Chemometric Analysis of Substituent Effects. VII. Inductive Effect as a Basic Substituent Effect. Isoeffect Substituent Constant

Abstract

The paper deals with chemometric analysis of the inductive effect. The notion of inductive effect is discussed, and unambiguous definitions are given for the notions of triad: reaction centre-basic skeleton-substituent, and the therewith connected definitions of inductive effect. For a quantitative description of inductive effect 7 types of chemical models were selected including noncyclic compounds, cyclic, and bicyclic compounds, derivatives of quinuclidine, 3-substituted benzoic acids, sulfonamides and pyridines. Altogether 139 sets of experimental data from literature have been used including altogether 1 294 points (9.3 points per set, 5 points at least) reflecting substituent effects of 34 substituents. It has been found that for a standard model the dissociation of substituted bicycloalkanecarboxylic acids only is satisfactory, all the other models reflecting also the mesomeric effects to variable extent (up to 10%). A distinctly different substitution behaviour was observed with 19F and 13C NMR chemical shifts of 4-substituted 1-fluoro- or 1-methylbicyclo[2.2.2]octanes. The earlier suggested model of substituent effects based on different way of transmission of substituent effects (3 classes) has been used for separating the inductive and mesomeric effects: it is mathematically presented as a set of straight lines with the intersection point at the so-called isoeffect substituent constant. Using the modified method of conjugated deviations a chemometric scale has been created for the inductive effect which agrees very well with the conventional scales given in literature; the only differences were observed for F and CH=O substituents (which are overestimated and underestimated, respectively, in literature). In the context given the inductive effect appears as a fundamental quantity forming a basis for quantitative description of other effects transferred by electrons.