The Self-Disproportionation of Enantiomers (SDE) of a-Pinene via Evaporation off Silica Gel and Foam Fractionation-Validation of the Plausibility of SDE via Gas Chromatography (GC) for a-Pinene

a-Pinene is an intriguing monoterpene as it has been reported to undergo the self-disproportionation of enantiomers (SDE) phenomenon via gas chromatography (GC), the only compound to decisively demonstrate this. Examples of the SDE involving the gaseous phase-sublimation aside-are extremely rare. Attempts to replicate the GC results were unsuccessful, though the authors argued convincingly for the difficulty of observing the phenomenon. However, we could effect for a-pinene SDE via evaporation off silica gel and by foam fractionation-albeit the SDE magnitude for both was only very slight-to confirm that a-pinene can undergo the SDE for processes involving a gaseous phase and thus validate the plausibility of the GC report. The indications are that the molecular associations responsible for the various SDE observations of a-pinene occur not in the gaseous phase or the bulk phase but rather in two-dimensional (2D) adsorbed monolayers and are not based on conventional functional group-based intermolecular interactions and instead are, most likely, as a result of homo- and heterochiral packing differences in the 2D monolayers-a well-known 2D chiral-based association packing effect. These are also the first reports of the occurrence of the SDE using an adsorptive bubble separation process (foam fractionation) and involving a gaseous phase other than sublimation, GC, and distillation.

Automated summary

a-Pinene is a unique monoterpene that undergoes the self-disproportionation of enantiomers (SDE) phenomenon in the gaseous phase, as shown by gas chromatography (GC). This study successfully replicated the SDE of a-pinene using evaporation off silica gel and foam fractionation methods, confirming the plausibility of the GC report. Molecular associations in 2D adsorbed monolayers, rather than conventional functional group-based interactions, are responsible for the observed SDE.