Model calculations of isotope effects. Part 5. Secondary hydrogen isotope effects in the solvolysis of isopropyl halides

Abstract

Model reaction systems in terms of reactant and transition state geometries and force fields have been developed to simulate S_N1 and S_N2 solvolyses of isopropyl halides. Equilibrium and kinetic isotope effects for α-deuterium (α-k_H/k_D) and β-deuterium (β-k_H/k_D6) substitution have been calculated, with transition state models being assigned structures using the ab initio geometry of the isopropyl cation as a basis. Values of α-k_H/k_D were found to be dependent on the identity of the leaving group and were, for ion-pair-like transition states, considerably larger than the values suggested by Shiner as ‘maximum’α-deuterium isotope effects in limiting solvolyses. The mechanistic consequences are examined. Solvent assistance to ionization appears to be a necessary component of simple secondary substrate solvolysis in that it is necessary to include a nucleophilically solvating water molecule in order to match the calculated and experiment values for the hydrolysis of isopropyl bromide.