Collect. Czech. Chem. Commun. 1999, 64, 1607-1616

Lower Bounds to Hammett σ Constants for meta and para Normal Substituents

João Carlos R. Reisa,*, Manuel A. P. Seguradob and Jaime D. Gomes de Oliveirac

a Departamento de Química e Bioquímica, Centro de Electroquímica e Cinética, Faculdade de Ciências, Universidade de Lisboa, 1749-016 Lisboa, Portugal
b Centro de Electroquímica e Cinética and Faculdade de Farmácia, Universidade de Lisboa, 1649-019 Lisboa, Portugal
c Centro de Electroquímica e Cinética and Instituto Superior de Engenharia de Lisboa, 1949-014 Lisboa, Portugal


Twenty-four pairs of meta and para Hammett σ constants recommended by IUPAC for normal substituents, i.e. dipolar groups without a lone pair of electrons (and with a full octet) in the atom next to the aromatic ring, were analysed with respect to their meta-para interrelationship. In terms of a previous hyperbolic model, the para/meta ratio of the universal electric effect is found to be 0.964 with a standard error of 0.028 estimated by Monte Carlo simulation. This value for benzoic acid derivatives supports the view that the universal electric effect, which is proposed to be termed the Electra effect, is transmitted through space. For normal substituents, it is demonstrated that the hyperbolic model predicts lower bounding values of σm and σp constants. It is proposed that organometallic substituents in which a metallic atom is bounded to the α carbon should not be considered as normal substituents. It is asserted that no normal substituent should exist for which either σm is less than -0.1 or σp is less than -0.3 in the Hammett σ scale.

Keywords: meta/para Ratio; Substituent constants; Substituent effects; Through-space interactions; Hammett equation; Hyperbolic model; Chemometrics.