Basic Organic Laboratory Methods and Techniques
I Distillation for Purification of Liquids: Since organic reactions inevitably give a variety of products in addition to any unreacted starting materials and the solvent, it is necessary to purify the final mixture after reaction. When the compounds involved are liquids, this is done by distillation. Every compound has a characteristic boiling point. As you will recall from general chemistry, the weaker the attractive forces between molecules, the higher the vapor pressure and the lower the boiling point. When the compounds have boiling points that differ by 75oC or more, they are usually separable by simple distillation, Figure 1.
Figure 1: Simple Distillation
Assemble the apparatus by first clamping the appropriate sized round bottomed flask to the ring stand. Add the mixture to be distilled, a boiling chip and then the take-off head. Carefully insert the thermometer in the thermometer adapter. Place the thermometer assembly in the adapter. Adjust the position of the thermometer so that the bulb of the thermometer is just below the take-off arm, as shown in Figure 1. Then place the condenser on the adapter. Hold it in place with a clip or rubber band. Attach hoses to the condenser; the lower hose should go to the water tap, the upper one to the drain. The water should flow up the condenser. Place the heating mantle on the distillation flask and hold it in place with an iron ring. There should be enough space so
that the mantle can be easily and rapidly removed when the distillation is over. The heating mantle is connected to a voltage regulator, never directly to the house current. The voltage regulator is used to adjust the heating rate. The function of the boiling chip is to prevent bumping of the heated liquids. Bumping occurs when the liquid becomes super-heated and then suddenly releases a large bubble of vapor, forcing liquid into the condenser. This occurs fairly often with organic liquids, so the boiling chip is added to ensure smooth boiling. When the compounds have boiling points closer together, it is necessary to use fractional distillation, Figure 2, in order to get a good separation.
Figure 2: Fractional Distillation
As is obvious from a comparison of Figures 1 and 2, the only difference between simple and fractional distillation is the addition of a column between the distilling flask and the adapter. This column may be empty but it is usually filled with some sort of packing: glass beads, glass helices, stainless steel sponge are common packings. These supply a great deal of surface for heat exchange to take place. The descending liquid covering the packing is heated by the ascending vapor and the more volatile component of the mixture in the liquid is revaporized. The less volatile component of the vapor recondenses and joins the descending liquid. As the vapor ascends the column, it becomes richer in the more volatile compound while the descending liquid
becomes richer in the less volatile compound. If the column is long enough and/or efficient enough, a fairly good separation of material is possible. The apparatus is assembled as before, placing the packed column between the distillation flask and the adapter. Be sure water circulates only in the condenser, not the fractionating column. Place a clip between the adapter and the fractionating column as well as between the adapter and the condenser. If necessary to keep the column straight, place a clamp on the fractionating column or the condenser, but it shouldn't be necessary. The heating mantle is connected to the voltage regulator as before, NOT directly to the house current.
Crystallization for Purification of Solids:
In a typical laboratory preparation producing a crystalline solid, the solid separates from the reaction mixture usually contaminated with small amounts of impurities from the reaction mixture. Purification is accomplished by dissolving the impure solid in a suitable solvent and then allowing it to recrystallize. The impurities are left behind in the solvent - or at least, you hope so. The general technique involves the following: 1. Select a suitable solvent. A good solvent will dissolve a moderate quantity of the solid in the hot solvent but only a small quantity in the cold solvent. 2. Dissolve the impure solid in the minimum amount of the hot solvent (or solvent mixture.) 3. If necessary, hot gravity filtration (see section VI, figure 5), through a fluted filter paper is used to remove insoluble impurities. Before carrying out a hot gravity filtration, add about 10% more solvent, heat to boiling and then filter. If the solution contains colored impurities, a small spatula full of charcoal is stirred into the hot solution before the hot gravity filtration. 4. The solution is allowed to cool. The dissolved material has a decreased solubility at lower temperatures and precipitates when the solution is cooled. If the solution cools slowly, the developing crystals tend to select the correct molecules from the solution to fit the crystal lattice. Rapid chilling will often trap impurities in the quickly forming crystal lattice. 5. Separate the crystals from the supernatant solution by suction filtration (see section VI, figure 6). 6. Wash the crystals with small amounts of cold solvent to remove the mother liquor. 7. Dry the crystals to remove the last traces of solvent. IV Heating Under Reflux:
In order to get an organic reaction to proceed at an appreciable rate, it is usually necessary to heat it. Direct heating of reactants usually leads to decomposition rather than reaction and a product that resembles asphalt rather than a clear colorless liquid or pretty white crystals. So the reactants are usually dissolved in a solvent, even sometimes when the reactants are liquids. The reaction temperature is then the boiling point of the solvent. However, since prolonged heating will simply boil off the solvent, there must be some means of returning the solvent vapor to the reaction flask. This is done by attaching a water-cooled condenser to the reaction flask. Then as the solvent boils, the vapor strikes the cooled walls of the condenser, is condensed and returns to the reaction flask. This process is called heating under reflux and is shown in Figure 3.
