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Abstract

The dissociation constants of nineteen ortho substituted benzoic acids have been determined in eight organic solvents (methanol, ethanol, acetone, dimethyl sulfoxide, dimethylformamide, acetonitrile, pyridine, 1,2-dichloroethane). The correlation between the σ_I, σ_R, and υ constants were unsuccessful due to neglecting the description of intramolecular hydrogen bond effect. The method of conjugated deviations has been applied to the results obtained and to those given in literature for ortho substituted benzoic acids (the dissociation constants, the reaction with diphenyldiazomethane, 33 sets), and values of three types of substituent constants have been determined for 29 substituents. The first of these substituent constants, σ_oⁱ, describes the electronic effects and was adjusted with the application of the isoparameter relation (σ_oⁱ as a function of σ_mⁱ) suggested in previous communications. This constant (after excluding the substituents NHCOCH₃ and OCOCH₃) correlates very well (R = 0.993) with the σ_I and σ_R constants. The second substituent constant, σ_HGⁱ, describes the interaction of the reaction centre (the oxygen atom of carboxylate anion) with the substituent, and it has non-zero values for the substituents OH, SH, NH₂, NHCH₃, NHCOCH₃, COOH, CONH₂, and SO₂NH₂. The third substituent constant, σ_Sⁱ, describes the steric effects and is not significantly related to any of the known quantities of this type. The set given was tested together with the triad of σ_I, σ_R, and υ on the definition set and on a set extended by other 28 sets of processes with ortho substituted compounds. On the whole, the set of substituent constants suggested explains 94.6% of variability of data, whereas only 66.0% are explained with the use of σ_I, σ_R, and u constants. Moreover, the tests have shown that the σ_oⁱ constant is not suitable for interpretations of processes involving direct conjugation between the reaction centre and substituent.