Carbon-13 N.M.R. Studies of Carbocations. III. An Investigation of the Variation of ipso-Substituent Chemical Shifts with Electron Demand in 1,4-Disubstituted Benzenes

Abstract

¹³C n.m.r. spectra have been obtained of a large range of 1(X),4(Y)-disubstituted benzenes in which X has been varied over a range of 25 substituents from NMe₂ to ⁺CHMe for each of the compounds where Y = H, OMe, Me, F, Cl, Br and CF₃. The ipso-substituent chemical shifts (ipso-SCS) for each of the latter (that is, the change in chemical shift (Δδ) of C4 by replacement of H by Y) have been shown to vary dramatically with the electron demand of the X substituent as measured by δ(C4). When plotted against &3948;(C4), the ipso-SCS of F and OMe both decrease linearly with increasing electron demand whilst those of Br and Cl show linear increases. Those of Me and CF₃ show discontinuities which indicate changes in the mechanism of interaction of these groups with the attached ipso-carbon. The variations in the ipso-SCS with electron demand of X are considered to be due to Y-induced variations in the sensitivity of the ipso-carbon to the effect of the para (X) substituent and not to through-conjugation effects. The results clearly show the fallacy of assuming that ¹³C substituent effects are constant.