Kinetic equations for reactions in concentrated aqueous acids based on the concept of "excess acidity"

Abstract

ROBIY A. COX and KEITH YATES. Can. J. Chem. 57,2944Chem. 57, (1979 Kinetic equations, applicable to A-1, A-S,2, and A-2 reactions in concentrated aqueous acids, are derived. The variation in reaction rate with varying acid concentration is treated in terms of the "excess acidity" of the medium (X-function), rather than in terms of Hammetttype acidity functions or the water activity. The parameters obtained are the medium-independent rate constant k,, in the aqueous standard state, as an intercept, and a slope parameter 1~1 ' ; hydration parameters (r-values) are also obtained, for A-2 reactions. The equations derived are shown to apply to A-1 acetal hydrolyses, A-SE2 electrophilic aromatic substitutions, and mixed A-21A-I ester hydrolyses. In a general discussion of available methods for analyzing rate data in these media, it is shown that the X-function method encompasses most, if not all, of the others, and that classical acidity functions are no longer necessary. Dans une discussion genirale sur l'ensemble des methodes disponibles d'analyse de donnees de vitesse en milieu acide on a montre que la methode de la fonction de X englobe la plupart si ce n'est pas toutes les autres methodes, et que les fonctions classiques d'acidite ne sont plus nkcessaires.[Traduit par le journal]

The hydrolyses of benzamides, methylbenzimidatium ions, and lactams in aqueous sulfuric acid. The excess acidity method in the determination of reaction mechanisms

“…For instance for benzamide, with r = 3 (the most likely value (17)) the plots are horizontal until about half-way and then deviate upward; with r = 1 they curve down before levelling out. In view of the consistent linearity found before with plots of this type (6)(7)(8) this can only mean that a gradual mechanism change is occurring at intermediate acidities.…”

“…It is involved in some data well at low acidity, but is not easy to interpret mechanistically (26). Because of this situation we decided to apply the excess acidity method, which is capable of providing considerable mechanistic information (6)(7)(8), to this process.…”

“…[2] (6). For an A-1 reaction the equation L21 log kI$ -log (CSHt/(CS + CSH+)) = (m* -1) m*X + log aN, + log ko is the same except that the term log a~, is not present (6). The observed pseudo-first-order rate constant is k$, and CsH+ and Cs are concentrations of protonated and unprotonated substrates, respectively; the protonation correction term containing these is negligible for full protonation.…”

“…X is the excess acidity, and aNu is the activity of the nucleophile or base involved in the A-2 reaction, water or bisulfate ion in sulfuric acid (6-8,3 1). m * and m* are slope parameters, expressing the sensitivity of the substrate and transition state, respectively, to the changing acidity (6), and ko is the medium-independent second-order rate constant. Although the equivalent equations for predominantly unprotonated substrates have been used extensively (6-8), eq.…”

“…All the activation parameters obtained in this study, using eqs. [2] and [3], are valid at 25°C in the aqueous standard state (6)(7)(8).…”