CHEMOMETRICAL ANALYSIS OF SUBSTITUENT EFFECTS .5. ORTHO EFFECT

The dissociation constants of nineteen ortho substituted benzoic acids have been determined in eight organic solvents (methanol, ethanol, ac etone, dimethyl sulfoxide, dimethylformamide, acetonitrile, pyridine, 1,2-dichloroethane). The correlation between the sigma(I), sigma(R), a nd upsilon constants were unsuccessful due to neglecting the descripti on of intramolecular hydrogen bond effect. The method of conjugated de viations has been applied to the results obtained and to those given i n literature for ortho substituted benzoic acids (the dissociation con stants, the reaction with diphenyldiazomethane, 33 sets), and values o f three types of substituent constants have been determined for 29 sub stituents. The first of these substituent constants, sigma(o)(i), desc ribes the electronic effects and was adjusted with the application of the isoparameter relation (sigma(o)(i) as a function of sigma(m)(i)) s uggested in previous communications. This constant (after excluding th e substituents NHCOCH3 and OCOCH3) correlates very well (R = 0.993) wi th the sigma(I) and sigma(R) constants. The second substituent constan t, sigma(HG)(i), describes the interaction of the reaction centre (the oxygen atom of carboxylate anion) with the substituent, and it has no n-zero values for the substituents OH, SH, NH2, NHCH3, NHCOCH3, COOH, CONH2, and SO2NH2. The third substituent constant, sigma(S)(i), descri bes 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 sigma(I), sigma(R) and upsilon 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 e xplains 94.6% of variability of data, whereas only 66.0% are explained with the use of sigma(I), sigma(R), and upsilon constants. Moreover, the tests have shown that the sigma(o)(1) constant is not suitable for interpretations of processes involving direct conjugation between the reaction centre and substituent.