Rotary evaporator (also called “rotavap“) are used to remove solvents from reaction mixtures and can accommodate volumes as large as 3 liters. They are found in almost every organic laboratory, since they allow performing this task very quickly. A typical rotary evaporator has a water bath that can be heated in either a metal container or crystallization dish. This keeps the solvent from freezing during the evaporation process. The solvent is removed under vacuum, is trapped by a condenser and is collected for easy reuse or disposal.
Most labs use a simple water aspirator vacuum on their rotavaps, so a rotavap cannot be used for air and water-sensitive materials unless special precautions are taken i.e. additional traps are used. In the lab, the house vacuum line, a circulation bath or a membrane pump are used as source for the vacuum (40-50 torr). The fact that a vacuum is usually applied to the setup means that the boiling points of the solvents are going to be significantly lower than at ambient pressure(see table below).
Since the flask is rotated during the evaporation process, the surface area is larger than normal which increases the evaporation rate significantly. The solvent is collected in a flask and can properly be disposed off afterwards (organic solvent waste). In addition, this method also avoids overheating of the target compound i.e. oxidation because lower temperatures are used. The same rules like for vacuum filtrations apply here in terms of the glassware and other precautions i.e no cracks on the flask, etc.
Distilled water should be used in the heating bath to minimize the scale build up in the bath which coats the thermistor and heating coils. It is very difficult to remove and reduces the efficiency of the bath. In addition, regular tap water will promote the growth of spectacularly disgusting algae colonies, particularly during the summer months. The best protocol is a regular exchange of the water.
To remove algae gunk from the inside of a coiled water condenser, the condenser has to be removed from the rotavap and the coil is soaked in a dilute nitric acid solution for a few hours. After carefully rinsing the insides, the rotavap is reassembled. All standard safety precautions should be followed when working with nitric acid!
The ground glass joint holding the flask does not need to be greased, but on rare occasions it (or the bump bulb) may get “frozen”. Some companies sell special joint clips that can free frozen joints simply by screwing them in one direction. If you are not lucky enough to have these and cannot release the joint you probably want to ask your teaching assistant for advice.
If a mechanical pump is used instead of an aspirator to produce a vacuum, a secondary trap has to be used to prevent that the solvent destroys the membrane or is absorbed in the oil.