Schlenk lines allow solvent and other volatiles to be removed without ever exposing the reaction mixture to atmospheric air or moisture. The low pressures offered by a high vacuum pump means that high boiling solvents such as toluene or chlorobenzene can be easily removed at room temperature, which may be crucial for temperature-sensitive compounds. An external liquid nitrogen trap is essential for removing large volumes of solvent and highly recommended for particularly nasty volatiles, or for solvents with freezing temperatures close to room temperature such as benzene or 1,4-dioxane.
Step 1: Seal the Schlenk flask under inert gas and disconnect it from the Schlenk line. Attach the flask to an external trap, then connect the external trap to the Schlenk line and slowly place under vacuum.
Step 2: Slowly lower the external trap into a dewar of liquid nitrogen to condense the solvent vapours.
Step 3: Slowly open the tap on the Schlenk flask (partially) making sure that the solution is stirring to prevent bumping. It may be necessary to open the tap fully or use a warm water bath to fully remove all solvent/volatiles.
Step 4: Once all of the solvent has been removed, close the tap on the Schlenk flask as well as the tap/stopcock connecting the external trap to the Schlenk line vacuum. Disconnect the Schlenk flask from the external trap and then cycle back onto the Schlenk line, slowly opening the tap when backfilling with inert gas. Disconnect the external trap from the Schlenk line and remove from the dewar of liquid nitrogen. Allow the solvent/volatiles to thaw and then discard into an appropriate waste container.
This same approach can also be used to partially concentrate solutions in vacuo prior to crystallisation.