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 σIB 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 σC– 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 σR– 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 σC– 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 σIB values of the para-substituted aryl derivatives and by the scarcity of data. The σIB set is linearly related to the set of σI constants obtained from substituted acetic acids. Both the σIB and the σC– sets account successfully for the acidity of para-substituted benzoic acids in H2O, in Me2SO, and in the gas phase: a rationale is given.