Collection of Czechoslovak Chemical Communications - digital archive 

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 pK_a 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 σ_m and σ_p 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 pK_a 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.

Keywords: Dissociation constants; Benzoic acid; ortho Effect; Latent variable; Chemometrics; Substituent effects; AISE.

References: 61 live references.