Alkyl Substituent Effect in the Deprotonation of Unsymmetrical Ketones

Abstract

The effect of various degrees of alkyl substitution on the relative rates of deprotonation from the two distinct sites in several unsymmetrical ketones in the gas phase is examined. The infrared multiple photon activation of an appropriately deuterium-labeled alkoxide ion generates the ion−molecule complex for the half-reaction of the bimolecular proton transfer process between an alkyl anion and an unsymmetrical ketone with one deprotonation site selectively deuterated. The resulting products are enolate ions generated by the removal of either a deuteron or a proton and, thus, are distinguishable by mass. The measurement of the enolate ion product ratios, along with an independent measurement of the kinetic isotope effect, allowed the kinetic effect of the alkyl environment on the relative proton transfer rates to be determined. The primary and secondary isotope effects are also estimated from the enolate ion product ratios. By examining the magnitude of the kinetic alkyl effect, the primary isotope effe...