A discussion of the Hammett acidity function. Study of some weak bases

Abstract

Important variations in the pK_a values of several weak bases have been obtained by using different acidity functions H_o valid for HCIO₄ and with the application of the Hammett equation; these discrepancies may be reduced by using a corrected form of the Hammett equation in which the ratio intercept slope is constant and coincides with the pK_a irrespective of the H₀ function utilized; the smallest standard deviations are observed within the range log I 1 to –1. The H₊ and H_R functions are not well behaved, even for cationic bases such as o-phenylenediamine, adenine, and guanine. All these results may be interpreted by a linear relationship among the activity coefficients.