Rationalisations among Heterocyclic Partition Coefficients Part 2: The Azines

Azine π‐values are discussed in terms of Δπ, which is the difference in π‐value from that expected for benzene. It is shown that Δπ is close to zero for alkyl and most halogen groups, but for polar substituents capable of hydrogen bonding may be as high as ca. 1.6. Except for peri ‐positions, these Δπ‐values may be correlated by a set of equations specific for different types of substituent position and containing terms which separately parameterise proton donor and acceptor ability. The rationale behind this treatment is justified in terms of the nature of the octanol‐water partitioning process and the manner in which electronic effects are expected to operate, in this context and that of the individual molecule. These equations will correlate those compounds for which they are expected to be applicable to Δ logP ± 0.12. Other topics discussed include: reasons for deviations among “irregular” substituents; the special problems of peri ‐positions; multi‐substitution; and some consequences of this analysis for other types of compound.