Day 1: Distillation and Gas Chromatography: WORK in PAIRS. Split evenly the limonene and carvone fractions for the derivatization step
Part A. Distillation
Before setting up the glassware as shown in Fig. 2 have your teaching assistant demonstrate how to connect to the vacuum pumping manifold. You should be able to reduce the pressure to 1-2 Torr (or less) in a closed system. At the pressure achieved, calculate the temperature at which the limonene and carvone should distill over. Carefully assemble the glassware as shown in Fig. 2. Lightly grease all joints as demonstrated by your TA. Be sure to include a heating mantle, a stirring plate and an ice-bath in the setup. Do not proceed until your TA has checked your setup. Test the vacuum on your system for any leaks without adding sample to the round bottom flask. Why is this important?
Pour 10 mL per student in your group (i.e. 20 mL for two students; save a small portion, about 5 drops, for gas chromatography and refractive index measurements) of your essential oil into the round bottom flask (for 20 mL oil use a 50 mL round bottom flask), and add a 1/2” stir bar.
- Check if the water-in and water-out connections are set correctly (see Fig. 2) and secured with the wire tubing clamp. Make sure you are using water tubing not vacuum tubing.
- Turn the water on in the condenser.
- Immerse only the first receiving flask into a mixture of ice and water until you have collected the limonene then cool all four receiving flask positioned under the outlet tubing of the cow in a mixture of ice and water. Why do you begin only cooling the one flask positioned to collect the first fraction?
- Put aluminum foil around the vigreux column and distilling flask, this will help raise the T of the system making the distillation proceed a bit faster.
- Turn on the magnetic stirrer and fully open the vacuum valve (note: pressure cannot be controlled with this valve. If an adjustment is necessary, please see the Instructor). The pressure should read less than 2 mm Hg (2 Torr) but greater than 1 mmHg (1 Torr) on the manometer. Determine the temperature the limonene and carvone fractions should begin to distill based on the pressure reading on the manometer, using the chart at the end of this experiment. Feel free to verify your answers with your TA. .
- Use the Variac dial (starting at setting 40); increase the temperature slowly at about 5 variac units every 5 minutes. Although some “explosive” bumping is expected, do not allow the heat to reach such a level that the column is flooded since this will dramatically decrease the efficiency of the separation.
Note: On a warm day the room temperature may be at or above the boiling point of limonene (ca 32 ºC) at reduced pressure. If the laboratory is warm be sure to check for limonene distillation before turning on the heat source.
When the oil begins to distill, the condensation can be seen inside the thermometer adapter, thus the progress of distillation can be monitored. The rate at which the temperature changes at the top of the column is significant. Be sure to record temperature information in your lab notebook.
To ensure an optimal separation, three fractions should be collected: the limonene fraction, an intermediate fraction that forms during the rapid increase in temperature after collection of the limonene fraction, and the carvone fraction.
To collect each fraction, rotate (under vacuum) the cow adapter such that the end of the bent outlet sits above the next empty receiver flask. Label flasks, for example, with letters A, B, and C.
When collecting the first fraction, chill only that one flask using a 100 mL beaker filled with ice. After collecting the first limonene fraction then cool ALL four collection flasks simultaneously with a large pyrex crystalizing dish filled with ice for the remaining collections. Why is it now important to cool all four flasks? At the end of the distillation use a glass Pasteur pipette to remove the material from your distillation flasks. Pouring it out will cause your product to be contaminated with stopcock grease, which may result in many extra peaks in your GC trace.
Figure 2. Apparatus used for Vacuum Distillation
- Vigreux column
- Vacuum connection
- 25mL (10 mL of oil) or 50 mL (15mL of oil) rond bottom flask
- Heating mantle
- Stirring plate
- 10 mL pear-shaped recieving flask
- Bucket with ice and water
- Keck clamps