Mechanisms of dioxin formation from the high-temperature oxidation of 2-bromophenol

Article abstract of DOI:10.1021/es048461y

The homogeneous, gas-phase oxidative thermal degradation of 2-bromophenol was studied in a 1 cm i.d., fused silica flow reactor at a concentration of 88 ppm, reaction time of 2.0 s, over a temperature range from 300 to 1000 °C. Observed products in order of yield were dibenzo-p-dioxin (DD) > 4,6-dibromodibenzofuran (4,6-DBDF) > 4-monobromodibenzofuran (4-MCDF), dibenzofuran (DF), 1-monobromodibenzo-p-dioxin (1-MBDD), naphthalene, bromonaphthalene, 2,4-dibromophenol, 2,6-dibromophenol, phenol, bromobenzene, and benzene. This result is in contrast to the oxidation of 2-chlorophenol, where the major product is 4,6-dichlorodibenzofuran (4,6-DCDF). 4,6-DBDF was observed in high yields in contrast to our previous results for the pyrolysis of 2-bromophenol, where 4,6-DBDF was not detected. The increase in 4,6-DBDF yields is attributed to hydroxyl radical being the major chain carrier under oxidative conditions, which favors hydrogen-abstraction reactions that lead to formation of 4,6-DBDF. However, DD is still the highest yield product under oxidative conditions because of the relative ease of displacement of Br? in the ring-closure reaction.

Full text of DOI:10.1021/es048461y

Environ. Sci. Technol. 2005, 39, 2128-2134
Previous works on the oxidation of chlorinated phenols
Mechanisms of Dioxin Formation
from the High-Temperature
Oxidation of 2-Bromophenol
under “slow combustion conditions” (i.e., T ) 300-600 °C,
reaction times between 10 s and 10 min) have reported the
formation of PCDFs as the major products (17-20). However,
in our recently reported studies on the high-temperature
pyrolysis of both 2-chlorophenol and 2-bromophenol we
reported the formation of PCDDs (dibenzo-p-dioxin (DD)
and 1-monochlorodibenzo-p-dioxin (1-MCDD) or 1-mono-
bromodibenzo-p-dioxin (1-MBDD)) was favored over the
formation of PCDFs (4,6-dichlorodibenzofuran (4,6-DCDF)
or 4,6-dibromodibenzofuran (4,6-DBDF)) (14, 21). There is
an apparent contradiction that suggests that molecular
oxygen is playing a large role in the product distribution and
PCDD/F ratio.
C A T H E R I N E S . E V A N S A N D
B A R R Y D E L L I N G E R *
Louisiana State University, Department of Chemistry,
Baton Rouge, Louisiana 70803
The homogeneous, gas-phase oxidative thermal degradation
of 2-bromophenol was studied in a 1 cm i.d., fused
silica flow reactor at a concentration of 88 ppm, reaction
time of 2.0 s, over a temperature range from 300 to
1000 °C. Observed products in order of yield were dibenzo-
p-dioxin (DD) > 4,6-dibromodibenzofuran (4,6-DBDF) >
4-monobromodibenzofuran (4-MCDF), dibenzofuran (DF),
1-monobromodibenzo-p-dioxin (1-MBDD), naphthalene,
bromonaphthalene, 2,4-dibromophenol, 2,6-dibromophenol,
phenol, bromobenzene, and benzene. This result is in
contrast to the oxidation of 2-chlorophenol, where the major
product is 4,6-dichlorodibenzofuran (4,6-DCDF). 4,6-DBDF
was observed in high yields in contrast to our previous results
for the pyrolysis of 2-bromophenol, where 4,6-DBDF was
not detected. The increase in 4,6-DBDF yields is attributed
to hydroxyl radical being the major chain carrier under
oxidative conditions, which favors hydrogen-abstraction
reactions that lead to formation of 4,6-DBDF. However, DD
is still the highest yield product under oxidative conditions
because of the relative ease of displacement of Br• in
the ring-closure reaction.
