Page 72 - Partition & Adsorption of Organic Contaminants in Environmental Systems

SUBSTITUENT CONTRIBUTIONS TO PARTITION COEFFICIENTS 63

Westall (1983) determined the K bw values of benzene and chlorinated ben-
zenes. For benzene (K ow = 135), the K bw value is about one-half the K ow value.
For trichlorobenzene (K ow 1050) and tetrachlorobenzene (K ow 4900) with
a significantly reduced water solubility, the K bw values become less than one-
fourth and one-fifth, respectively, of the corresponding K ow values. By contrast,
for a series of highly water-soluble small organic acids, amines, and alcohols,
Collander (1951) found that the K bw values are greater than the correspon-
ding K ow values when K ow £ 10; the biggest deviation, by a factor of about 3.5,
is with the most water-soluble solute (K ow 0.03). These characteristics are
consistent with the different solvent polarities of butanol and octanol and the
related solvent–water mutual solubilities. For most sparingly water-soluble
compounds, the K bw values would thus be small fractions of the K ow values; for
a wide variety of solutes, the K bw values should fall into a shorter range than
the K ow or K hw values. A similar result is expected for solutes in other organic
phase–water mixtures where the organic solvent or medium is relatively polar
in nature. With this consideration, the partition uptake of relatively water-
insoluble solutes by such highly polar organic phases as proteins, cellulose, and
carbohydrates should be very weak relative to the partition uptake by the oily
substances such as petroleum hydrocarbons, waxes, and biological lipids. In a
later section we will have an opportunity to look into the partition character-
istics of solutes in the lipid–water system and see which solvent–water system
examined so far best mimics solute partition behavior in the lipid–water
system.

5.5 SUBSTITUENT CONTRIBUTIONS TO
PARTITION COEFFICIENTS

The concept of substituent contribution to the partition coefficient of a sub-
stituted molecule with respect to a parent molecule was introduced by Fujita
et al. (1964) in medicinal chemistry and pharmaceutical science for estimating
the K ow values of some drugs and other chemicals in the absence of their exper-
imental values. It has gained relatively good success when applied to small and
structurally simple molecules but has had less success when extended to more
complicated molecules. To understand the feasibility and limitation of the
concept, one must unravel the physical basis associated with the contribution
of a substituent to the partition coefficient of a reference (parent) molecule.
Elucidation of the relevant factors on the substituent effect on K ow enables
one to understand not only the observed effect and limitation in the
octanol–water system but also the associated effect and limitation in other
solvent–water systems.
According to the convention adopted by Fujita et al. (1964), when a sub-
stituent X is incorporated into a parent molecule by replacing one of its
H atoms, the impact on the partition coefficient of the substituted molecule is
termed p X , which is defined as