Octanol-water partition congenerity

Two approaches to quantify/fQ, i.e., to establish a quantitative relationship between the structural features of a compoimd and its properties, are described in this section quantitative structure-property relationships (QSPR) and linear free energy relationships (LFER) cf. Section 3.4.2.2). The LFER approach is important for historical reasons because it contributed the first attempt to predict the property of a compound from an analysis of its structure. LFERs can be established only for congeneric series of compounds, i.e., sets of compounds that share the same skeleton and only have variations in the substituents attached to this skeleton. As examples of a QSPR approach, currently available methods for the prediction of the octanol/water partition coefficient, log P, and of aqueous solubility, log S, of organic compoimds are described in Section 10.1.4 and Section 10.15, respectively. 

Trone, M. D., Leonard, M. S., Khaledi, M. G. Congeneric behavior in estimations of octanol-water partition coefficients by micellar electrokinetic chromatography. Anal. Chem. 2000, 72, 1228-1235. 

The hydrophobicity term as measured by log P, the log n-octanol/water partition coefficient, indicates that the ability of an odorant to partition from the medium in which it is dissolved into the atmosphere and its ability to partition through mucus and membreine layers to reach olfactory receptor sites is highly correlated to odor intensity. Results of this study also indicated that within a congeneric series, the analogs with the highest volatilities are not necessarily the most intense odorants. 

One of the most frequently used physico-chemical descriptors of similarity is the logarithm of the octanol/water partition coefficient (log P, or log Kq, ) and its significance and application is well documented (Hansch and Leo 1979). While it has been recognized that any reliable prediction can only be made within a congeneric series of compounds (Rekker 1985), the a priori definition of congenericity is still problematic in many cases. Consequently, many QSAR equations are limited to very narrowly defined sets of substances and cannot be generalized. 

It is suggested that when assessing the environmental fate or toxicity of organic chemicals, especially those of congeneric series, such as PCBs, it is useful to gather and critically review their basic properties including solubility, vapour pressure, octanol/water partition coefficient and Henry s law constant. This approach is illustrated for three series of chemicals, the chlorobenzenes, the polychlorinated biphenyls and the chlorinated dibenzo-p-dioxins. 

Chlorinated aromatic hydrocarbons, congeneric series, chlorobenzenes (CBs), polyclorinated biphenyls (PCBs), polychlorinated dibenzo-p-dioxins (PCDDs), solubility, vapour pressure, octanol/water partition coefficient, Henry s law constants, correlations, chlorine number, molar volume. 

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