In this paper, the thermal degradation of 2-bromophenol
under oxidative conditions is reported for a reaction time of
2.0 s over the temperature range from 300 to 1000 °C and we
compare these results to the results from previous studies
of the pyrolysis of 2-bromophenol (14) as well as oxidation
and pyrolysis of 2-chlorophenol (22).
Experimental Section
All experiments were performed using a high-temperature
flow reactor system referred to in the archival literature as
the System for Thermal Diagnostic Studies (STDS). The
detailed design has been published elsewhere (23). In short,
the STDS consists of a high-temperature, 1 cm i.d., fused
silica flow reactor equipped with an in-line Varian Saturn
2000 GC/MS. The flow reactor is housed inside a furnace
located inside a Varian GC, where the temperatures sur-
rounding the reactor are controlled. Pressure inside the
reactor is also maintained at 1.00 ( 0.15 atm. Gas-phase
products are cryogenically trapped at the head of the GC
column in preparation for chemical analysis.
To maintain a constant concentration of 88 ppm, 2-mono-
bromophenol (2-MBP) (Aldrich) was injected into a 20% O2
in helium gas stream by a syringe pump through a vaporizer
maintained at 280 °C. Gas-phase samples of 2-MBP then
were swept by the 20% O2 in helium flow through heated
transfer lines (300 °C) into a 35 cm long, 1.0 cm id., fused
silica tubular flow reactor where the temperature was
maintained between 300 and 1000 °C in individual experi-
ments. The 20% O2 in helium flow rate was varied with
temperature, so that the residence time within the reactor
was held at 2.0 s. The unreacted 2-MBP and thermal
degradation products were then swept through a heated
transfer line to another Varian GC, where they were cryo-
genically trapped at the head of a CP-Sil 8 phase capillary
column (30 m, 0.25 mm i.d., 0.25 µm film thickness). To
separate the individual reaction products the column was
temperature programmed from -60 to 300 °C at 15 °C/min.
Detection and quantification of the products were obtained
using a Varian Saturn Mass Spectrometer which was operated
in the full-scan mode (40-650 amu) for the duration of the
GC run. The length of each experimental run was ap-
proximately 45 min.
Introduction
Over the past decade there has been an increase in concern
over the risk of environmental exposure to brominated flame-
retardant-containing materials (1, 2). Many of these materials,
such as electronic or “E-wastes”, find their way to waste-
treatment facilities where they are burned (3-7). They are
also subject to thermal degradation during accidental fires
(8). Because of their chemical composition and combustion
inhibition properties, they are prone to forming products of
incomplete combustion, including polybrominated dibenzo-
p-dioxins and dibenzofurans (PBDD/Fs).
Previous research has indicated that the presence of
bromine during the combustion of hazardous wastes in-
creases the production of PBDD/Fs and well as polychlo-
rinated dibenzo-p-dioxins and dibenzofurans (PCDD/Fs) and
mixtures of brominated and chlorinated dibenzo-p-dioxins
and dibenzofurans (PXDD/Fs) (2, 9). It has also been
established that brominated phenols and brominated flame
retardants, e.g., polybrominated biphenyl ethers (PBDEs),
are known precursors to PBDD/Fs (2, 3, 10-12). More
importantly, some studies have shown that brominated
phenols form more PBDD/Fs than the analogous chlorinated
phenols form PCDD/Fs (10, 13, 14). With this knowledge, it
is important to note that the toxicity of the PBDD/Fs has
been shown to be similar to the analogous PCDD/Fs (15, 16).
Product concentrations were calculated based on the
calibrations with standards of the products (Aldrich and
Cambridge Isotope Lab) and the peak area counts from the
chromatogram. The yields of the products were calculated
using the expression
yield ) {[product]/[2-MBP]0} × 100
where [product] is the concentration of the particular product
formed (in moles) and [2-MBP]0 is the initial concentration
* Corresponding author phone: (225) 578-6759; fax: (225) 578-
3458; e-mail: barryd@lsu.edu.
