Empirical relations between σ I, σ oR and taft σ* values of alkyl, acyl, benzoyl and substituted phenyl groups

Abstract

The X(X) values 1 of the halogens (which resemble the Pauling electronegativities) and of some oxa substituents can be interpreted in terms of the inductive and resonance parameters σ I and σ o R according to the regression equation ▪ and η* R=η(X)−η(R) it is found that for some substituted methyl, phenyl and benzoyl groups [σ*] X R=αη X R where α equals −10.6 and −10.9 for R = Me and R = Ph, CHO and PhCO respectively. Thus [σ*] X Rand η x r represent Taft σ* and [σ I(X)−σ o R(X)] values relative to that of the parent R group. The hydroxyl frequencies of phenol, and benzoic, acrylic, acetic and formic acids measured in dilute carbon tetrachloride solutions correlate with σ I(X) and σ o R(X) according to the equations v(OH) = −423.0 σ I(X) + 3654.7 v(OH) = −270.0 σ 0 R(X) + 3586.7 where X = Ph, PhCO, CH 2=CHCO, MeCO and HCO. From these results, it is inferred that the σ* values of substituents having an α sp 2 hybridized carbon atom are proportional to σ 0 R according to the equation σ*(X) = 3.97 σ 0 R(X) + 1 New σ I σ o R and σ* values of some acyi, benzoyl and substituted phenyl groups are presented.