Tautomer interconversion of 2,4-pentanedione during gas chromatography on an oxidized cyano-modified capillary column

The tautomerization of 2,4-pentanedione (acetylacetone) is examined on a microbore column containing an acid-modified stationary phase made by oxidizing a commercially available cyano-modified column. This stationary phase is found to provide separation of the two tautomers, which allows the kinetic and thermodynamic properties of the on-column interconversion to be investigated. The enol-to-keto tautomerization is found to occur primarily in the stationary phase, being enthalpically driven. By treating the column as a reactor, the interconversion is investigated as a function of temperature. Monitoring the loss of the more gas-stable 'enol' tautomer makes it possible to extract an energy of activation for the net tautomerization (42.7 kj/mol), because the reaction is found to obey pseudo first-order kinetics. Simple peak-shape analysis of the major component (enol), which is used commonly in treatments of peak tailing, provides insight into the nature of the retention processes of the two tautomers as well as information on chromatographic optimization.