How to Purify by Distillation at Reduced Pressures (Vacuum Distillation)

How it works: A reduced-pressure (vacuum) distillation is performed at a reduced pressure using a water aspirator or a mechanical pump. Since the boiling points of liquids are depressed at reduced pressure, organic compounds can be distilled at lower temperatures. This is desirable because when high temperatures are used for distillation at atmospheric pressure, distillation time is extended, efficiency is compromised, and compounds may decompose. For best results, a vacuum distillation should be carried out at a pressure where your compound boils between 45 ^oC and 180 ^oC. 0.1 mmHg is appropriate for many organic compounds.

Keep in mind: distillations can be performed at temperatures exceeding 180 ^oC but many compounds are unstable and one has to be careful about bath selection (certain oil baths cannot be used at temperatures exceeding 200 ^oC without risk of fire!).

If you know the boiling point of your substance at atmospheric pressure (or any pressure), use a Nomograph (such as the one found of Sigma-Aldrich’s website) to get an estimate of its boiling point at a lower pressure. Note that Nomographs are reasonably good for less polar compounds but tend to underestimate the boiling points of polar compounds.

A list of items you will need to carry out a vacuum distillation is below:

• Ground glass thermometer
• Vacuum distillation piece
• A round bottom distilling flask
• A stir bar
• A tear drop- or pear-shaped receiving flask
• Vacuum grease
• Aluminum foil or glass wool
• A Bunsen burner head (necessary for achieving pressures slightly above those of your vacuum source)
• A McLeod gauge head (necessary for pressure measurement)

A picture of some of these items is below:

A procedure to carry out a vacuum distillation is provided below.

1. To a round bottom flask equipped with a stir bar, add the organic liquid to be purified.
   • CAUTION: Be sure to check all glassware being used for cracks (especially star cracks and fine cracks) as this can cause the glassware to implode when placed under vacuum.
   • Use the smallest possible round bottom flask (so it is ~ ⅔ full) to ensure you get the highest yield possible.

   

2. Set up a vacuum distillation apparatus as shown below.





Notes:

• Each joint must be greased to ensure there are no leaks and a low pressure can be obtained.
• The flask and collection flask should be secured with a clamp to the back of the fume hood.
• Cold water should travel from bottom to top of the condenser.
• Thick-walled tubing appropriate for holding low pressure should be used. Using incorrect tubing will result in the tubing collapsing under reduced pressure. The recommended tubing can be found on Fisher.
• The tubing attached to the vacuum distillation piece should be connected to a dual-port vacuum manifold which is, in turn, connected to a vacuum pump.



3. Once the distillation is set up, the manifold port must not be open to the vacuum when it is turned on. This will result in your mixture bumping, and you will likely have to set up the distillation again.
   • Make sure the ports leading to the nitrogen ports on the manifold are closed if using a manifold as depicted above.

4. OPTIONAL: For distillations at higher pressures, use a Bunsen burner head attached to another line to introduce a small bleed. On a third line, attach a McLeod gauge (or another pressure measurement device) to determine the overall pressure. Generally speaking, by simply attaching the Bunsen burner and having the manifold open to this port, the pressure will go to ~0.5- 1.0 mmHg on a system that typically pulls 0.1 mmHg.





5. Once a vacuum is being pulled, carefully open the port to the vacuum distillation piece.
   • At this point the mixture will start to bubble, especially if there is residual solvent in the flask.
   • Gradually open the vacuum port more over time as traces of solvent and low-boiling impurities are removed.

6. Once all the residual solvent and low-boiling impurities are removed (absence of bubbling) and the vacuum port is fully opened, heat can be applied to the distillation.





7. Use a heating mantle or oil bath to heat the organic mixture to the appropriate temperature.



• You should start by setting the temperature to distill the lowest-boiling compound first, to aid in separation if there are higher-boiling impurities that will get collected.
• Note the ring of condensate rising slowly up the distillation piece.
• If the ring of condensate stops rising, increase the temperature slightly.
8. If material does get collected before your desired compound, continue collecting the distillate until the temperature on the thermometer reaches the boiling point of the compound and remains there while a few drops are collected.
• At this point, to switch receiving flasks, remove the distillation from the heating element and open another vacuum port on the manifold to air.
• This will allow you to replace the receiving flask with a clean one to collect the pure distillate.
• Make sure to re-grease the joint before putting the new flask on the distillation piece.
9. Once the new receiving flask in attached, close the vacuum port that is open to air to return the distillation setup to reduced pressure.
10. Lower the distillation back into the hot oil bath and increase the temperature of the heating element to distill the target compound.
11. Distillation will commence once the flask reaches the appropriate temperature.



• After the first few drops of liquid start to fall down the condenser, the temperature should remain nearly constant.
• To obtain the optimal distillation, the distillation rate must be constant and relatively slow
• This is the temperature that should be recorded as the boiling point of the compound.
• If the distillation is occurring very slowly, wrap the distillation piece with aluminum foil or glass wool to protect it from cold drafts.
12. Once the distillation is complete, remove the distillation piece from the heating element, and open another vacuum port on the manifold to air to remove the receiving flask and turn off the vacuum pump.
13. Transfer the pure organic compound to a clean, labeled vial for safekeeping.
14. Make sure to remove grease from all the pieces of glassware used before typical cleaning.