THE MECHANISM AND THERMODYNAMICS OF TRANSESTERIFICATION OF ACETATE-ESTER ENOLATES IN THE GAS-PHASE (VOL 172, PG 25, 1998)

In solution, base-catalyzed hydrolysis and transesterification of este rs are initiated by hydroxide-or alkoxide-ion attack at the carbonyl c arbon. At low pressures in the gas phase, however, transesterification proceeds by an attack of the enolate anion of an acetate ester on an alcohol. Fourier transform mass spectrometry (FTMS) indicates that the reaction is the second-order process: -CH2-CO2-R+R'-OH-->-CH2-CO2-R'R-OH and there is little to no detectable production of either alkoxid e anion. Labeling studies show that the product and reactant enolate a nion esters undergo exchange of hydrogens located alpha to the carbony l carbon with the deuterium of R'-OD. The extent of the H/D exchange i ncreases with reaction time, pointing to a short-lived intermediate. T he alcoholysis reaction rate constants increase with increasing acidit y of the primary, straight-chained alkyl alcohols, whereas steric effe cts associated with branched alcohols cause the rate constants to decr ease. Equilibrium constants, which were determined directly from measu rements at equilibrium and which were calculated from the forward and reverse rate constants, are near unity and show internal consistency, In the absence of steric effects, the larger enolate is always the fav ored product at equilibrium. The intermediate for the transesterificat ion reaction, which can be generated at a few tenths of a torr in a ta ndem mass spectrometer, is tetrahedral, but other adducts that are col lisionally stabilized under these conditions are principally loosely b ound complexes. Published by Elsevier Science B.V.