Absolute proton affinities of some substituted toluenes: the additivity rule of thumb for ipso attack

The problem of the ipso protonation of toluene and its predominantly disubstituted derivatives was considered by the MP2(fc)/6–31G ** //HF/6–31G * +ZPE(HF/6–31G * ) theoretical model. The substituents involved covered a wide range of different donor–acceptor capabilities. It is shown that the calculated MP2 ipso proton affinities of substituted toluenes follow mutatis mutandis the same additivity rule which was found earlier to be operative in polysubstituted benzenes, naphthalenes and biphenylenes. The additivity equation is both intuitively appealing and useful, being able to offer quantitative estimates of the proton affinity by very simple calculation. It is based on the concept of the increment, which in turn describes the influence of a single substituent on the proton affinity. Any substituent behaves as a rule as if the other were non‐existent, thus giving rise to the independent substituent approximation (ISA). The performance of the additivity rule of thumb is very good, as evidenced by the average absolute deviation of 1 kcal mol −1 . Larger deviations are possible, but they rarely occur, being indicative of a difference in interactions between substituents in the initial neutral base and in the final cationic conjugate acid. Finally, it follows as a corollary of the present analysis that protonation ipso to the CH 3 group is never thermodynamically the most favourable site of proton attack in the benzene ring, provided that there is a single unsubstituted carbon atom within the aromatic moiety. The relevance of ipso protonation in persubstituted benzenes is briefly discussed. © 1998 John Wiley & Sons, Ltd.