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2128 ENVIRONMENTAL SCIENCE & TECHNOLOGY / VOL. 39, NO. 7, 2005
10.1021/es048461y CCC: $30.25
2005 American Chemical Society
Published on Web 02/17/2005

TABLE 1. Percent Yield of Products of Gas-Phase Oxidation of 2-Bromophenola
temp (°C)
product
350
400
450
500
550
600
650
700
750
800
850
900
2-bromophenol
dibenzo-p-dioxin
1-bromodibenzo-p-dioxin
dibenzofuran
4-bromodibenzofuran
4,6-dibromodibenzofuran
naphthalene
1-bromonaphthalene
phenol
2,4-dibromophenol
2,6-dibromophenol
benzene
99.2 74.7
69.3
66.9
54.7
22.2
36.2
17.0
16.7
16.6
0.07
0.90
0.82
2.40
0.08
0.10
0.61
0.03
0.03
0.02
0.02
4.07
8.01
0.06
0.64
0.46
1.25
0.06
0.06
1.90
3.30
0.03
0.11
0.17
0.22
0.03
1.09
0.18
0.02
0.05
0.10
0.11
0.02
0.22
0.12
0.06
1.36
3.27 12.6
0.04
0.08
0.13
0.15
0.12
0.02
0.52
1.69
0.06
0.16
0.58
1.89
0.04
0.04
0.37
0.04
0.04
0.002
0.005
0.06
0.46
0.07
0.22
1.14
0.08
0.00 5
0.02
0.04
0.02
0.006
0.06
0.008
0.03
0.04
0.11
0.06
0.10
0.07
0.17
0.03
0.01
0.02
0.02
0.03
0.007 0.005
0.03
0.01
bromobenzene
phenylethyne
0.005 0.004 0.03
0.007 0.005
a Percent yield ) {[product]/[2-MBP]0} × 100.
of 2-MBP (in moles) injected into the reactor. Multiple runs
were performed for each temperature to ensure the repeat-
ability of the experiments. Once the experimental procedure
was fully developed, the repeatability of the experiments was
within 10%.
Products (other than PBDD/Fs) were identified based on
the NIST mass spectral library as well as the GC retention
times and mass spectra of the standards for each product.
Product concentrations were calculated based on the cali-
brations with standards of the products (Aldrich and
Cambridge Isotope Lab) and the peak area counts from the
chromatogram.
Standards for PBDD/Fs with less than four bromines were
not available. Concentrations of observed PBDD/Fs are
reported based on calibrations for the analogous PCDD/F.
This is a reasonably accurate approach as the peak area counts
for various chlorinated and brominated aromatics and
PCDD/Fs and PBDD/Fs were compared, and it was found
that the difference in calibration factors for brominated
aromatic hydrocarbons and chlorinated aromatic hydro-
carbons varied less than 10%.
Only three chromatographic peaks were observed that
were tentatively identified as PBDD/F based on their mass
spectra: 1-bromodibenzo-p-dioxin, 4,6-dibromodibenzo-
furan, and 4-bromodibenzofuran. The mass spectral library
match qualities for each of these species were 264, 326, and,
248, respectively. These products are the same three PBDD/
Fs that were anticipated based on the predicted pathways
from previous research on formation of PCDD/F from the
analogous 2-chlorophenol (22). In the 2-chlorophenol study
the PCDD/F standards were available to confirm the iden-
tifications based on GC retention time and mass spectral
pattern. Although standards were not available to confirm
the identifications of PBDD/F, we are confident in the
assignments based on the following: combination of mecha-
nistically anticipated product formation; comparison of GC
retention times, mass spectral response, and mass spectral
patterns of chlorinated and brominated hydrocarbons; and
previous studies of the formation of PCDD/F from 2-chlo-
rophenol (22).
