Distillation and Preparative GC

Preparation: Reading: Mayo, et al. pp. 71-72, 77 (bottom) and 82-84. Look up and record here the boiling points from the Handbook in the section Physical constants of Organic Compounds: 1,1,1- trichloroethane (under "ethane" subheading "1,1,1- trichloro") ______ and toluene ________.

Students will be assigned separation methods. Methods 1-5 are separations of a mixture of 1,1,1- trichloroethane and toluene by distillation. Method 6 is a separation of heptanal from cyclohexanol by preparative gas chromatography ("Prep GC"). The first separation may not be very successful, so be prepared to repeat the procedure at least once. Students will share information on each separation with the rest of the class. 

The purpose is to a) become familiar with various techniques for the separation of liquid mixtures and b) to compare class data of composition (purity) and yield (efficiency) of the fractions separated to evaluate each method.

Procedure:

Method 1 uses the semimicroscale level as shown on p.82; the apparatus is to be checked out from the stockroom. To the receiving flask, add 10 mL of water and make a mark. Add another 20 mL to establish a 30 mL mark. Empty and rinse the flask with acetone, then allow the acetone to drain from the flask before attaching it to the apparatus. Place 75 mL of the mixture and a boiling chip in the distilling flask (pot). Make sure all connections are made firm as toluene is quite flammable. Have your instructor check the system before you light the burner. Collect three fractions: A, the first 30 mL of distillate; then I, the next 10 mL of distillate; and B until 1 mL remains in the pot. After each fraction is collected, measure its volume more accurately with a graduate cylinder and transfer it to a beaker marked either A, I or B. Analyze each fraction by GC; and share the results with the rest of the class. Tabulate: Fraction, Temp. range, Volume, % 1,1,1-Trichloroethane, %Toluene.

Method 2 is the same as method 1 with the addition that the distillation will be conducted under reduced pressure. Attach the "leave open" connection to a water trap (stockroom) with a heavy walled hose. Test to see that the aspirator (fig 5.21) pulls a vacuum; a well functioning aspirator will reduce the pressure from atmospheric (760 mm) to about 25 mm inside the apparatus. Attach the trap and the aspirator with a heavy walled rubber hose when all is ready. In order to collect the fractions, you will need to remove the flame, then slowly disconnect a hose (do not turn off the aspirator) to allow air to return to the system; change the receiving flask, reattach the hose, and resume the distillation. Analyze the fractions as for Group 1. Tabulate: Fraction, Temp.range, Volume, % 1,1,1- Trichloroethane, %Toluene.

Method 3 is a distillation using a Hickman still according to Fig. 5.10 (but do not clamp the thermometer that sticks directly into the sand bath). Place 4 mL of the mixture in the 5 mL conical vial. Use a magnetic stirring vane. Surround the neck connecting the conical vial and the still head with a "cloak" of aluminum foil that reaches down to the sand bath. The capacity of the receiver is about 1 mL. With a Pasteur pipet calibrated with a 1/2 ml mark, transfer the distillate every 1/2 mL from the receiver into one of three clean containers labeled A (the first 1 1/2 ml), I (the next 1 ml) and B (however much else distills). Note the temperature in the still and the sand bath at every transfer (borrow or check out a second thermometer, but do not leave the thermometer in the sand bath.) Analyze fractions A, I and B once they are collected. Enter data for each ½ ml in the table: volume transferred, temp at still head, temp of sand bath, into which fraction (A, I or B) collected, % 1,1,1-Trichloroethane, %Toluene.

Method 4 uses a Hickman still as above but a wad of steel wool is inserted between the vial and the still head. Tabulate the data as in Group 3.

Method 5 is a fractional semimicroscale distillation with a distillation column like that shown on page 86. The amounts of material and instructions are the same as for Group 1. You may refer to the procedure on page 91 for additional instructions. Analyze the fractions as instructed for Group 1.

Method 6 is a separation of heptanal and cyclohexanol using preparative GC as outlined in Mayo, p. 71. This separation is carried out with teams of students. Each team will use two collection tubes, one of them marked with a bit of tape. Weigh both tube to the nearest mg. Make at least 4 injections of the mixture. After each collection, reweigh each collection tube to follow the accumulation of each fraction in each tube (do not empty the tube between injections.) After the final collection, 1 µL of each fraction will be re-analyzed by GC to determine its purity. To calculate the yield for each fraction, use the initial and final weights of each tube and follow the calculations on p.72 but do not forget include the fact that several injections were made.

All students are to circulate and observe all the separation experiments in progress. Remember that the fractions collected by each method are to be run on the gas chromatograph. Report your data on the table outlined on the chalkboard so that the entire class can evaluate and compare the data. Record all the data in your book once it is complete. Teams sharing chromatograms measure, calculate and report the data on the chalkboard before they make photocopies. Distillation fractions (methods 1-5), once analyzed, can be returned to the 1,1,1-Trichloroethane/Toluene bottle for re-use.

Example tables of separation data:

Typical information in the notebook for students in Group 3 or 4:

These data are summarized for the chalkboard as follows:

Typical data from each team in Group 6 reported on the chalkboard:

In your report compare the separation methods using the data collected.
1. Which method is the best for separation of a mixture?
2. If heptanal and cyclohexanol (the mixture for method 6) were used for methods 1-5, would they be separated with higher purity than with 111-trichloroethane - toluene? Why or why not?
3. Simulate an entry (for fractions A and B, purities and recovery) that would be reported on the chalkboard for a perfect separation.

Reference:

"Mayo et al.": Mayo, D.W., Pike, R.M., Butcher, S.S. and Trumper, P.K. Microscale Techniques for the Organic Laboratory; Wiley: New York, 1991

Rev. January, 2001