2013: Advent Chemistry: Rotary Evaporation

by on December 5, 2013
A rotary evaporator

A rotary evaporator

Consider this: You’ve got a flask of liquid. In the flask is some solvent, and dissolved in it (along with probably some leftover bits and pieces of raw material that you don’t care about) is your product. How do you get the product out?

You can’t filter it out – it’s dissolved. You can’t turn it into something insoluble, because it’s already what you want it to be. So you use one of the oldest tricks in the book. When you can’t take the product out of the solvent, you take the solvent out of the product.

It’s the same process as collecting sea salt. You dry it out until all that’s left is the part you want. The technique depends on your product being either a solid at room temperature, or a visibly different liquid that won’t evaporate significantly at the temperatures you’re going to use to drive the solvent off. Usually you separate liquids with a different bit of kit we’ll cover another day, but sometimes you do have to deal with liquids that mix – like alcohol and water, instead of like oil and water – and rotary evaporation will work for some of those mixtures.

To use a rotary evaporator, you put your solution in a round-bottomed flask and attach it to the bottom of a series of tubes. The series of tubes will function to re-condense and capture the solvent before it can get into the vacuum pump, which will also be of use if you mess up and end up sending your product up the spout along with the solvent – it means you can start over.

You turn on the vacuum first, because it’s significantly helpful in making your flask stay attached to the tubes, turn on the associated water bath to a fairly low temperature, and lower the flask half-way into the water. Then you turn the dial to start rotating the flask.

The flask spins through the water. The part of the flask at the bottom heats up and the rest cools down, and the rotation turns this into a smooth consistent warmth. Helped along considerably by the low pressure the vacuum pump causes, the solvent starts evaporating. It is always tempting to turn the temperature up and make it go faster, but rarely advisable. If you let it boil, which is also considerably easier at low pressure, all your lovely crystals of product will end up inside the tubes, and may not make it as far as the solvent trap to be reclaimed. I have often found it helpful with finicky
solutions to lift the flask back out of the water and use my cupped hand as the heat source.

If you are patient and careful and a little bit lucky, you end up with all the solvent over on the far side of the tubing, and your product all tidy in the flask you made it in. How you then remove it efficiently from the round-bottomed flask is a problem I’ve yet to solve. I usually end up scratching at it awkwardly with a spatula and swearing.

Leave a Reply