FIGURE 1. “Dioxin” products from the gas-phase oxidation of 2-MBP.
[2-MBP]o ) 88 ppm in helium. Gas-phase reaction time of 2.0 s.
O•, H•, and Br•. The Chemkin Equil code was used to calculate
the concentrations of these species over a range of reaction
temperature from 300 to 1000 °C (25). The initial inputs for
the calculations were the same as the experimental runs in
that the initial 2-MBP concentration was held at 88 ppm and
the initial O2 concentration was held at 20%. Over this
temperature range the major molecular species are CO2, H2O,
HBr, and Br2. Other species of interest included in the
•
calculation were OH, O•, H•, Br•. HO2•, H2, and CO.
Results
The temperature dependence of the oxidative thermal
degradation of 2-MBP and the yield of “dioxin” products are
presented in Figure 1 and Table 1 for a reaction time of 2.0
s. The non-dioxin products are presented in Figure 2 and
Table 1. Figures 1 and 2 are presented on a semilogarithmic
scale in which the percent yields of products (or percent
yield of unconverted 2-MBP) are presented on a logarithmic
scale versus temperature. The thermal degradation of 2-MBP
initially increased gradually from 350 to 600 °C, achieving
99% destruction at 800 °C.
The heats of reaction, ∆Hrxn, for key steps in product
formation pathways were calculated using AM1, semiem-
pirical molecular orbital formalism. The calculations were
performed using the MOPAC computation program that is
contained within the ChemBats3D Pro computer application
(24). Without experimental benchmarks the calculated ∆Hrxn
cannot be considered to be completely accurate. They are
shown to assess the likelihood of potential parallel pathways.
Pseudo-equilibrium calculations were performed to es-
The predicted PBDD/F products, dibenzo-p-dioxin (DD),
1-bromodibenzo-p-dioxin (1-MBDD), and 4,6-dibromodi-
benzofuran (4,6-DBDF), were all observed for the oxidation
of 2-MBP (cf. Figure 1 and Table 1). Other detected PBDD/F
products were 4-bromodibenzofuran (4-MBDF) and diben-
zofuran (DF). The two PBDD products, DD and 1-MBDD,
were observed between 400 and 850 °C, reaching maximum
yields of 22.2% and 0.15% at 550 °C, respectively. 4,6-DBDF
and 4-MBDF were detected between 450 and 850°C, reaching
maximum yields at 650 °C of 2.40% and 0.82%, respectively.
•
timate the concentrations of reactive species such as OH,
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paper on the thermal degradation of 2-MBP under pyrolytic
condition, ∆Hrxn for reactions with H• and Br• was discussed
(14). It was determined that the most favorable reactions for
generating the bromophenoxyl radical were the abstraction
of hydrogen by H• or Br•. With the addition of O2 one can
easily generate •OH and O• that can also abstract hydrogen
via highly exothermic reactions (eqs 2 and 3)
C6H4BrOH + •OH f
C6H4BrO• + H2O ∆Hrxn ) -39 kcal/mol (2)
C6H4BrOH + O• f
C6H4BrO• + •OH ∆Hrxn ) -22 kcal/mol (3)
Rate coefficients based on analogous reactions with
phenol for eqs 2 and 3 are k2 (1000-1150 K) ) 6.0 × 1012
cm3/mol/s (28) and k3 (340-870 K) ) 1.28 × 1013 exp(-2900/
RT) cm3/mol/s (28). We used equilibrium calculations and
other formalisms from the literature to estimate the •OH and
O• concentration in our system (26, 28). Using these
concentrations and the rate expressions given above (using
Ea (eq 1) ) ∆Hrxn ) 79 kcal/mol), the rate of eq 2 is ∼200×
faster than eq 3 and a factor of 30 faster than eq 1. Thus, eq
2 is the dominant source of phenoxyl radical under oxidative
conditions.
