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 sig ma'(X)(m) being valid only for meta substituents, is the only parameter tha t can be recommended for general use. For local use the Delta(B), Delta(Ph) , and Delta(An) values, defined in this study, are recommended.