Hickman head

Experiment 7A is designed to be performed with semimicroscale glassware using a conventional distillation apparatus. A microscale alternative with a Hickman head is given in Experiment 7B however, the scale is the same in both cases, and the experiment can be performed more easily wifh the semimicroscale glassware. 

This experiment can also be performed using a Hickman head, although it is not as easy to monitor the volume of distillate. To perform a simple distillation, refer to Technique 14, Figure 14.7B. For a fractional distillation, see Technique 15, Figure 15.2. For both distillations, use a 10-mL round-bottom flask and a Hickman head with a side port (Technique 14, Figure 14.4B). Attach a water-cooled condenser on top of the Hickman head. It is helpful to tilt the apparatus slightly (5-10 degrees) in the direction of the side port so that the liquid in the reservoir of the Hickman head will flow toward the side port. 

Follow the procedure given in Experiment 7A, except that it will be necessary to transfer the distillate from the Hickman head to a 10-mL graduated cylinder to collect data for the distillation temperature and total volume of distillate. This must be done frequently so that data can be taken at 0.5-mL intervals, as indicated in the procedure. Because you will not be able to count drops, you should try to distill at a rate of three to four minutes per mL distillate. It is important to remove as much distillate as possible each time you make a transfer. Otherwise, the next sample of distillate will be contaminated by the leftover liquid. 

Choose either Experiment 14A or Experiment 14B, but not both. The semimicroscale procedure requires the use of equipment not found in the typical microscale kit a 20-mL round-bottom flask, a distillation head, and a vacuum takeoff adapter. The purpose of Experiment 14B is to allow an alternative to the use of a Hickman head for the distillation step. 

Be careful when handling methylene chloride. It is a toxic solvent, and you should not breathe it excessively or spill it on yourself. To complete the distillation in a reasonable time, boil the mixture as rapidly as possible without allowing the boiling mixture to rise above the neck of the Hickman head. This requires that you work with careful attention during the distillation procedure. The distillation requires 1-2 hours. 

As the distillation proceeds, use a Pasteur pipette (5M-inch) to transfer the distillate from the reservoir of the Hickman head to a 15-mL screw-cap centrifuge tube. If you are using a Hickman head with a side port, you can easily remove the distillate by opening the side port and withdrawing the liquid. If your Hickman head... 

The temperature during the distillation may be monitored either with a thermometer or a stainless steel temperature probe. If a stainless steel probe is used, it must be used in conjunction with either a digital thermometer or one of the Vernier devices (see Technique 13, Section 13.4 and Technique 14, Figure 14.12). Your instructor will provide instructions about the method that you will use. Insert the thermometer (or probe) so that the bulb is level with or slightly below the cap connecting the Hickman head to the air condenser. Also use a thermometer to monitor the temperature of the heat source. Cover the top of the sand bath (if used) with a square of aluminum foil with a tear from the center of one edge to the middle. 

As distillate condenses in the Hickman head, transfer the liquid from the reservoir to a preweighed 3-mL conical vial. If your Hickman head does not have a side port, it will be necessary to use a 9-inch Pasteur pipette. In the latter case, it is helpful to bend the tip of the pipette slightly by heating it in a flame. The distillate can then be removed without removing the thermometer. Be sure to cap the conical vial used for storage each time after you transfer the distillate. Continue to distill the mixture, and transfer the distillate to the vial until the temperature in the Hickman head increases above 78°C or until the temperature in the Hickman head drops several degrees below 78°C and remains at this lower temperature for 10 minutes or more. You should collect about 0.4 mL of distillate. The distillation should then be interrupted by removing the apparatus from the heat source. 

When the solution is dry, transfer it to a clean, dry, 3-mL vial using a Pasteur pipette and distill it (aluminum block about 140°C) using a clean, dry Hickman still (Technique 14, Figure 14.5). Each time the Hickman head becomes full, transfer the distillate to a preweighed conical vial using a Pasteur pipette. 

When distillation is complete, remove as much distillate as possible from the Hickman head and transfer it to the 3-mL conical vial. Then, using a Pasteur pipette with the tip slightly bent, rinse the sides of the inside wall of the Hickman head with 1.0 mL of saturated sodium chloride. Do this thoroughly so that as much liquid as possible is washed down into the well of the Hickman head. Transfer this liquid to the 3-mL conical vial. 

In this experiment, 1.0 mL of saturated sodium chloride is used to rinse the Hickman head after the initial distillation. Why is saturated sodium chloride, rather than pure water, used for this procedure and the subsequent washing of the organic layer ... 

