Distillation and Preparative GC

Students will be assigned separation methods.  Methods 1-3 are separations of a mixture of 1,1,1-trichloroethane and toluene by distillation.  Method 4 is a separation of heptanal from cyclohexanol by preparative gas chromatograph (Prep GC).  Students will share information on each separation with the rest of the class.

Required reading:  Mayo,  pp. 57-63, 72-83. 

Purpose:  

•    To  become familiar with various techniques for the separation of liquid mixtures  
•    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. 73; 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, %ethyl acetate, %Toluene.

Method 2 is a distillation using a Hickman still according to Fig. 5.5, p 74 (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, % ethyl acetate %Toluene.

Method 3 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 4 is a separation of heptanal and cyclohexanol using preparative GC as outlined in Mayo, p. 64. 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 (20ml each) 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.64 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 1-3), once analyzed, can be returned to the ethyl acetate/Toluene bottle for re-use.

Example tables of separation data:

Typical information in the notebook for method 2 or 3:

These data are summarized for the chalkboard as follows:

Typical data for method 1 reported on the chalkboard:

In your report compare the separation methods using the data collected.

1.  Clearly show % recoveries, %compositions and retention times calculations for method 1, 2 or 3 and GC.  Show the measurements on the graphs. Report your data in a table format as shown in the procedure.
2.  From the assigned reading, which separation method is the best method?  Does your class data support the prediction?  Explain in detail how you conclude this finding using your class data.

3.  What are the two types of GC detectors mentioned in the reading?  Could both types of detectors be used for the preparative GC and analytical GC methods?  List the pros and cons of each of the detectors for each of the GC methods.
4. If heptanal and cyclohexanol (the mixture for method 4) were used for methods 1-3, would they be separated with higher purity than with ethyl acetate - toluene? Why or why not?
5. Simulate an entry (for fractions A and B, purities and recovery) that would be reported on the chalkboard for a perfect separation.

6.  List the possible sources of errors for GC and distillation techniques.  Indicate how the errors would affect the results.

7.  Which distillation method is the best for separation of a mixture?  Explain your reasoning.

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. October, 2006