Model calculations of isotope effects. VIII. Deprotonation of nitroethane

Abstract

Transition-state models for the base-promoted deprotonation of nitroethane have been designed, and primary and secondary hydrogen-deuterium kinetic isotope effects have been calculated. Comparison of the results with experimental values of the primary isotope effects allows no firm conclusions to be reached concerning probable transition-state structures. However, the secondary α-deuterium isotope effect comparison disqualifies from consideration those transition states in which rehybridization of C_α and delocalization of the partial negative charge by the nitro group keep pace with the extent of deprotonation. Transition-state models wherein C_α is carbanionic and essentially pyramidal yield theoretical isotope effects lying within the experimental range.