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A simple re‐evaluation of the hammett ϱσ relation

1959; Royal Netherlands Chemical Society; Volume: 78; Issue: 10 Linguagem: Inglês

10.1002/recl.19590781009

1878-7096

H. van Bekkum, P. E. Verkade, B. M. Wepster,

Protein Structure and Dynamics

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.