An isotope effect maximum for proton transfer between normal acids and bases

Kinetic isotope effects were determined for the reaction between p-methylbenzaldehyde and methoxylamine in the presence of thirteen carboxylic acid catalysts and ammonium ions using deuterium oxide as the solvent for k/sub D/ measurements. The ratio of k/sub H//k/sub D/ = 1 for the relatively strong acids, CNCH/sub 2/CO/sub 2/H and HCO/sub 2/H, rise to a maximum approaching 3 for CNCH/sub 2/CH/sub 2/NH/sub 3//sup +/, O(CH/sub 2/CH/sub 2/)/sub 2/NH/sub 2//sup +/, and C/sub 6/H/sub 5/CH/sub 2/NH/sub 3//sup +/, and decrease to less than 2 for CH/sub 3/OCH/sub 2/CH/sub 2/CH/sub 2/NH/sub 3//sup +/, and CH/sub 3/CH/sub 2/CH/sub 2/NH/sub 3//sup +/. The authors state that the maximum observed can not be the result of solvent and secondary isotope effects, but must be caused by a rate determining step involving the proton transfer from the catalyst to the alkoxide oxygen of the first-formed zwitterion intermediate. From a discussion of the pKa's of the proton donor and the protonated proton acceptor (for these systems, pKa = 0), the authors conclude that the experimental data are consistent with a model in which encounter, proton transfer, and separation occur at approximately equal rates.