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 σ0‐scale shows that the interrelation between σ 40 and σ 40/σ 30 can 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 4n0 (σ 4n0 − γ0)/(σ 4n0 − 2γ0) = λ0 σ 3n0 and σ 4s0 = γ0 + λ0 σ 3s0 are 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 σ0 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.