A critical examination of the use of Hammett’s σ(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 σ′(X)m being valid only for meta substituents, is the only parameter that can be recommended for general use. For local use the ΔB, ΔPh, and ΔAn values, defined in this study, are recommended.
A. Brändström, J. Chem. Soc., Perkin Trans. 2, 1999, 1847 DOI: 10.1039/A904818I
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