Distillation and Preparative GC

Preparation: Reading: Mayo, et al. pp. 71-72 77 (bottom) 82-84. Look up and record 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 are divided into groups. Groups 1-5 will separate a previously analyzed mixture of 1,1,1- trichloroethane b.p. ? and toluene, b.p. ? by various distillation methods. Group 6 will separate heptanal from cyclohexanol by preparative gas chromatography ("Prep GC"). Your instructor will assign the separation method you will carry out. The first distillation is not likely to be very successful, so be prepared to repeat the procedure at least once.

The purpose is to become familiar with various techniques for the separation of liquid mixtures and to compare the performance of the methods by examining the yield (efficiency) and composition (purity) of the fractions separated.

Procedure:

Group 1 will distill the mixture using 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 frist 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.

Group 2 will duplicate the procedure for Group 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.

Members of Group 3 will individually set up a Hickman still according to Fig. 5.10. (use 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.

Member of Group 4 will individually set up a Hickman still as above but insert a wad of steel wool between the vial and the still head. Tabulate the data as in Group 3.

Each pair of students in Group 5 will and set up and run 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.

Teams of students in Group 6 will separate a mixture of heptanal and cyclohexanol as shown on page 71 using preparative GC. 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 purtiy. 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 group are to be run on the gas chromatograph. Report your data on the table outlined on the chalkboard so that the entire classs can evaluate and compare the data. Record all the data in your book once it is complete. Distillation fractions (Groups 1-5), once analyzed, can be returned to the 1,1,1-Trichloroethane/Toluene bottle for re-use.

Example tables of separation data:

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

These data are reported on the chalkboard as follows:

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

In your report compare the separation methods using the data collected. Which method is the best for separation of a mixture? Is the mixture for Group 6 more, or less, difficult to separate?  What would have been the ideal percents in your table?  Compare the total amount of liquid obtained from the separation to the initial amount of mixture.

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. June, 1999