A vacuum distillation is used when the boiling point of the compound (or the solvent) is too high in order to distill the compound (or the solvent off) without significant decomposition. The setup is similar to a micro-scale or semi-macroscale distillation. However, there are some important differences as well.

1. When a vacuum distillation is performed, a spin vane or spin bar has to be used instead of boiling stones, which do not work in vacuo. Alternately, a small boiling capillary can be used.

2. The setup is connected to the vacuum source (house-vac, aspirator) via thick-walled tubing, which can withstand the pressure difference. Either a vacuum adapter is used (semi-macroscale), or a drying tube on the top of the air condenser for a microscale setup (shown below). The glassware should fit together well in order to guarantee a good vacuum. A little vacuum grease on the ground-glass joints often improves the quality of the vacuum significantly.



3. The entire glassware has to be crack-free and the setup secured using a metal clamp.

4. When performing a vacuum distillation, the following sequence should heating and evacuation should be used:

a. All volatile solvents are evaporated off at ambient pressure.

b. The remaining liquid is allowed to cool down to room temperature and then transferred into the flask (or conical vial). A spin bar (or spin vane) is added. The rest of the distillation setup is assembled making sure that there are no leaks.

c. The stirrer is turned on and the entire setup is slowly evacuated without heating and cooling. This way, the low boiling components in the mixture evaporate away first.

d. The temperature is slowly raised while maintaining stirring and the vacuum. The water flow in the water-jacketed condenser is initiated (or the Hickman head is cooled using wet paper towels) in order for the boiling liquid to condense in the receiving flask (or Hickman head).

e. If the Hickman head fills up prior to finishing the distillation, the setup is raised from the hotplate to allow for some cooling. The tubing is disconnected from the vacuum source and then the liquid in the removed. Afterwards, the vacuum setup is re-assembled and the distillation continued until almost the entire compound is distilled.


5. Things to think about:

a. What is the boiling point of diethyl ether at p=30 mmHg? Which consequences does this have for the vacuum distillation?
b. Why is too much lube not any helpful in the distillation?
c. What happens if the student heats up his mixture first and then applies the vacuum?
d. Which advantage does the vacuum distillation have over a normal distillation for propyl benzoate?