Charge-Transfer Complexes for Isolation/Separations: Lewis Acid Type

Introductory Base Ideas

The KiloMentor emphasis in process development is on the simplification of the work-up, isolation, and purification of the desired product coming out of a process step.  This is where more of a typical process’s time is expended. Another reason to emphasize this phase is because this is where chemical database searching is poorly organized to help choose the better methodology.

Separations on the molecular level are achieved by phase switches such as liquid-liquid extractions (ie methanol\hexane), acid-base extractions, and physical changes of state (like crystallization, distillation, sublimation, etc). Purification results because the conditions for the phase switch are so controlled that impurities are relatively poorer at making the switch and are thereby left behind.

Charge Transfer Complexes as a Phase Switch

The possible use of preferential formation of charge-transfer complexes in separation and purification is attractive because the property used to achieve the switch is not that of a particular functional group, like the acidity of a carboxylic acid or the basicity of an amine, but is a function of the entire aromatic system and all its substituents. It is more of a molecular property.  Furthermore, because of this, even molecules that have no serviceable functional groups, like naphthalene, for example, can form charge transfer complexes.

Electron-deficient compounds that are known to form pi-type charge-transfer complexes with aromatic compounds include tetracyanoethylene, 3,5,7-trinitroflorenone,  tetranitrofluorenone, dicyanomethylene, trinitrofluorene, tetranitrofluorenone, trinitrobenzene, and bromotrinitrofluorenone. Usually to form a solid thermally stable complex the aromatic system must comprise at least two rings, but tetracyanoethylene forms complexes even with benzene. Newman has reported that the compound 4-bromo-2,5,7-trinitrofluorenone forms a complex with benzene that can be dried under a vacuum of 1-2 mm mercury at room temperature but was decomposed by heat before reaching 100C.

Also, in order to achieve higher stability the substituents must not be ones that can interfere with a close approach to at least one of the faces of an electron-donating ring. As an illustration, a methyl aryl substituent is well tolerated but an ethyl group is not.  This sensitivity to small, what would seem to be inconsequential, variants is part of the attraction to using such a method.

There are very few examples of isolation/purifications using charge-transfer complexes. The only one I could find was a report that a charge-transfer complex was used to remove naphthalene from petroleum distillate. 

No one it seems has succeeded in using one of these electron-deficient complex formers as a component in a reactive distillation to reduce the volatility of one aromatic substance in the presence of another or of an aromatic substrate in the presence of a non-aromatic one. In order to be effective in this situation even a preferential association of one substance over another might be sufficient to be practicable.