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3.5 Evaporation

3.5.1 Open-Dish Evaporation

Solvent can be evaporated by placing the solution in an open container (an Erlenmeyer, evaporating dish, beaker, vial). The container is set on a heat source (steam bath, hot plate, heating mantle, sand bath) and the solvent boiled off. (If the solvent is water, use a heat source other than a steam bath.)

The problem with open-dish evaporation is that the solvent is released into the air. Open-dish evaporation should always be done in a hood if the solvent is anything other than water. Even in a hood, however, vapors are released into somebody's air. If the solvent is a hazardous compound (for instance, methylene chloride), it is probably better to choose another method of solvent removal.

3.5.2 Reduced-Pressure Evaporation

You can accomplish evaporation from a solution quickly by placing it in a side-arm flask, sealing the flask, and then applying vacuum. Under vacuum (reduced pressure) liquids vaporize and boil off at lower temperatures; effectively, the solvents come off a lot faster when under vacuum than at atmospheric pressure. A small (25 or 50 mL) side-arm flask fitted with a rubber stopper is used to strip off small (5-10 mL) amounts of solvent.

3.5.2 Rotary Evaporators

Rotary evaporators, or rotovaps, are standard equipment in most organic chemistry research labs. These evaporators are designed to remove solvent rapidly from solutions. The motor in the rotovap turns the flask rapidly, providing a greater surface from which evaporation can occur and speeding up the process. Cooling coils in the rotovap condense the vapors and drop them into a collection flask so that they can be recycled or properly disposed. The roto-vap is connected to a vacuum source, and again, this speeds up the evaporation process.

There are three tubing outlets on the rotovap, one for a vacuum source and two for the cooling coils. If these outlets are not already connected, use vacuum tubing to connect the outlet that evacuates the rotovap chamber to the vacuum source. Use a piece of Tygon tubing to connect one cooling coil outlet to the cold water faucet, and use another piece of Tygon tubing to connect the other cooling coil outlet to the drain.

Place the solution to be evaporated in a round bottom flask, then connect the flask to the rotovap. Do not fill the flask more than about one-third full. Use a Keck clip to secure the flask to the apparatus. Make sure the water flow to the cooling coils is turned on. Turn on the motor so that the flask rotates. Usually the flask containing the solution to be evaporated is warmed by a water bath;

Make sure the vent at the top of the cooling coils is closed (horizontal position).
Turn the vacuum outlet on the vacuum system on and turn on the machine.
Then move the flask down with the button and slowly increase the spin velocity.
As the solvent evaporates, you may notice a lot of frothing and bubbling in the evaporating flask. If it starts bubbling out of the flask, you can open the vent a little (see picture above) to release some of the pressure. The (unwanted) solvent condenses on the cooling coils and drips down into the collection flask.
When the solvent has evaporated, turn off the motor that turns the flask and turn the vacuum outlet to closed. But do NOT FORGET, that to eliminate the final residues of solvent, you have to increase to maximum the vacuum outlet and also increase the spin rotation before turning off everything.
Slowly open the vent to release the pressure in the roto-vap chamber (vertical position).

Remove the flask from the roto-vap.

Afterwards, you should clean up the round flask with a piece of paper.
And measure the weight of the crude product, substracting the difference between the final result and the flask initial weight.
Never forget to identify correctly your flask, cover it with the rubber top and place it on the fridge.


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