Removal of solvents by simple distillation is a tedious procedure, complicated by setup and cleaning of the apparatus. A major advance in efficiency is achieved by using a rotary evaporator ("rotovap"). This cuts the distillation time for eliminating ether from a couple of hours down to 20 minutes or so.
The basic theory is that the apparatus works by vacuum distillation. Rotating the "pot" (on the left in the picture) provides agitation, maintains contact with a warming bath (hot water, usually), and maximizes the surface area of the liquid you are evaporating. However, the cost of doing this is that the seal (located in the white box housing the motor) is a likely spot for air leaks to occur. Cooling the collecting flask minimizes the amount of solvent vapor that gets into the water stream from the aspirator; in many cases use of a dry ice trap is needed to avoid contaminating wastewater with organic solvents.
An efficient spiral condenser (right) allows us to collect the distilled solvent. Normally, the desired product has a low enough vapor pressure that it will not distill (unless you have the water bath too hot!)
The company that manufactures many rotary evaporators has a number of publications describing the principles of design. However, this is a marketing tool, and they do ask you for contact information before they give it to you. The material is in PDF format, so you also need the free Adobe Acrobat reader.
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We have four rotovaps set up. Two are reserved for "H"
people and two for "P" people, to allow rapid use and minimize
cross-contamination. When the ether has all been removed (no more condensate forms), vent the vacuum (the valve on the top of the condenser), turn the spinning motor off, carefully remove the pot, and transfer your crude compound into the flask you will use in the vacuum distillation. Rinse the large rotvap pot with 10 mL ether to recover as much of your compound as possible, and clean it for the next student. |
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