Figure 3: Reflux
The apparatus is assembled by clamping the reaction flask to the ring stand, leaving enough room to place a heating mantle, as in the distillations discussed previously. Add the reactants, a boiling chip or magnetic stir bar and the condenser. If a magnetic stir bar is used, don not use an iron ring to hold the heating mantle. The heating mantle is held in place by the electric stirrer. The water flows in the bottom of the condenser and out the top. Make sure the water is flowing in the condenser before turning on the heating mantle. As before, the heating mantle is connected to a voltage regulator. The voltage regulator is set to provide a gentle, constant boiling. V Separation of Non-miscible Liquids:
Since a great many organic liquids are virtually insoluble in water (the lower molecular weight alcohols and acids are exceptions), one often encounters a situation where there are two liquid phases: an organic phase and an aqueous phase. If inorganic compounds have been used, water is useful to remove them, since inorganic reagents are frequently ionic and ionic compounds are more soluble in water than the less polar organic liquids. If an acid catalyst is used, a basic aqueous wash is used to remove the acid after the reaction has finished. However, it is necessary to separate the two phases afterwards. This is accomplished with a separatory funnel, Figure 4.
Figure 4: Separatory Funnel
When washing an organic solution with water or an aqueous solution, both solutions are placed in the separatory funnel, which should be no more than 2/3 to 3/4 full. Stopper the flask and hold the stopper in place with one hand, holding the stopcock with the other. Your lab instructor will demonstrate the proper technique. Holding the stopper firmly in place, tip the funnel so that the stopcock end is pointing up and vent the funnel by opening the stopcock. Make sure the funnel isn't pointed at anyone. Close the stopcock. Shake the funnel 2 or 3 times, invert and vent. Repeat this procedure several times. Replace the funnel on the ring, remove the stopper and allow the layers to separate. The bottom layer is drawn off through the stopcock. When the top layer is removed, it is poured out the top. Be sure the stopper is not on the funnel when the liquid drains through the stopcock. It is a good idea to keep all layers in labeled flasks until you are sure you have your product.
VI Filtration Techniques For rapid filtration of impurities from a concentrated solution, use hot gravity filtration. This involves filtering through a fluted filter paper in a funnel set in a beaker set on a hot plate. The beaker should contain a small amount of solvent (Figure 5). For isolation of crystals from a cold solution, use suction filtration. In suction filtration, a heavy-walled flask with a side-arm is clamped to a ringstand. It is fitted with a rubber adapter in the mouth of the flask. A Buchner funnel is placed in the adapter. A Buchner funnel is a porcelain funnel with a perforated disk sealed inside. A piece of precut filter paper that just fits inside and covers all the holes in the disk is placed in the funnel. The side arm is attached with rubber tubing to a vacuum source and the vacuum turned on to full. The filter paper is moistened with the same solvent as the one containing the crystals to be filtered. The solution with the crystals is poured rapidly into the funnel. The crystals are washed with small amounts of cold solvent (Figure 6).
Figure 5: Hot gravity filtration
Figure 6: Suction filtration
VII. Melting Point Determination:
The boiling point of a liquid is used both to characterize it and as a measure of its purity. The melting point of a solid is used in the same way. For a liquid, the boiling point can be measured as the purification process, distillation, takes place; for a solid, the purification process, crystallization, and the melting point determination are separate procedures. A pure crystalline organic compound has a sharp melting point; i.e., it melts over a small range in temperature, 0.5' - 1.0 oC. Soluble impurities lower the melting point and increase the range over which the solid melts. The melting point is determined by heating a small amount of the solid slowly in a special apparatus equipped with a heating element and a magnifying eyepiece for observing the crystals. Two temperatures are recorded: the first is the temperature where the first drop of liquid is observed in the crystals. The second is the temperature when the last crystals melt. When a melting point range is reported, these two temperatures are the values given. It is important to heat slowly to maintain equilibrium. If you heat too quickly, the melting point will be low, with a large range, not because the material is impure but because the heating occurs faster than the system can record it. A heating rate of about 2 oC/min in the range of the melting point should be used.
Figure 7 : Melting Point Apparatus