Substituent effects in aromatic detritiation reactions.

The literature on substituent effects in electrophilic aromatic hydrogen exchange reactions and in naphthalene compounds is reviewed A number of halogen-and methyl- (or methoxyl-) substituted benzenes, labelled with tritium in specific positions, and of substituted 1-tritionaphthalenes, have been prepared and their rates of detritiation in anhydrous trifluoroacetic acid have been measured by a simple scintillation counting method.The results provide a measure of the reactivities of the various aromatic positions which were labelled with tritium, and hence of the effects on these positions of the various substituents. The results for the naphthalenes are discussed in terms of the transmission of effects by conjugative, inductive and field effects and of steric (peri) interactions. The conclusions are considered in relation to the small amount of data available on the more familiar aromatic substitutions (such as bromination and nitration) and the more numerous results on side chain reactions, and some anomalies are noted. The order of decreasing interaction between the 1-position and those positions with which it can conjugate directly is 2>4>7>3. For halogen-substituted toluenes and polymethylbenzenes the deactivating effect of a chloro-or bromo-substituent on the meta position has been found to vary with the reactivity of the system: this is also true for the tritiated chloroanisoles but here the results are complicated by the fact that some interaction occurs between the methoxy-substituent and the trifluoroacetic acid.