Formation of Phenol, Bromobenzene, Benzene, 2,4-
Dibromophenol, and 2,6-Dibromophenol. The formation
of phenol is likely due to the exothermic displacement of
bromine by H•(∆Hrxn )-29 kcal/mol). The temperature range
at which phenol is detected is much lower than for previous
results of 2-MBP under pyrolytic conditions (14). This is due
to the early onset of reaction of 2-MBP under oxidative
conditions and the oxidation of phenol at higher temper-
atures.
FIGURE 2. “Non-dioxin” products from the gas-phase oxidation of
2-MBP. [2-MBP]o ) 88 ppm in helium. Gas-phase reaction time of
2.0 s.
The final product, DF, was not detected until 550 °C and
achieved a maximum yield of 0.90% at 650 °C. No brominated
dioxin products were detected above 900 °C.
Non-PBDD/F products were also detected for the oxida-
tion of 2-MBP (cf. Figure 2 and Table 1). Initially, at 400 °C
2,4-dibromophenol, 2,6-dibromophenol, and naphthalene
were observed. 2,4-Dibromophenol and 2,6-dibromophenol
achieved maximum yields of 0.07% and 0.17% at 550 °C,
respectively. Naphthalene remained at a relatively constant
yield from 400 to 700 °C and then decreased in yield at 900
°C, where it was no longer detected. Phenol, benzene, and
phenylethyne were detected between 550 and 750 °C,
achieving their respective maximum yields of 0.61%, 0.02%,
and 0.007% at 650 °C. Bromobenzene was observed between
550 and 750 and 825-925 °C with local maximum yields of
0.02% at 650 °C and 0.06% at 900 °C.
This result is very similar to that observed for pyrolysis
and oxidation of 2-MCP (17, 22). The yield of phenol for the
oxidation of 2-MBP is slightly higher than the yield for 2-MCP,
which reflects the relative ease of bromine displacement
compared to chlorine displacement due to the 15.5 kcal/mol
lower carbon-bromine bond energy (29).
Bromobenzene and benzene are formed with much lower
yields than phenol. These lower yields are due to the slightly
endothermic displacements of hydroxyl by H• to form
bromobenzene (∆Hrxn ) 2 kcal/mol) and hydroxyl from
phenol by H• to form benzene (∆Hrxn ) 4 kcal/mol). However,
bromobenzene reaches a maximum at 650 and 900 °C. The
lower temperature maximum is due to the displacement of
hydroxyl from 2-MBP by H•. The higher temperature maxi-
mum of bromobenzene is due to well-documented molecular
growth pathways resulting from fragmentation of 2-MBP into
C2 species (30-32).
Discussion
The formation of dioxin products (DD, 1-MBDD, and 4,6-
DBDF) indicates that stable phenoxyl radicals are formed in
significant yields through loss of the hydroxyl hydrogen. The
formation of aromatics (phenol, bromobenzene, and ben-
zene) indicates that simple substitution reactions are oc-
curring. The formation of 2,4-dibromophenol and 2,6-
dibromophenol indicate evidence of bromination of 2-MBP.
The formation of larger aromatic molecules at low temper-
atures (naphthalene and bromonaphthalene) is the result of
reactions involving the release of CO from the phenoxyl and
bromophenoxyl radicals that will then recombine to form
naphthalene and bromonaphthalene.
2-MBP Decomposition. The addition of oxidative de-
struction pathways with the addition of molecular oxygen
results in the decomposition of 2-MBP initiating at 600 °C
rather than 650 °C, the temperature observed under pyrolytic
conditions (14). The decomposition of 2-MBP can, in
principle, be initiated by loss of the phenoxyl hydrogen by
unimolecular, bimolecular, or possibly other low-energy
pathways (including heterogeneous reactions). Unimolecular
decomposition of the oxygen-hydrogen bond (eq 1) is rapid
with a reported rate coefficient for phenol of k1(430-930 °C)
) 3.2 × 1015 exp(-86 500/RT) s-1 (26, 27). The direct
bimolecular reaction with O2 via reaction 1a is endothermic
by 28 kcal/mol and viable only as minor initiation reaction
2,4-Dibromophenol and 2,6-dibromophenol are produced
from bromination of the 2-MBP. Since displacement of
hydrogen by Br• is endothermic, the direct reaction of Br•
with 2-MBP is unlikely. The formation of dibromophenol is
instead due to recombination of phenoxyl radicals and Br•.
