Determination of Hammett sigma constants by intramolecular hydrogen bonding

Abstract

THE extensive literature that exists concerning Hammett's σ-constants has been subjected to several recent reviews, including a compilation and examination of available data1, a discussion of the significance and limitations of the concepts involved2, and an attempt to deduce best values and probable errors for the σ-values3. In most cases σ-values have been determined by examination of equilibrium constants or reaction-rate constants of substituted benzene derivatives, thus requiring consideration of product states or transition states. In the few instances where examination of the native (unreacted) molecule has led to a determination of σ (via physical measurements), the site examined has invariably been attached directly to the ring carrying the groups under study, thereby still exerting some influence (other than electronic) on the effect measured as well as excluding the ortho-position for steric reasons. © 1958 Nature Publishing Group.