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The H M O method is a simple but powerful approach toward ex plaining the stabilities, physical properties, and chemical reactivities of organic π-systems. The σ-electronic frameworks of these systems are relatively uninteresting in a theoretical sense since the properties of σ-bonds do not vary significantly from structure to structure. The H M O results that one can obtain easily are admittedly crude and approximate, being based on a num ber of gross assumptions or approximations. However, these results are capable of explaining and predicting a large am ount of interesting chemistry and seldom if ever give answers that are clearly in disagreement with either chemical experience or chemical intuition.If we start with the equationin which is the electronic Hamiltonian operator, including all electronic and nuclear interaction terms, this is completely insoluble for any organic molecule containing π-electrons (recall the form of this operator for ethylene). Therefore, it is necessary to make several basic assumptions and to use Postulate V to obtain approximate "eigenvalues" and approximate "eigenfunctions" for these π-systems. 27 II Hückel Molecular Orbital TheoryUsing Postulate V, we have for each ij/j J dx =

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]

. 61, 2225 (1983).The subject of acidity functions is reviewed, with emphasis on rnodcrn developrncnts. H o and othcr scales determined in acid media, using the Hammett cancellation assumption (zero'th order approximation) are discussed bricfly. The extension to strongly basic systems is also documcnted. Literature references to acidity function scalcs in aqueous acid, aqueous acidlorganic solvcnt, and aqueous base media are tabulated, togcther with those in aqueous baselorganic solvent mixtures at both variable and fixed base concentration. Over 400 acidity functions are referenced, including several electrochemical scales. Various approaches to developing universal functions from the data are discussed. Proton activity scales, based on the Yates-McClelland method of pKBH+ determination, and on electrochemical HoF scales, are prominent among these. Carpentier's method is mentioned, and the first-order approximation, assumed linearity among log activity coefficient ratios, is used to introduce the Bunnett-Olsen and excess acidity (X-function) methods. A brief critical discussion of which method to use is provided. Future developments anticipate the second-order approximation, with a quadratic dependence of log ionization ratios upon X-functions. The review cites 337 individual references. [Traduit par le journal]

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