On the magnitude of primary isotope effects for proton abstraction from carbon

Abstract

Primary deuterium isotope effects have been measured for the loss of an ..cap alpha.. proton from 4-(4-nitrophenoxy)-2-butanone. The isotope effect data showed that no significant change in transition-state structure occurred in the same region of oxyanion catalyst pK/sub a/ range that does exhibit a large change in ..beta.. value. This conclusion confirms the previous assertion that the dramatic curvature in the Broensted plot for proton abstraction by oxyanions from carbon is due to solvation. The effect upon k/sub H/ / k/sub D/ of varying the identity of the atom acting as the proton acceptor was examined. Catalysts having oxygen, nitrogen, carbon, and sulfur as the acceptor atom were included. An inverse correlation was found between the magnitude of the isotope effect for proton abstraction by X/sup -/ and the energy change upon conversion of HX to DX. With use of the Marcus equation as a basis, an equation was derived for predicting values of k/sub H/ / k/sub D/ which not only relies on the ..delta..pK/sub a/ value but also incorporates the zero-point energy difference of both the reactants and the products as measured by infrared stretching frequency shifts upon deuteration. This equation demonstrates improved correlation between calculated and experimentalmore » values of k/sub H/ / k/sub D/ taken from the literature. 48 references.« less