A simple re‐evaluation of the hammett ϱσ relation

Abstract

Abstract Starting from ten selected σ values, a large number of ϱ values is calculated; these ϱ values in their turn serve to calculate other σ values. The results prove that the concept of “duality of σ constants” is erroneous. In those cases in which a substituent enters into mesomeric para interaction with the reaction centre, a multiplicity of (exalted) σ values is observed; this holds for +M as well as for −M substituents. In the absence of mesomeric para interaction σ values are reasonably constant; the mean of such σ values (σ n ) is listed. It is demonstrated that σ values are, in general, not suitable for judging mesomeric para interactions, and should be replaced by the (free) energy difference term involving not only the exaltation of σ, but ϱ and T as well. Several important applications of this concept are discussed, illustrating the way in which the ϱσ relationship can be most profitably used for mechanistic and predictive purposes.