Octanol/water partition coefficients Kow: A critical examination of the value of the methylene group contribution to log Kow for homologous series of organic compounds

Abstract

The literature data for the values of the octanol/water partition coefficient KOW are examined critically, specifically in relation to the methylene group increment in logKOW – here called the lipicity L – for homologous series. With simply substituted alkanes, the plots of L versus the methylene group number m are linear, following the form: L=α+βm (Collander equation). The slope parameter β represents the methylene group increment, which widely is expected to be constant on the simplest theoretical grounds, and assumed to be so in most practical applications. The Collander equation behaviour for some 84 homologous series and subseries, ranging in complexity from the alkanes up to the alkyladenines and alkyl galactosides, is presented graphically. Compounds with α,ω-disubstituted alkyl chains give nonlinear Collander plots. The remaining series give linear Collander plots, but the methylene group increment β is not constant, the variation being statistically significant, with the distribution essentially normal (Gaussian) and with the mean value β¯=0.52 and standard deviation σ(β)=0.06. The literature data from other solvent/water systems – ethoxyethane (diethyl ether), and the two alkanes heptane and hexadecane – show similar behaviour. Most significantly, the fact that the methylene group increment β is not constant casts doubts on the applicability of the linear free energy approach, and of the “fragmental methods” that are widely used in interpreting and predicting partition coefficients. More generally, the graphical approach used is essential in a proper treatment of correlations of this kind; the graphs form an atlas that shows at a glance the partition coefficient behaviour for these series, revealing anomalies in the literature data that need to be rectified, and gaps that need to be filled.