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34 of 38 tetrachlorodibenzofurans (T4CDFs), 22 of 28
pentachlorodibenzofurans (P5CDFs), 12 of 16 hexa-
chlorodibenzofurans (H6CDFs), all four heptachloro-
dibenzofurans (H7CDFs), and octachlorodibenzofuran
(OCDF). A universal response factors was used (DF
response factor).
not listed. The reader should refer to Table 1 for these
products. The basis for these groupings is discussed
next, and then an example calculation is provided.
The relative rate constants in Table 3 are listed in six
groups. In group 1, eight relative rate constants mea-
sured for PCDF congeners with different numbers of Cl
atoms are presented. Phenol is the most potent PCDF
precursor. Overall, two conclusions can be drawn. First,
PCDF formation is favored from less chlorinated phe-
nols. Steric and electronic effects associated with chlo-
rine substitution suppress phenoxy radical dimerization.
Second, PCDF formation is favored from phenols with
chlorine substitution at meta (3 and 5) positions rather
than ortho (2) and para (4) positions, likely due to an
electronic effect that favors phenoxy radical dimeriza-
tion at sites ortho/para to chlorine substitution.
4. Results and discussion
4.1. Derivation of reaction order and relative rate
constants
Preliminary experiments were performed at 600 ꢁC
using an equal molar mixture of phenol, 4-chlorophenol
(4-CP), and 2,4-dichlorophenol (2,4-DCP) in benzene
at four dilutions to estimate reaction order n. The ex-
pected products from these phenols (see Table 1) were
formed. In all experiments, DF was produced in greatest
amount, indicating that chlorine substitution decreases
the rate of phenoxy radical coupling. Reaction orders
for PCDF product formation obtained from these
experiments ranged from 1.2 to 2.0, as shown in Table 2.
Here, we use an average value of 1.6 for the model. This
compares with a reaction order of PCDD formation
from 2,4,6-T3CP of 1.3 measured by Sidhu et al. (1995).
Overall, the reaction order increases as phenol chlorine
content increases, suggesting that suppression of phen-
oxy radical coupling (Eq. (4)) due to chlorine substitu-
tion results in phenoxy radical decomposition (Eq. (3))
becoming the dominant phenoxy radical consumption
channel.
In group 2, six relative rate constants measured for
PCDF isomers formed from phenols with different
numbers of Cl atoms are presented. In general, PCDF
isomers formed from phenols with different numbers of
Cl atoms are favored over isomers formed from phenols
with similar numbers of Cl atoms. This may be due to
steric effects associated with a parallel plane approach
geometry of reacting phenoxy radicals, as proposed by
Nakahata and Mulholland (2000). As in group 1, phe-
nols with chlorine at meta positions are observed to
produce higher PCDF yields than phenols with chlorine
at ortho and para positions.
In group 3, relative rate constants for different PCDF
isomers formed from condensation of the same phenol
pairs are presented. 3-Chlorophenol and/or 3,4-dichloro-
phenol produce multiple isomers due to their having
both ortho sites unchlorinated and their lack of sym-
metry. There are relative rate constants for 31 PCDF
formation reactions represented in group 3. For exam-
ple, the relative rate constant for PCDF formation
reactions of 3-chlorophenol at its 2 versus 6 positions
with phenol, 2- and 4-chlorophenol, 2,3- and 2,4-di-
chlorophenol, and 2,3,4-trichlorophenol are all esti-
mated to be 0.8. This result is based on experimental
data with the three monochlorophenol reactants. Reac-
tion at the 6 position of 3-chlorophenol is favored over
reaction at the 2 position, whereas reaction at the 2
position of 3,4-dichlorophenol is favored over reaction
at the 6 position except when 1,9 CDF isomers are
formed.
Published results from flow reactor experiments sin-
gle phenol reactants and equal molar mixtures of up to
four phenol reactants (Yang et al., 1998; Nakahata and
Mulholland, 2000; Mulholland et al., 2001) were used to
obtain 40 independent relative rate constants for deter-
mining ratios of PCDF products (Eq. (12)). The other 95
relative rate constants are determined from similarities
in the molecular structure, considering reaction pathway
factors such as steric, statistical, and electronic factors.
The relative rate constants and reaction groupings are
listed in Table 3. For the rows in which more than one
set of reactions are represented, the PCDF products are
Table 2
Reaction order n from a mixture of phenol, 4-chlorophenol,
and 2,4-dichlorophenol
In groups 4, 5, and 6, relative rate constants are
presented for formation of various PCDF isomers from
trichlorophenol, dichlorophenol, and monochlorophe-
nol reactants, respectively. Again, these values demon-
strate that PCDF isomers from phenols with chlorine at
meta sites are favored over PCDF isomers from phenols
with chlorine at ortho and para sites, and that formation
PCDF isomers with 1,9 sites chlorinated is suppressed.
As an example of how to use Table 3, consider the
distribution of the 28 T3CDF isomers. A total of 28
Phenol reactants
PCDF products
Reaction order n
Phenol + phenol
Phenol + 4-CP
DF
2-MCDF
1.2
1.6
1.5
1.8
1.9
2
Phenol + 2,4-DCP
4-CP + 4-CP
2,4-DCDF
2,8-DCDF
2,4,8-T3CDF
2,4,6,8-T4CDF
4-CP + 2,4-DCP
2,4-DCP + 2,4-DCP