kilomentor | 28 December, 2009 19:17
Dipolar aprotic solvents such as N-methylpyrollidone, dimethyl formamide, N-methyl formamide, dimethyl acetamide and dimethyl sulfoxide are often drowned out with water and then extracted to isolate organic products. No cheap and convenient method has been worked out to separate these polar organics from the bulk of the water and return them to an anhydrous condition suitable for reuse.
Kilomentor at his present employment does not have access to a chemical laboratory where experimentation could be done so the following idea has not been tested, but on the basis of the physical properties of the chemicals it might be workable.
Diisopropyl ether (DIPE) forms an azeotrope with water that is reported to boil at 62.2 C. This is a heterogeneous azeotrope that according to the Chemical Rubber Handbook splits into a water-poor DIPE upper phase and a water rich lower phase. Addition of DIPE therefore to one of these higher boiling solvents and water, and boiling of the ternary mixture under a Dean-Stark trap with continuous return of the top DIPE phase should gradually separate a lower water rich phase which could be periodically drained away. The high boiling dipolar aprotic solvent that is being dried should theoretically be confined to the still pot at the low azeotropic boiling point. In the real laboratory situation, however, a small amount of dipolar solvent vapour entrained in the reflux stream could be all that is needed to prevent the distillate from separating into two phases in the trap and this would scupper the procedure. It is crucial for a practical process that the DIPE be recycled since the distillate is 97% DIPE and only 3% water. Recycling is essential to be able to remove a large amount of water using only a small amount of DIPE.
Other solvents that boil above 100 C that can potentially be separated from water and dried are: nitromethane, acetic acid, dioxane, ethylene diamine, sulfolane and isoamyl alcohol.
After the water has been completely removed continued distillation will separate the DIPE. Even if small amounts of DIPE might remain they are usually unreactive. If particular importance they are inert towards organometallic reagents.For safety remember that DIPE needs to be worked with under inert gas to prevent the accumulation of explosive peroxides. The solvent very readily forms peroxides.