Field and charge-transfer theory for the quantitative correlation of substituent effects in aromatic molecules. Part IV. Non-additivity in electrophilic substitution reactions

By the theoretical model described in Part III the additivity principle for substituent effects should fail in aromatic electrophilic substitutions. The non-additivity is predicted to arise because the charge-transfer effect of each substituent depends on the inductive power of all the substituents. Here the non-additivity is calculated for several series of polyhomosubstituted benzenes and heterodisubstituted benzenes. The results indicate that, although it should provide a good approximation when applied to polyalkylbenzenes, the additivity principle should lead to considerable errors when applied in the general case. From the calculations a number of predictions are made concerning the pattern of non-additivity. The outcome of empirical checks on the theoretical predictions gives confidence that the FCT theory of non-additivity may provide a much better theoretical tool than the additivity principle.