Ortho Effect in Dissociation of Benzoic Acids with Electron-Accceptor Substituents Using the AISE Theory; Relation to para Substitution and Solvent

Abstract

Potentiometric titration at 25 °C has been used to measure the dissociation constants of 2- and 4-substituted benzoic acids with electron-acceptor substituents in methanol, N,N-dimethylformamide, acetonitrile, and acetone. But for a few exceptions, no significant differences have been found between the dissociation constants measured and those given in literature. Using the measured pKa values, those of 3-substituted derivatives taken from literature (measured in the same solvents) and those of 3- and 4-substituted derivatives measured in water, the so far missing substituent constants σi of NO and CH3SO substituents have been determined by Alternative Interpretation of Substituent Effects (AISE). The previously published relationships and the σi constants have been used to calculate the σm and σp constants for the given substituents and compare them with literature. The agreement was the better, the more reliable the corresponding Hammett substituent constant was. Using a non-linear regression model, we have analysed the structure of matrix of pKa values of ten 2-X-benzoic acids (X is electron-acceptor substituent) measured in the given organic solvents and in water. It has been found that in non-aqueous solvents the conjugate base of benzoic acid is stabilised by intramolecular hydrogen bond if X is COOH and SO2NH2 or electrostatic force if X is CH3SO. On the other hand, in water the carboxylate group is out of the plane of the benzene ring due to interaction between the solvated reaction centre and substituent, this being the case with all the substituents except for X = CHO, CH3CO, NO, and CN. In all the solvents used, intramolecular ring closure takes place between the carboxylate group and substituent X = CH3CO, CHO, and NO, this phenomenon being the most important in water.