Collection of Czechoslovak Chemical Communications - digital archive 

Abstract

Three model compounds have been selected to study the relationship between ortho and para substitution: benzoic acid, phenol, and aniline. Sixteen substituents have been chosen involving also those capable of potential interaction between ortho substituent and the reaction centre. For the combinations given, literature presents 25 pairs of data obtained by measuring a particular process for both the ortho and para substituted derivatives. The missing dissociation constants of 16 ortho substituted benzoic acids in water and ethanol and 16 para substituted benzoic acids in dimethyl sulfoxide and pyridine have been measured by potentiometric titration. The data matrices were submitted to analysis by the methods of projection of latent structures (PLS) and principal component analysis (PCA). It has been found that the substituent effects from ortho and para positions have the same character unless the ortho substituents interact with the reaction centre. Such interactions can change the experimentally found value by as much as 20% of its magnitude. The most significant interaction is a hydrogen bond formation. Out of the three models studied the most extensive interactions are present in benzoic acid, whereas almost none were observed in aniline. The capability of donation of electron pair to a hydrogen bond decreases in the substituent series COCH₃ > SO₂CH₃ > NO₂. The capability of donation of proton to a hydrogen bond with electron-pair donor decreases in the substituent series OH > NHCOCH₃ ≈ SH > NH₂ > SO₂NH₂.

Keywords: Substituent effects; ortho Effect; Dissociation constants; PLS; PCA; Benzoic acid; Phenol; Aniline; Chemometric.