A Novel and Possibly Versatile Method for Separating of Aldehydes Alone

For 40 years I have been thinking about commenting on this article published in the Chemical and Pharmaceutical Bulletin in 1980. In that year Shunsaku Ohta and Masao Okamoto published a three-page communication that taught a simple method for extracting only aldehydes into an aqueous layer and then recovering them in pure form and high yield. I expected to find more complete details later along with experimentation to support a hypothesis for the mechanism of action and I expected many subsequent applications of the method. Nothing could be further from reality. There does not seem to have been any further work or use!

What the authors taught in Chem. Pharm. Bull. 28(6) 1917-1919 (1980) was that a 1.2 M 6-aminohexanoic acid sodium salt solution could quantitatively carry aldehydes, from mixtures of substances comprising at least one aldehyde dissolved in either diethyl ether or diisopropyl ether, into an aqueous phase. Then, after separating the aqueous and organic solvent layers, the aldehyde could be liberated by acidifying the aqueous phase to pH 4-6 and back extraction into an organic phase….. free of non-aldehydes (including ketones). 

6-aminocaproic acid (6-aminohexanoic acid) is cheap. It is the monomer for making nylon! 

The data in this communication shows that the method is not completely selective for aldehydes. Cyclopentanone was partly selected by the reagent, even though cyclohexanone was completely excluded.  Aliphatic aldehydes gave emulsions but these were cleared by adding some isopropanol.

So this procedure seems very practical. Of course, it may not work! Perhaps that is why nothing more has been written about it. But surely it is worth investigating further.

The authors pictured the isolation as proceeding through the formation of the imine, the covalent bond of which pulled the aldehydic moiety into water courtesy of the sodium carboxylate functionality on the other end of the reagent. The authors do not offer any explanation, however, of why the equilibrium so greatly favors the imine. 

Also left hanging- how high can the molecular weight of the aldehyde be and still have it successfully transferred to the aqueous phase? What organic solvents can be used besides diethyl ether or diisopropyl ether? All remains clouded.