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 d isubstituted derivatives was considered by the MP2(fc)/6-31G*//HF/6-3 1G + ZPE(HF/6-31G*) theoretical model. The substituents involved cove red a wide range of different donor-acceptor capabilities. It is shown that the calculated MP2 ipso proton affinities of substituted toluene s follow mutatis mutandis the same additivity rule which was found ear lier to be operative in polysubstituted benzenes, naphthalenes and bip henylenes. The additivity equation is both intuitively appealing and u seful, being able to offer quantitative estimates of the proton affini ty by very simple calculation. It is based on the concept of the incre ment, which in turn describes the influence of a single substituent on the proton affinity. Any substituent behaves as a rule as if the othe r were non-existent, thus giving rise to the independent substituent a pproximation (ISA). The performance of the additivity rule of thumb is very good, as evidenced by the average absolute deviation of I. 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. Finall y, it follows as a corollary of the present analysis that protonation ipso to the CH3 group is never thermodynamically the most favourable s ite of proton attack in the benzene ring, provided that there is a sin gle unsubstituted carbon atom within the aromatic moiety. The relevanc e of ipso protonation in persubstituted benzenes is briefly discussed. (C) 1998 John Wiley & Sons, Ltd.