Evaluation of the polar-inductive and mesomeric effects exerted by para-substituted phenyl rings on contiguous functionalities

Abstract

The para C-13 shifts of the phenyl ring in PhCH₂Ar (I) is used as a monitor to evaluate, in terms of the previously defined σ_IB constants, the polar-inductive effect exerted by some para-substituted aryl rings. Analogously, the para C-13 shift of the phenyl ring in PhNHAr (IV) is used as a monitor to evaluate, in terms of the previously defined σ_C^– constants, the blended polar-inductive and mesomeric effects exerted by such para-substituted aryl rings. The same para C-13 data offer access, through a biparametric (DSP) treatment, to σ_R^– constants, which account for the mesomeric effects exerted by the same substituents. C-13 Shifts data are also reported for the 4′-substituted sodium aryl(phenyl)amides PhN^–Ar (VII) which have been prepared in Me₂SO by deprotonation of PhNHAr. The σ_C^– values just obtained account successfully for the para C-13 shifts of the phenyl ring of (VII), for the para C-13 shifts of the phenyl ring of phenylhydrazones of para-substituted benzaldehydes, and for the acidity of aryl(phenyl)amines measured in Me₂SO–H₂O by Dolman and Stewart. The success of the biparametric treatment is limited by the small range of the σ_IB values of the para-substituted aryl derivatives and by the scarcity of data. The σ_IB set is linearly related to the set of σ_I constants obtained from substituted acetic acids. Both the σ_IB and the σ_C^– sets account successfully for the acidity of para-substituted benzoic acids in H₂O, in Me₂SO, and in the gas phase: a rationale is given.