Gas-phase acidities and sites of deprotonation of 2-ketones and structures of the corresponding enolates

Abstract

The gas-phase acidities of seven different 2-alkanone molecules have been investigated using pulsed ionization high pressure mass spectrometry (HPMS). From the temperature dependence of the equilibrium constant for proton-exchange reactions, the enthalpy and entropy changes for these reactions were determined. These experimental values agreed well with the G3(MP2) calculated values. From the thermochemistry of the proton-exchange reactions a gas-phase acidity scale, relative to acetone, was constructed for 2-ketones up to and including 2-decanone. Furthermore, an absolute gas-phase acidity scale is presented which is anchored to the gas-phase acidity value for acetone determined by Bartmess et al. [J. Am. Chem. Soc. 101 (1979) 6046]. The entropy changes associated with the proton-exchange reactions were all found to be very close to zero which suggests that there is no intramolecular solvation occurring in any of the enolate ions over the temperature range studied. The G3(MP2) calculations predict that the primary and secondary enolates of butanone, formed by deprotonation at either C1 or C3, respectively, have nearly identical basicities. However, as the 2-ketone chain length increases, the secondary enolate becomes more stable, with respect to the primary enolate. These theoretical predictions are in agreement with the stability ordering for enolates (2°≥1°⪢3°) found experimentally by Chyall et al. [J. Am. Chem. Soc. 116 (1994) 8681].