off the lower layer. In this case, this is the organic layer and the layer you want to save. Dispose of the aqueous layer in the aqueous waste carboy. (In some cases, for instance if you are using diethyl ether, the upper layer will be the desired organic layer.)

Solid Drying Agents
Final traces of water are removed by treating the organic solution with a drying agent. A drying agent is an inorganic salt which readily takes up water to become hydrated. Several such salts are used routinely.
drying agent capacity speed applications
calcium chloride, CaCl2 high medium used for hydrocarbons
calcium sulfate, CaSO4 (Drierite) low fast generally useful
magnesium sulfate, MgSO4 high fast not used for very acid-sensitive compounds
potassium carbonate, K2CO3 medium medium not for acidic compounds
sodium sulfate, Na2SO4 high slow generally useful
Of the five drying agents in the above table, magnesium sulfate is a fine powder and the rest are of a larger particle size. Calcium chloride, magnesium sulfate, and sodium sulfate are the three most commonly used agents. Usually, a wash with saturated sodium chloride solution to remove the bulk of the water is done before treating with a solid drying agent.

Procedure
Add a small amount of the solid drying agent directly to the organic solution. Swirl the solution.  Observe the drying agent; if it is all clumped together, add more. Click on the picture link at the right to see how a solution with drying agent looks when it is clumped and when it is free-flowing.  Continue swirling and observing the solution for 5-15 minutes, adding more drying agent only until a fresh addition no longer forms clumps

Each drying agent will have a slightly different appearance when “clumped” and practice will make you better at judging whether or not the inorganic salt is wet or dry. There is no set “rule” as to how much drying agent needs to be added. The amount required depends on the amount of water in the solvent solution which you are drying, and this amount varies from experiment to experiment. Use as much as it takes to dry the solution. In most cases, drying is as complete as it will get in 20 minutes. When drying is complete, you need to remove the dried organic solution from the drying agent. Usually gravity filtration is used to remove the drying agent or decanting off the dried solution.