Kinetic isotope effects. Part 6.—Ionization of 2-carbethoxycyclopentanone

Abstract

Measurements are reported on the kinetics of the base-catalyzed detritiation of 2-carbethoxy-cyclopentanone over the temperature range 15–50°C. The catalysts were the solvent (water or deuterium oxide), chloroacetate ions and fluoride ions. The results for the water-catalyzed bromination of the proto-ester, obtained spectrophotometrically by observing the absorption band of bromine at 3 900 Å are in good agreement with previously-determined values. The detritiation results, taken in conjunction with the previously measured rates of deprotonation and dedeuteration, serve to confirm the original interpretation, viz., that the results for proton transfer indicate a considerable tunnelling contribution especially when the catalyst is the fluoride ion. The Swain correlation factor in this case is 1.32, whereas values higher than 1.44 are obtained for deuterium oxide and chloroacetate ion catalysis. These latter findings are accompanied by abnormal values of A_T/A_D and E_T–E_D and are discussed in terms of the mechanism of the reaction.