A Proposal for Separating a Mixture of Water and a High Boiling Dipolar Aprotic Solvent from a Drown-Out Precipitation.

Frequently in organic synthesis, a product synthesized in dimethylformamide (DMF), dimethylsulfoxide (DMSO) or another high boiling dipolar aprotic solvent is precipitated by adding a substantial excess of water. At present, this drowned-out aqueous solvent mixture cannot be economically recovered but needs to be sent for destruction.  This is wasteful and environmentally questionable. It is also more expensive than burning a completely combustible, purely organic waste. A drown out also usually raises the point of maximum volume thereby reducing throughput.

These difficulties separating away dipolar parotid liquids are not only because there is a strong affinity between these higher boiling organics and the water but also, because after a drown-out there is just such a high proportion of water. 

 Pyridine Treatment

Let us do a thought experiment. Imagine what would happen if pyridine, which forms a binary, minimum-boiling azeotrope with the water in the mixture, is added. Pyridine makes a stronger hydrogen bonding possibility available to the water.  The result may be that pyridine will form an azeotrope with water, which can be removed as the low bp. azeotrope at 92.6 C.  If this were to be the result the still-pot residue would be DMF. A further advantage of this method of purification would be that the potential pyridine impurity left in the residual DMF  is aprotic and is therefore unlikely to interfere in reactions requiring anhydrous conditions. That is, the recovered DMF is aprotic and can probably be reused.

But wait a minute, we now have a large amount of a homogeneous solution of pyridine and water; the pyridine/water azeotrope, which is 57% pyridine and 43% water. Are we better off or have we just changed one problem for another while spending more time and more money?

Pyridine Recycling

The difference is that this mixture can be separated into two liquid phases by the addition of sodium hydroxide.  The separated pyridine layer can be further dried with solid sodium hydroxide or can be used in a wet condition to purify more DMF. The alkaline water can be used in the plant to neutralize an aqueous acid fraction from any other process in preparation for sending it to the sewer.

Imagine a mixture containing 90% water and 10% DMF. Add to this a convenient portion of pyridine and distill the water/pyridine azeotrope away from the DMF containing mixture. In the distillate vessel, the azeotrope comes into contact with a reservoir of solid sodium hydroxide or perhaps just liquid caustic. The distillate separates into a strongly basic aqueous lower layer and an upper pyridine layer. The pyridine layer is led back into the still pot to remove more water/pyridine as an azeotrope. When all the water is transferred out the head temperature rises and the pyridine is distilled leaving purer DMF separated from all the water! If there is any pyridine residue in the DMF it does not contain any available hydrogens and may be suitable for reuse as is. The pyridine fraction is drummed off for reuse and the aqueous alkali kept to neutralize another plant waste.