ISOTOPE EXCHANGE AND SOLVOLYSIS IN BASIC METHANOLYSIS OF ARYL ESTERS . MOLECULAR INTERPRETATION OF FREE ENERGIES ENTHALPIES AND ENTROPIES OF ACTIVATION

The free energies of activation for methoxide-catalyzed methanolysis of aryl acetates and carbonates in methanol are not linearly related to the free energies of ionization of the corresponding phenols; instead the rate process becomes less sensitive to substituent as the electron-withdrawing power of the substituent increases. This is not due to a change in rate-determining step from phenoxide elimination of the tetrahedral intermediate with poor leaving groups to carbonyl addition with good leaving groups, because methoxyl-labeled aryl methyl carbonates, CH3*OCO2C6H4X, undergo basic methanolysis without methoxyl exchange regardless of the character of X (X = p-CHO, H, P-CH3O). A plot (slope 1.8) of free energies of activation vs. (0.80.F + R), where Fand R are the field effect and resonanee constants of Swain and Lupton, is linear, consistent with the attribution of the curvature in the other plots to greater importance of the resonance interaction in the equilibrium than in the rate process. Alternatively, the length of the transition-state nucleophile-carbonyl bond may increase with electron withdrawal in the leaving group. The substituent effect appears neither in the enthalpy of activation alone nor in the entropy of activation alone, but rather both vary and there is not a linear relation between them. The substituent points in the enthalpy-entropy plane may be represented by vector sums of isergonic (“isokinetic”) relations, one having a slope of 330˚K, presumably resulting from substituent effects on solvation, and one having a very large slope, presumably resulting from substituent effects on internal energy. The solvation effects essentially disappear from the free energies of activation at 298˚ (near 330˚) and the free energy relations thus arise from internal energy interactions. © 1969, American Chemical Society. All rights reserved.