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kilomentor | 09 April, 2007 04:37
The need for solvent exchanges
The need for solvent exchanges in the sense of displacing one solvent by another without passing through a liquid free state practically does not exist outside of process chemistry. At laboratory scale, when one solvent needs to be replaced with another, the solution contents are placed in a r.b. flask, set spinning on the vacuum rotary evaporator with appropriate heating and strong condensing efficiency. When the first solvent has been completely evaporated then the required new solvent is added and the solutes brought back into solution by swirling and scrapping.
On scale, evaporation to dryness is not possible without caking and possibly charring. Even if it were possible to avoid degradation, the layer of non-volatile residue would become so thick on the reactor's wall that heat transfer to complete the evaporation would be made impractically. Combined with this difficulty, at low volumes in a normal reactor stirring becomes ineffective. Thus solvent replacements must be done without completely removing the liquid phase at any point.
As an exercise let us consider solvent replacements among a dozen of the most common solvents. This examination is a logical analysis. None of the more complex multistage switches have been experimentally verified. The only inputs are known miscibilities, boiling points and the data from binary azeotrope tables.
The solvents are listed with their boiling points. I have named the list, Common Reaction Solvents, because they are not all solvents of choice for process chemistry. Chloroform for example would not be used today in a chemical process and hexane because of its flash point is questionable.
Methylene chloride 39.6
Acetone 56
Chloroform 61.2
Methanol 64.5
Tetrahydrofuran 66.0
Hexane 68.7
Ethyl acetate 77.1
Ethanol 78.3
Isopropanol 82.2
Toluene 110.6
Dimethylformamide 153.0
Dimethylsulfoxide 189.0
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