Hyperbolic Model for the meta – para interrelationship in benzene derivatives

A new approach to the long‐standing problem of interrelating meta and para substituent constants is presented. An analysis of the unified σ 0 ‐scale shows that the interrelation between σ   4 0and σ   4 0 /σ   3 0can be modelled by a pair of conjugate rectangular hyperbolae, one for normal (n) and the other for special (s) substituents. The latter are characterized by a lone electron pair in the first atom. The equations σ   4n 0(σ   4n 0− γ 0 )/(σ   4n 0− 2γ 0 ) = λ 0 σ   3n 0and σ   4s 0= γ 0 + λ 0 σ   3s 0are derived and discussed in terms of Taft's separation of mesomeric and non‐mesomeric effects. Asymptotic values λ = 0.960 γ = −0.225 were obtained by non‐linear least rectangles fitting. A nonnegligible mesomeric contribution to σ 0 constants for normal substituents is predicted by the hyperbolic model. The present results are at variance with Exner's analysis of the meta ‐ para interrelationship in benzene compounds with normal substituents. This divergence is ascribed to opposite views concerning the role of the π‐inductive effect.