Scheme 1 depicts the formation of 2,4-dibromophenol and
2,6-dibromophenol by Br• attack at the resonance-stabilized,
ortho- or para-carbon sites of the bromophenoxyl radicals
(∆Hrxn ) -29 kcal/mol). Subsequent tautomerization results
in the formation of the respective dibromophenols (∆Hrxn
)
-17 kcal/mol) (33). The dibromophenols were also detected
in our previous study of 2-MBP under pyrolytic conditions
(14). However, they were observed over a narrower tem-
perature range and lower yields (14).
On the basis of our pseudo-equilibrium calculations at
700 °C, the concentrations of Br2 (9.4 × 10-6 mol) and Br• (9.6
× 10-7 mol) are, respectively, 3 and 1 orders of magnitude
higher than the concentration of HBr (5.4 × 10-8 mol). This
C6H4BrOH f C6H4BrO• + H• ∆Hrxn ) 79 kcal/mol (1)
C6H4BrOH + O2 f
C6H4BrO• + HO2• ∆Ηrxn ) 31 kcal/mol (1a)
Bimolecular propagation reactions under oxidative con-
ditions include attack by H•, Br•, •OH, and O•. In our previous
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monaphthalene has been traditionally ascribed to molecular
growth pathways involving largely C2 fragments (30-32).
However, the low-temperature onset of formation of naph-
thalene (400 °C) suggests a pathway that does not require
the complete fragmentation of 2-MBP. In our previous work
on the pyrolysis of 2-MBP we presented a reasonable pathway
for the formation of naphthalene from the 2-bromophenoxyl
radical through elimination of CO to form a cyclopentadienyl
radical (14). The recombination of two cyclopentadienyl
radicals has been previously shown to be a favorable pathway
for formation of naphthalene (34, 35). A similar low-
temperature route to the formation of naphthalene from the
recombination of bromophenoxyl radicals is described in
Scheme 2 (vide infra) as a competitive pathway to the
formation of 4,6-DBDF. This formation of naphthalene is
based on a previously proposed pathway of the recombina-
tion of two chlorophenoxyl radicals to form naphthalene
(36).
SCHEME 1. Reaction Mechanism for the Formation of
2,4-Dibromophenol and 2,6-Dibromophenol from
2-Bromophenoxyl Radical
is in contrast to the results for 2-MCP, where the concentra-
tion of HCl (1.7 × 10-5 mol) was greater than that of Cl2
(1.3 × 10-6 mol) and Cl• (1.5 × 10-7 mol) (22) and our
calculations for the pyrolysis of 2-MBP for which the HBr,
Br2, and Br• concentrations were 4.2 × 10-7, 2.0 × 10-12, and
2.7 × 10-10 mol, respectively. The addition of oxygen creates
•OH, which converts HBr into water and Br•, the latter being
in equilibrium with Br2 (28). The increased yield of bromi-
nated products under oxidative conditions is likely due to
the release of strong brominating agents, Br•, as well as the
increase in bromophenoxyl radical concentration.
The yields of naphthalene and bromonaphthalene are
significantly lower under oxidative than pyrolytic conditions,
most probably due to the more rapid rate of oxidation of the
cyclopentadienyl radical (14). Also, with the increase in the
concentration of brominated phenoxyl radicals, the rate of
PBDD/F formation will increase in competition with elimi-
nation of CO. The oxidation rate of naphthalene is also
increased. Thus, the concentration of naphthalene is dra-
matically lowered and never becomes a major product as it
did under pyrolytic conditions.
