Activation and thermodynamic parameters of nucleophilic substitution. Reaction of alkyl halides with solid salts under phase-transfer catalysis conditions

The paper is concerned with the effect of temperature and the nature of a solid ionophore (MCl, where M = Li, Na, K, Rb or Cs) on the rate and equilibrium constants in the process of substitution of chlorine for bromine in hexyl bromide. The non-linearity of the log k vs 1/T, dependence is shown to be linked with a change in the physical meaning of the constants with temperature variations. This is attributed to the occurrence of a strong exothermic process consisting in the formation of a kinetically independent intermediate, that precedes the limiting stage. A mechanism including the formation of two stable ternary complexes coordinated on the solid-phase surface, one of which is structurally close to the starting reagents and the other to the final reaction products, is proposed. The observed activation energies, enthalpies of formation of the ternary complex and activation energies of the limiting stages for the forward and reverse substitution processes were calculated. Thermal effects of the substitution in the presence of various solid MCl species were determined from the thermodynamic data and observed activation energies for both reaction types. The substitution is shown to involve a limiting stage represented by a transition between the ternary complexes. A linear relationship between the activation energy for the limiting stage and the solid ionophore crystalline lattice was established, suggesting incorporation of the solid salt molecule into the transition reaction state. Potential energy changes in the course of substitution in the presence of the various MCl solid salts are discussed. The Bell—Evans—Polyani principle is shown to hold for these studies.