Hydrophobicity Parameter of Diazines (1) Analysis and Prediction of Partition Coefficients of Monosubstituted Diazines

The octanol/water partition coefficient (P) of a number of monosubstituted diazines was measured. The composition of the π value of substituents, the increment in the logP value accompanied with the introduction of substituents, was examined in terms of physicochemical substituent parameters and correlation analysis. The diazine-π value of substituents was generally higher than the pyridine-π value of corresponding substituents, indicating that the intramolecular electronic interactions between the ring-N atoms and substituent are more pronounced than those in substituted pyridines in governing the logP value of the molecule. Except for 2-substituted pyrimidines, the π value of substituents in each series of monosubstituted diazines was in general nicely correlated with the π value of the corresponding substituents in substituted pyridines along with electronic parameter terms representing “bidirectional” electronic effects on the relative solvation of the ring-N atom(s) and the hydrogen-bondable substituents with partitioning solvents according to the procedure proposed previously for the analysis of the π value in disubstituted benzenes and monosubstituted pyridines. Keeping in mind that 2-pyrimidines substituted by hydrogen-bondable groups sometimes behave as outliers, the correlations were believed to be usable for prediction of logP value of monosubstituted diazines.