ALKYL SUBSTITUENT EFFECT IN THE DEPROTONATION OF UNSYMMETRICAL KETONES

The effect of various degrees of alkyl substitution on the relative ra tes of deprotonation from the two distinct sites in several unsymmetri cal ketones in the gas phase is examined. The infrared multiple photon activation of an appropriately deuterium-labeled alkoxide ion generat es the ion-molecule complex for the half-reaction of the bimolecular p roton transfer process between an alkyl anion and an unsymmetrical ket one with one deprotonation site selectively deuterated. The resulting products are enolate ions generated by the removal of either a deutero n or a proton and, thus, are distinguishable by mass. The measurement of the enolate ion product ratios, along with an independent measureme nt of the kinetic isotope effect, allowed the kinetic effect of the al kyl environment on the relative proton transfer rates to be determined . The primary and secondary isotope effects are also estimated from th e enolate ion product ratios. By examining the magnitude of the kineti c alkyl effect, the primary isotope effect, and the secondary isotope effect, we learn about the transition state for proton transfer.