The Hickman Head. Two types of Hickman head (also called a Hickman "still") are shown in Figure 14.4. One of these variations has a convenient opening, or port, in the side, making removal of liquid that has collected in it easier. In operation, the liquid to be distilled is placed in a flask or vial attached to the bottom joint of the Hickman head and heated. If desired, you can attach a condenser to the top joint. Either a magnetic spin vane or a boiling stone is used to prevent bumping. Some typical assemblies are shown in Figures 14.5 and 14.7. The vapors of the heated liquid rise upward and are cooled and condensed on either the walls of the condenser or, if no condenser is used, on the inside walls of the Hickman head itself. As liquid drains downward, it collects in the circular well at the bottom of the still. 

External Monitoring of Temperature. The simple assembly using the Hickman head shown in Figure 14.5 does not monitor the temperature inside the apparatus. Instead, the temperature is monitored externally with a thermometer placed in an aluminum block. 

Carefully notice the position of the thermometer in Figures 14.7 and 14.8. The bulb of the thermometer must be placed in the stem of the Hickman head, just below... 

The reflux ratio is defined as the ratio of the number of drops of distillate that return to the distillation flask compared to the number of drops of distillate collected. In an efficient column, the reflux ratio should equal or exceed the number of theoretical plates. A high reflux ratio ensures that the column will achieve temperature equilibrium and achieve its maximum efficiency. This ratio is not easy to determine in fact, it is impossible to determine when using a Hickman head, and it should not concern a beginning student. In some cases, the throughput, or rate of takeoff, of a column may be specified. This is expressed as the number of milliliters of distillate that can be collected per unit of time, usually as mL/min. 

At the microscale level, water separation can be achieved using a standard distillation assembly with a water condenser and a Hickman head (Figure 15.15). The side-ported variation of the Hickman head is the most convenient one to use for this purpose, but it is not essential. In this variation, you simply remove all the distillate (both solvent and water) several times during the course of the reaction. Use a Pasteur pipette to remove the distillate, as shown in Technique 14, Figure 14.6 Because both the solvent and water are removed in this procedure, it may be desirable to add more solvent from time to time, adding it through the condenser with a Pasteur pipette. 

Azeotropic distillation is also used in other types of reactions, such as ketal or acetal formation, and in enamine formation. The use of azeotropic distillation is illustrated in the formation of 2-acetylcyclohexanone (Experiment 43) via the enamine intermediate. Toluene is used in the azeotropic distillation of water. The Hickman head is used as a water separator. 

A basic apparatus similar to the one shown in Figure 16.1 may be used for microscale vacuum distillations. As is the case for simple distillation, this apparatus uses the Hickman head as a means to reduce the length of the vapor path. The major difference to be found when comparing this assembly to one for simple distillation (Technique 14, Figure 14.8) is that the opening to the atmosphere has been replaced by a connection to a vacuum source (top right-hand side). The usual sources of vacuum are the aspirator (Technique 8, Section 8.5), a mechanical vacuum pump, or a "house" vacuum line (one piped directly to the laboratory bench). The aspirator is probably the simplest of these sources and the vacuum source most likely to be available. However, if pressures below 10-20 mm Hg are required, a vacuum pump must be used. 

Thermometer Placement. If a fhermometer is used, be sure that the bulh is placed in the stem of fhe Hickman head just below the well. If it is placed higher, it may not he surrounded by a constant stream of vapor from the material being distilled. If fhe fhermometer is not exposed to a continuous stream of vapor, it may not reach temperature equilibrium. As a result, the temperature reading would he incorrect (low). 

Assemble the apparatus as shown in Figure 16.1. It should be held with a clamp attached to the top of the Hickman head and placed above the aluminum block. 

NOTE If you expect the temperature of the distillation to rise above 150°C, omit the threaded cap and 0-ring between the conical vial and Hickman head. They will melt at high temperature. 

Increase the temperature of the heat source until you begin to see distillate collect in the well of the Hickman head. (Observe very carefully liquid may appear almost "magically" without any sign of boiling or any obvious reflux ring.)... 

Partially disassemble the apparatus and remove the fraction with a Pasteur pipette, as shown in Technique 14, Figure 14.6A. (If you have a Hickman head with a side port, you may simply open the side port to remove the fraction. This is shown in Figure 14.6C.)... 

Semimicroscale. The apparatus shown in Technique 14, Figure 14.10, may also be used to perform a steam distillation at the microscale level or slightly above. This apparatus avoids the need to empty the collected distillate during the course of the distillation as is required when a Hickman head is used. 

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