Estimation of electronic effects in aromatic systems

A critical examination of the use of Hammett's sigma(X) values is presented. The problems due to the possible existence of two forms of benzoic acids have been discussed as well as the problems with resonance. A parameter sigma'(X)(m) being valid only for meta substituents, is the only parameter that can be recommended for general use. For local use the Delta(B), Delta(Ph), and Delta(An) values, defined in this study, are recommended.