Nontraditional interpretation of deviations from the bronsted relationshipL effect of structural and solvation factors on the equilibrium and kinetic characteristics of the CH acidity in polar basic nonaqueous media

摘要

A concept is suggested for explaining deviations of certain CH acids from the linear Bronsted relationship. The major attention is given to ion-dipole interaction between the molecules of a polar aprotic solvent and dissolved carbanions.The concept is based on correcting the experimental pK_a values by eliminating the contributions from electrostatic solvation using data on the ionization constants of CH acids in the gas phase (DELTAH~g). By the examples of acid dissociation of methyl CH bonds under conditions of established equilibrium in DMSO and kinetic data for deuterium exchange in a solution of an alkali metal amide in liquid ammonia, it was shown that for a series of compounds PhM(CH_3)_(n-1) and M(CH_3)_n (M=N,O,P,S,CO), and also for some other methyl derivatives all kinds of deviations from the Bronsted relationship are realized (those originating from specific conditions of stabilization of the transition state in the course of reaction; those due to electrostatic solvation of the equilibrium carbanions; and the deviations resulting from combined effect of the kinetic and equilibrium factors). Also, the Bronsted relationships were obtained for ionization of CH acid series covering compounds with aliphatic, quasiaromatic, and aromatic CH bonds (the deuterium exchange was performed in liquid ammonia without a catalyst). The Bronsted coefficient alpha 0.65 in this relationship suggests a higher degree of deprotonation of the CH bond in the transition state as compared to the catalytic exchange (alpha 0.3). The revealed difference is due to the different effect of electronic factors: In the catalytic deuterium exchange, the major contribution to the activation free energy (log kappa_D) is made by the resonance effects of substituents, whereas in the noncatalytic deuterium exchange the inductive effect prevails.