Formation of Naphthalene and Bromonaphthalene.
Formation of polycyclics such as naphthalene and bro-
SCHEME 2. Pathways for Formation of DD, 1-MBDD, and 4,6-DBDF
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SCHEME 3. Pathways for the Formation of 4-MBDF
The formation of naphthalene for the oxidation of both
2-MCP and 2-MBP occurred over a similar temperature range.
However, higher yields of naphthalene were observed for
the oxidation of 2-MCP than for 2-MBP (22). This may be
due to the higher concentration of bromophenoxyl than
chlorophenoxyls radicals, leading to an increased rate of
formation of PBDD/F by radical recombination processes at
the expense of CO elimination, leading to naphthalene
formation.
Formation of Dibenzo-p-dioxin, 1-Bromodibenzo-p-
dioxin, 4,6-Dibromodibenzofuran, 4-Bromodibenzofuran,
and Dibenzofuran. Scheme 2 summarizes previously identi-
fied reaction pathways to DD, 1-MBDD, and 4,6-DBDF from
reaction of the different mesomers of 2-bromophenoxyl
radical.
pathways then undergo tautomerization followed by dis-
placement of hydroxyl to form 4-MBDF. Under oxidative
conditions the latter pathway is more favorable, while under
pyrolytic conditions the former is dominant.
Oxidation versus Pyrolysis of 2-MBP. DD is the major
product of both pyrolysis and oxidation of 2-MBP. However,
the yield of DD is 4 times greater for oxidation than for
pyrolysis (14). This is primarily due to the increase in
bromophenoxyl radicals at lower temperatures for oxidative
conditions, which react to form PBDD/Fs. Under pyrolytic
conditions bromophenoxyl radicals form at higher temper-
atures, where their rate of decomposition is greater and yields
of PBDD/F are reduced. The yield of 1-MBDD is 5× greater
under oxidative conditions than pyrolytic conditions. Its
formation is again facilitated by the increase in bromo-
phenoxyl radicals. However, the presence of •OH facilitates
hydrogen abstraction in pathway 2a, which further promotes
formation of 1-MBDD.
Pathway 1a in Scheme 2 depicts the mechanism for DD
formation; the oxygen-centered radical mesomer recombines
with the carbon- (bromine substituted) centered radical
mesomer to form a keto-ether. Following abstraction of
bromine by H• or •OH, DD is formed by intra-annular
elimination of Br•. Another possible pathway for the formation
of DD is through a radical-molecule reaction, pathways 1b,
shown in parentheses below the radical-radical pathway in
Scheme 2. This reaction depicts the oxygen-centered radical
mesomer reacting with 2-MBP via Br• displacement to form
a bromohydroxy diphenyl ether (HDE) followed by abstrac-
Detection of 1-MBDD is also observed as low as 400 °C.
This is a dramatically lower than the 650 °C formation
temperature observed under pyrolytic conditions (14). This
suggests another pathway is involved in the low-temperature
formation. In our previous study of the oxidation of 2-MCP
similar results were observed where 1-MCDD was detected
as low as 400 °C (22). We suggested that at lower temperatures
1-MCDD can be formed by a unimolecular pathway following
the formation of the keto-ether intermediate via radical-
radical recombination. An analogous pathway is proposed
for formation of 1-MBDD (cf. pathway 2a in Scheme 2).
Alternately to the abstraction of hydrogen by •OH in pathway
2a in Scheme 2, a simple, intra-ring, single-proton tau-
tomerization results in the formation of a hydroxyl-diphenyl
ether intermediate that can then form 1-MBDD by inter-
ring elimination of HBr. This proposed mechanism is based
on the similar observation of naphthalene at temperatures
as low as 400 °C. Previous work has proposed that after
recombination of chlorinated phenoxyl radicals to form the
diketo intermediate, the formation of naphthalene shown in
pathway 3b in Scheme 2 is unimolecular (39). Following
recombination the resulting intermediate eliminates two CO
moieties, resulting in the formation of bicyclopentadienyl.
