The octanol–water partition coefficient of aromatic solutes: the effect of electronic interactions, alkyl chains, hydrogen bonds, and ortho-substitution

Abstract

The calculation from structure of the hydrophobic parameter, log P(octanol–water), involves the addition of fragment values (or of π constants to parent log P) plus correction factors for interactions not present in the standard state from which the f or π values were determined. In this paper the important correction factors for multply substituted aromatic solutes are classified as: electronic, negative ortho, hydrogen bonding, and alkyl–aryl. The electronic factor F_σ is best treated as a continuous function in a manner similar to Hammett's ρσ product. Both field and resonance components appear to be present in the electronic effect. σ and ρ values for 50 substituents are reported in a generalized structural form which makes possible estimation of many others. While the other factors F₀,F_HB, and F_αφ are probably continuous functions also, they are conveniently treated as ‘quantized’. Calculated in this way, the standard deviation for nearly 400 solutes amounts to less than twice the estimated error in their measurement, and thus a more precise estimation of these effects is unwarranted at this time. The overall equation is: observed log P= additive log P+ρσ– 0.29F₀+ 0.63F_HB– 0.15F_αφ.