engleski

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 is considered by the MP2(fc)/6-31G**// HF/6-31G* + ZPE(HF/6-31G*) theoretical model. Substituents involved cover 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 formula is both intuitively appealing and useful being able to offer quantitative estimates of the proton affinity by the "back of an envelope" calculation. It is based on the concept of the increment, which in turn describes influence of a single substituent on the proton affinity. Any substituent behaves as a rule as if the other were nonexistent thus giving rise to the independent substituent approximation (ISA). Performance of the additivity rule of thumb is very good as evidenced by the average absolute deviation of 1 kcal/mole. Larger deviations are possible, but they occur rarely 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 corrolary of the present analysis that the protonation ipso to the CH3 group is never thermodynamically the most favourable site of the proton attack in the benzene ring, provided there is a single unsubstituted carbon atom within the aromatic moiety. Relevance of the ipso protonation in persubstituted benzenes is briefly discussed.

silacycloproparenes; ab initio calculations; B3LYP calculations; bond alternations; reversed Mills-Nixon effect

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