Rate–equilibrium relationships based on the CH-acidity constants of oxocarbenium ions, for proton transfer from hydronium ion to α-methoxystyrenes: evidence for perfect synchronization between bond cleavage, bond formation, and positive-charge delocalization

Abstract

The CH-acidity constants in water for the oxocarbenium ions derived from ring-substituted α-methoxystyrenes have been calculated from the following literature data. (i) Equilibrium constants for oxocarbenium-ion formation from the corresponding acetals; (ii) equilibrium constants for the acetalto-enol ether process in methanol; (iii) Gibbs free energies of transfer of acetals and enol ethers from methanol to water. The plot of the logarithms of hydronium-ion-catalytic rate constants against the pK_a values of the intermediate ions exhibits a slightly curved relationship, with a mean slope β_s= 0.58, which can be accounted for by the Marcus equation with an intrinsic barrier of ca. 15 kJ mol^–1 and work terms W_r= 44 kJ mol^–1 and W_p= 52 kJ mol^–1. The agreement of all these parameters with those derived when the catalyst is changed indicates that C–H bond formation and O–H bond cleavage are synchronous concerted primitive changes. Separation of substituted-ring polar effects and direct resonance interactions from the overall substituent effects on rates and equilibria also shows that there is perfect synchronization between proton transfer and positive-charge delocalization.