Studies of substrate selectivity in aromatic iodination and other substitution reactions reinforce previous conclusions about the nature of the mechanism of electrophilic aromatic substitutions

Abstract

We have investigated substrate selectivity as a probe to distinguish between electron-transfer (ET) and conventional polar mechanisms of electrophilic aromatic substitution. Selectivity toward mesitylene and durene, in competition experiments, has been determined for iodination, bromination, acetylation, mercuriation and thallation reactions under the same experimental conditions. In all cases mesitylene, i.e. the substrate with the higher σ-basicity, was more reactive than durene; a similar behaviour was shared by two other pairs of substrates, namely, mesitylene/naphthalene and m-C₆H₄(OMe)₂/p-C₆H₄(OMe)₂, where again the more reactive substrate within each pair was that with the higher σ-basicity. These findings suggest that the structure of the transition state of the above reactions resembles that of the σ-complex, and would therefore endorse the conventional polar mechanism of electrophilic aromatic substitution. Only for the nitration reactions were the experimental results too ambiguous to allow a definite mechanistic conclusion to be reached.