Kinetic and equilibrium aspects of the effects of salts and nonelectrolytes upon the ionization equilibrium of tris-p-methoxyphenylmethanol in aqueous solution at 25°

Abstract

The rates of formation and disappearance of the R₃C⁺ ion in aqueous solution at 25° are measured in the presence of several electrolytes and nonelectrolytes, the effects of which upon H_R, also reported, can therefore be interpreted in terms of these two opposing reactions. (It is first verified that the rate of disappearance of the ion is not complicated by general base catalysis.) Additives generally influence the forward and reverse reaction rates in opposite ways and thus produce still more marked effects upon the equilibrium [R₃C⁺]/[R₃COH] ratio and H_R.

Electrolytes and nonelectrolytes also have large and varied effects upon the solubility of R₃COH, but although these resemble the effects of the additives upon the rate of formation of the ion, no simple correlation with the latter emerges. Instead a good correlation is found between the rate of formation of R₃C⁺ and the Hammett function H₀, implying that the reasons for the peculiarities in the H_R scale are to be sought in effects upon the reverse reaction. Perchlorates [which like (CH₃)₄NCl behave similarly to the nonelectrolytes in their effects upon the solubility of R₃COH] have a very marked effect upon the reverse reaction even at quite low concentrations and this behaviour (interpreted tentatively in terms of a ‘structure enforced association’ between R₃C⁺ and ClO₄^–) is believed to hold the key to the absence of a simple correlation between H_R and water activity for strong acids.