Effect of substituents in aromatic hydrogen exchange : an examination of the Additivity Principle

Abstract

The rates of detritiation in trifluoroacetic acid have been measured for all the ring-hydrogen sites of the polymethylbenzenes. Results for toluene and the xylenes are in agreement with those reported in the literature, but the rate for benzene is slower than that obtained by earlier workers. The results show a general trend towards increasing departures from the Additivity Principle with increasing reactivity. The predictions of the Principle are consistently too high but deviations are found to be inexplicable in terms of substituent-reaction site steric interactions. Several modifications of the Principle are investigated. The N.M.R. spectra of the polymethylbenzenes have been examined in carbon tetrachloride and in n-hexane. Solvent shifts previously attributed to steric hindrance have been shown to correlate much better with the chemical shift. For both ring and methyl protons satisfactory prediction of chemical shifts by an additive relationship is possible only if large second order interactions between adjacent methyl substituents are admitted. It appears possible that such interactions may affect the behaviour of substituents in the detritiation reaction.