Hydrophobicity parameter of diazines. III: Relationship of partition coefficients of monosubstituted diazines and pyridines in different partitioning systems.

The logarithm of the 1-octanol-water partition coefficient value (log Poct) was compared with those from CHCl3-water (log PCL) and di-n-butyl ether-water (log PE) for (di)azines substituted singly by nonhydrogen-bonding and hydrogen-accepting substituents (2-substituted pyrazines, 2-substituted pyrimidines, 5-substituted pyrimidines, and 2-substituted pyridines). The difference between log Poct and log PCL for diazines was primarily governed by the number of hydrogen-bonding sites in the substituent. For 2-substituted pyridines, the difference in the hydrogen-bonding association of the ring N-atom with octanol from that with CHCl3 was also significant. In the relationship between log Poct and log PE, the hydrogen-bonding solvations of the ring N-atom(s), as well as the hydrogen-accepting substituent with octanol, should be taken into account because the butyl ether acts as a nonhydrogen-bonding solvent.