Range of validity of the Hammett equation: acidity of substituted ethynylbenzenes

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Abstract
Acidities of 19 meta- and para-substituted ethynylbenzenes were calculated at the B3LYP/6-311+G(d,p) level and correlated within the framework of the Hammett equation with the calculated acidities of equally substituted benzoic acids. The substituent effects were decomposed in terms of isodesmic reactions into those operating in the anions and in the uncharged molecules. Characteristic deviations from the Hammett equation were found for para-substituents, both for acceptors and donors; the former can be interpreted by the resonance formula only with an electron sextet. With reference to the series of ionization reactions investigated previously, it was possible to reinvestigate the validity of the Hammett equation on the basis of calculated reaction energies using a more homogeneous data set than had been ever accessible from the experimental reactivities. The equation was fulfilled for all meta-substituents with a higher accuracy than commonly attainable with the experimental data. When para-substituents were included, deviations occurred according to the character of the functional group: When this group was an acceptor, the donor substituents showed deviations and vice versa. Another series of reactions proceeding between uncharged groups bonded directly on the benzene ring was investigated in the same way: The Hammett equation held with a similar precision, although its original range of validity was surpassed. The properties of a set of common substituents were investigated by principal component analysis and cluster analysis. There is a fundamental difference between uniform acceptors and more discriminated donors but clustering is not so strong to depreciate common statistical analysis.