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À propos de : Background of the Hammett Equation As Observed for IsolatedMolecules: Meta- and Para-Substituted Benzoic Acids        

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  • Background of the Hammett Equation As Observed for IsolatedMolecules: Meta- and Para-Substituted Benzoic Acids
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  • Fundamental model compounds for the Hammett equation, meta- and para-substituted benzoicacids, were investigated by the density functional theory at the B3LYP/6-311+G(d,p) level. Energiesof 25 acids and of their anions were calculated in all possible conformations and from them theenergies of the assumed mixture of conformers. Relative acidities correlated with the experimentalgas-phase acidities almost within the experimental uncertainty, much more precisely than in thecase of previous calculations at lower levels. Dissection of the substituent effects into those operatingin the acid molecule and in the anion was carried out by means of isodesmic reactions startingfrom monosubstituted benzenes. Both effects are cooperating in the resulting effect on the acidity;those in the acid molecule are smaller but not negligible. They are also responsible for somedeviations from the Hammett equation (through-resonance of para donor substituents) and for theweaker resonance in the acid molecule in meta derivatives; in the anions the inductive and resonanceeffects are almost equal. On the other hand, the cooperation of effects in the acid and in the anionmakes the relative acidity more sensitive to electron withdrawing and is probably one of the reasonswhy the Hammett equation is so generally valid.
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