John Bromilow

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

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.