On the basis of their similarity to the chlorophenoxyl radicals,
naphthalene can be formed by the bromophenoxyl radicals
in a similar manner. Naphthalene is then formed by the
rearrangement pathways previously proposed in the literature
(14, 34, 35). Once the diketo intermediate is formed, the entire
process is unimolecular. This can explain the high yields at
low temperatures before the radical pool has developed.
However, above 500 °C the radical pool increases rapidly
and bimolecular pathways involving H• and Br• abstraction
begin to dominate the formation of 1-MBDD and other
PBDD/F products. The formation of DD and 4-MBDF at 400-
450 °C is attributed to the lower temperature formation of
the bromophenoxyl radical precursor.
•
tion of hydrogen by OH. Finally, DD is formed by intra-
annular displacement of Br•. It has been previously suggested
that this radical-molecule reaction is too slow to account
for the observed yields of the DD (37-39).
Formation of 1-MBDD, shown as pathway 2a in Scheme
2, is initiated by recombination of the oxygen-centered radical
mesomer and the carbon (hydrogen)- centered radical
mesomer to form a keto-ether. Following loss of hydrogen
to form the phenoxyl phenyl ether (PPE), ring closure to
form 1-MBDD occurs through intra-annular displacement
of Br•. Pathway 2b depicts an alternate unimolecular pathway
for the formation of 1-MBDD.
Pathway 3a depicts a possible pathway to 4,6-DBDF
formation. Initially, for both pathways 3a and 3b two carbon-
hydrogen-centered radical mesomers react to give the diketo
dimer. The dimer can follow the upper pathway, 3a, by
abstraction of hydrogen by •OH and then undergo tau-
tomerization followed by displacement of •OH to form 4,6-
DBDF. Pathway 3b is the alternative pathway to formation
of naphthalene through CO and Br• elimination (36).
We believe that DF is simply a recombination of unbro-
minated phenoxyl radical formed from decomposition of
phenol (40). The reaction proceeds by mechanisms analogous
to those shown for formation of 4,6-DBDF in Scheme 2.
Scheme 3 depicts proposed pathways for the formation
of 4-MBDF. Two pathways are depicted: (1) the carbon
(hydrogen)-centered radical mesomer recombines with the
carbon (bromine)-centered radical mesomer or (2) the carbon
(hydrogen)-centered radical mesomer recombines with an
unbrominated carbon-centered phenoxyl radical to form a
diketo dimer. For the first pathway H• abstracts bromine,
One product not observed under pyrolytic conditions,
4,6-DBDF, was detected in high yields under oxidative
conditions (14). This behavior is similar to that observed for
2-MCP (22). With the addition of oxygen, •OH becomes the
•
and in the second OH abstracts another hydrogen. Both
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2132 ENVIRONMENTAL SCIENCE & TECHNOLOGY / VOL. 39, NO. 7, 2005

major carrier over H•. Hydroxyl radical facilitates highly
exothermic hydrogen-abstraction reactions in pathways 2a
(-46 kcal/mol) and 3a (-47 kcal/mol), resulting in the
formation of 1-MBDD and 4,6-DBDF, respectively. However,
the abstraction of bromine by •OH is 40 kcal/mol endothermic
and not favorable. Thus, the increase in •OH concentration
increases the rate of 1-MBDD and 4,6-DBDF formation but
does not increase the rate of DD formation, which requires
abstraction of bromine.
under EPA Contract 9C-R369-NAEX, EPA Grant R828166, and
the Patrick F. Taylor Chair foundation.
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•
The addition of OH to the system increases the rates of
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•
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•
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Acknowledgments
We gratefully acknowledge the assistance of our colleagues,
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Received for review September 29, 2004. Revised manuscript
received December 28, 2004. Accepted January 11, 2005.
ES048461Y
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