QUANTITATIVE-ANALYSIS OF SOLVENT EFFECTS ON THE RATE OF HETEROLYSIS REACTIONS - RELATIVE IMPORTANCE OF CATION AND ANION SOLVATION

A number of heterolysis reactions which are insusceptible or less susceptible to hydrogen bonding by protic solvents are exemplified. It is emphasized that reactions of this type are as important as those of the hydrogen bond-susceptible type that are prone to be taken as a common feature of heterolysis reactions. Evidence that a primary factor in determining the type of reaction is the relative importance of cation and anion solvation is given by introducing a two-parameter equation for solvent effects on the rate: DELTA G(double dagger) = DELTAG(double dagger)DMSO - a (chi+ + bchi- - 1) in which chi+ and chi- are scales for cation and anion solvation for a particular solvent, respectively. These scales have been derived from purely kinetic data for eight solvents; methanol, ethanol, dimethyl sulfoxide (DMSO), N,N-dimethylformamide, acetonitrile, pyridine, nitrobenzene, and acetone. The equation applies satisfactorily to twelve typical heterolysis reactions. The coefficients a and b represent the sensitivity to solvent change and the relative importance of cation and anion solvation, respectively. When b = 1, the rate in DMSO is equal to that in methanol, and thus heterolysis reactions can be classified: b > 1, hydrogen bond-susceptible; b < 1, hydrogen bond-insusceptible.