Environmental Engineering Reference
In-Depth Information
specific sub-unit and that they are additive. Hence we can write
log
K
ow
=
b
j
+
B
k
.
(3.51)
j
k
Thefragmentconstantsarefundamentaltothesub-unit
j
whilethestructuralfactors
relate to specific intermolecular forces between the sub-units. The value of
b
j
for a
specific subunit will be different based on which other atom or sub-unit it is attached
to. For example, a
Cl atom attached to an alkane C has a different
b
j
from the one
attached to an aromatic C. Furthermore, a
−
−
Cl attached to an unsaturated
−
C
≡
unit
will be different from the one attached to a
−
C
=
unit. In effect the
π
-electron cloud of
the
unit. Substituents attached to an
aromatic C unit typically make less of a contribution to log
K
ow
than those attached to
an alkane C atom. One should also expect widely different contributions if a nonpolar
groupsuchas
−
C/unit reacts differently from the one in a
−
C
=
−
−
Clatominthemolecule.Theintermolecularforces
among fragments and subunits are characterized by the
B
factor. The more complex
the stereochemistry of the molecule, the less contribution it makes toward log
K
ow
.
The interactions between polar moieties in the molecule give rise to electronic factors.
Increasing polarity invariably decreases the contribution toward log
K
ow
.
For almost any new compound manufactured today, one can obtain log
K
ow
in this fashion. Computer software exists to compute log
K
ow
using this method
without even knowing the structure of the molecule. The reliability of the method
is, however, questionable for structurally complex molecules. Nevertheless, it is an
acceptable method of estimation for many environmentally significant compounds
for which the structures are well established. Appendix 4 lists the
b
and
B
factors for
some typical fragments. A detailed listing of the parameters is available in Lyman
et al. (1990).
Since experimental values are always the most useful ones, another approach to
obtaining log
K
ow
exists that utilizes the log
K
ow
of known compounds. This is done
by adding or subtracting appropriate
b
and
B
values from the parent compound.
CH
3
replacesthe
=
±
b
j
±
log
K
ow
(
new
)
log
K
ow
(
parent
)
B
k
.
(3.52)
j
k
There are a set of specific rules that must be followed to estimate the octanol-water
partition coefficients for a given molecule based on the specific values of
b
and
B
listed in the literature. The topics by Hansch and Leo (1979) or Lyman et al. (1990)
give more details on complex structures and a greater depth of discussion.
E
XAMPLE
3.10 E
STIMATION OF
L
OG
k
ow
U
SING
F
RAGMENT
C
ONSTANT
M
ETHOD
Let us determine the log
K
ow
values of the following compounds: hexane, chloroben-
zene, cyclopentane, and chlorobiphenyl using data from Appendix 3. The first
information we need in each case is the correct molecular structure.
Search WWH ::
Custom Search