Reactions performed in dipolar aprotic solvents such as N-methylpyrollidone, dimethyl formamide, N-methyl formamide, dimethylacetamide, or 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 the dipolar aprotic to an anhydrous condition suitable for reuse.
On the basis of the physical properties of the chemicals, the following might be workable but KiloMentor has seen no experimental work to substantiate it.
KiloMentor has argued that diisopropyl ether can safely be used at scale because better precautions and practices are taken than in the laboratory. Diisopropyl ether (DIPE) forms an azeotrope with water that is reported to boil at 62.2 C. This is a heteroazeotrope. That means that the vapour is in equilibrium with two immiscible liquid phases. According to the Chemical Rubber Handbook, DIPE and water form an azeotrope that on condensation 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 the continuous return of the top DIPE phase might realistically gradually separate a lower water-rich phase which could be periodically drained away. The high-boiling solvent that is being dried would theoretically be confined to the still pot.
In the real-life situation, however, a small amount of the high-boiling solvent could co-distill. Enough of this vapour, entrained in the reflux stream, could scupper the procedure by making the distillate a single phase, so this idea would need to be thoroughly tested for each different dipolar aprotic solvent. Nevertheless, if it works and your facility has unused distillation capacity, solvent recovery could be profitably practiced.
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 using DIPE are nitromethane, acetic acid, dioxane, ethylenediamine, sulfolane, and isoamyl alcohol.
After the water has been completely removed continued distillation will drive over the DIPE itself. Even if small amounts of DIPE remained in the recovered dipolar aprotic solvent they are usually unreactive. Of 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 but fortunately, plant processing is invariably done under inert gas.
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