Fractional Distillation

Source: Laboratory of Dr. Nicholas Leadbeater — University of Connecticut 

Distillation is perhaps the most common laboratory technique employed by chemists for the purification of organic liquids. Compounds in a mixture with different boiling points separate into individual components when the mixture is carefully distilled. The two main types of distillation are "simple distillation" and "fractional distillation", and both are widely used in organic chemistry laboratories.

Simple distillation is used when the liquid is (a) relatively pure (containing no more than 10% liquid contaminants), (b) has a non-volatile component, such as a solid contaminant, or (c) is mixed with another liquid with a boiling point that differs by at least 25 °C. Fractional distillation is used when separating mixtures of liquids whose boiling points are more similar (separated by less than 25 °C).

This video will detail the fractional distillation of a mixture of two common organic solvents, cyclohexane and toluene.

1. Set-up of Fractional Distillation Apparatus

Table 1. Fractional distillation apparatus components.
Note: all glassware, with the exception of the graduated cylinders, should have ground-glass joints

1. Gather all of the equipment needed to assemble the fractional distillation apparatus (Table 1) and place them in a fume hood.
2. Place the heating mantle and stir plate at the foot of the retort stand, sitting on top of a screw jack.
3. Using the screw jack, elevate the heating mantle to a height of 20 cm.
4. Attach a clamp on the retort stand. Secure the round-bottom flask to the retort stand using the clamp so that the flask sits snuggly in the heating mantle.
5. Remove the screw jack and the heating mantle, putting them aside for later.
6. Add a magnetic stir bar to the round-bottom flask.
7. Add a fractionating column to the top of the round-bottom flask, securing it to the retort stand with a clamp.
8. Add a "Y" adaptor to the top of the fractionating column, securing it to the retort stand with a clamp and to the fractionating column with a Keck clip.
9. Add a condensing column to the downward sloping arm of the "Y" adaptor, securing it with a Keck clip. For additional stability, the condenser can be secured with a clamp to a second retort stand.
10. Connect tubing from the water source to the lower connection on the condenser and attach the water return tube to the higher connection on the condenser. Ensure the tubing is securely fitted to the condenser (consider using copper wire to hold the tubing in place).
11. Connect a collection adaptor to the open end of the condenser and secure the connection with a Keck clip.
12. Have three 25 mL graduated cylinders available for collection of the distillate.

Figure 5. Apparatus for fractional distillation.

2. Preparation for the Distillation

1. Remove the round bottom flask from the assembly by lowering it to the base of the retort stand.
2. Place a stemmed funnel into the top of the round-bottom flask, and add the liquid to be distilled. Do not fill the flask more than half full.
3. After filling the flask, remove the funnel.
4. Raise the flask once again and place the flask back in its original position on the retort stand.
5. Using the thermometer adaptor, place the thermometer into the remaining open port on the "Y" adaptor. The thermometer should be added last, as it is most susceptible to breakage.
6. Position the bulb of the thermometer so that it is just below the side arm of the "Y" adaptor. This ensures an accurate reading of the vapor temperature.
7. Replace the heating mantle and screw jack to their original position, snuggly enveloping the round-bottom flask.
8. Place a graduated cylinder underneath the condenser adaptor, ready to collect the distillate.

3. Performing the Distillation

1. Increase the temperature of the heating mantle gradually until the liquid in the round-bottom flask begins boiling.
2. As soon as boiling commences, hold the temperature for 2 min.
3. Note the ring of condensate rising slowly up the fractionating column. The rise should be gradual, which ensures proper separation of the mixture components across the theoretical plates. If the ring of condensate stops rising, increase the temperature slightly. It should take at least 5 min for the ring to reach the top of the column.
4. Once the first drops of liquid start to fall down the condenser, the temperature should remain nearly constant. Record the temperature as each 2 mL of distillate collects in the graduated cylinder.
5. Adjust the heat accordingly to allow for 1—2 drops/s from the condenser into the graduated cylinder.
6. Each time the graduated cylinder reaches 4 mL, quickly replace it with an empty one.
7. Pour the distillate into a clean, labeled vial for safekeeping.
8. If the distillation is occurring very slowly, wrap the column with aluminum foil or glass wool to protect it from cold drafts.
9. When the temperature begins to drop significantly, nearly all the cyclohexane has been distilled. Increase the temperature until the toluene starts to boil and distill. Again, record the temperature as each 2 mL of distillate collects.
10. Before the entire content of the flask evaporates, stop heating the flask. It is important to ensure that the flask does not become dry, which might cause it to crack.
11. Remove the heating mantle by lowering the screw jack to the base of the retort stand
12. Analyze the contents of the vials of distillate to confirm the purity using a chosen technique, such as ¹H NMR spectroscopy.