Nucleophilic radical substitution reaction of triazine herbicides with polysulfides

Article abstract of DOI:10.1021/jf049505p

Triazine herbicides are among the most widely used herbicides in the United States. Many triazine compounds are relatively stable under natural conditions and have become prominent contaminants in hydrologic systems. It was previously reported that chloro-s-triazine compounds were rapidly dechlorinated in water by polysulfides, and the reaction was assumed to be aromatic nucleophilic substitution (S_NAr). In this study, we evaluated the effect of free radical inhibitors on the reaction rate of polysulfides with herbicides atrazine, simazine, and cyanazine. The reaction was significantly inhibited by radical scavengers oxygen and 1,4-benzoquinone, suggesting involvement of free radicals in the reaction. Spectral analysis of the reaction mixture using electron spin resonance showed that after the reaction, the free radical concentration in polysulfide solution substantially decreased. These evidences indicate that radical sulfur anions may also be involved in the reaction, likely via a free radical substitution reaction (S_RN1) mechanism. Amendment of sodium tetrasulfide significantly reduced the leaching of atrazine or simazine from packed sand columns. Therefore, polysulfide salts may be potentially used to remove residues of triazine herbicides in environmental media.

Full text of DOI:10.1021/jf049505p

J. Agric. Food Chem. 2004, 52, 7051−7055 7051
Nucleophilic Radical Substitution Reaction of Triazine
Herbicides with Polysulfides
WEICHUN YANG, JAY J. GAN,* SVETLANA BONDARENKO, AND WEIPING LIU
Department of Environmental Sciences, University of California, Riverside, California 92521
Triazine herbicides are among the most widely used herbicides in the United States. Many triazine
compounds are relatively stable under natural conditions and have become prominent contaminants
in hydrologic systems. It was previously reported that chloro-s-triazine compounds were rapidly
dechlorinated in water by polysulfides, and the reaction was assumed to be aromatic nucleophilic
N
substitution (S Ar). In this study, we evaluated the effect of free radical inhibitors on the reaction rate
of polysulfides with herbicides atrazine, simazine, and cyanazine. The reaction was significantly
inhibited by radical scavengers oxygen and 1,4-benzoquinone, suggesting involvement of free radicals
in the reaction. Spectral analysis of the reaction mixture using electron spin resonance showed that
after the reaction, the free radical concentration in polysulfide solution substantially decreased. These
evidences indicate that radical sulfur anions may also be involved in the reaction, likely via a free
radical substitution reaction (SRN1) mechanism. Amendment of sodium tetrasulfide significantly reduced
the leaching of atrazine or simazine from packed sand columns. Therefore, polysulfide salts may be
potentially used to remove residues of triazine herbicides in environmental media.
KEYWORDS: Chloro-s-triazine herbicides; SRN1 reaction; polysulfides; dechlorination; remediation
well-known that radical anions •S3- and •S4- are the most
INTRODUCTION
reducible chemical species in polysulfides (5, 6). For instance,
Halogenated organic compounds (HOCs) have been heavily
used as solvents, degreasing agents, and pesticides. Many of
these compounds are recalcitrant to degradation in the environ-
ment, and a great effort is being made to prevent further
pollution and to decontaminate environmental media that are
already polluted from previous uses (1). Currently, however,
there are few safe and effective decontamination methods for
these chemicals. Chemical treatments based on oxidation
reactions and physical means are often nonselective and may
damage the target environmental systems. Therefore, it is
necessary to identify new selective approaches to decontaminat-
ing these compounds in environmental media (2).
•
-
•
-
in the cyclic voltammetry test, S3 and S4 reduction peaks
were observed at all temperatures and all scan rates (5).
Recently, Rossi et al. (7) reviewed the nucleophilic substitution
reactions by the electron transfer (ET) pathway. For hetero-
aromatic compounds, competition between an ET process and
a SNAr process can occur mainly when more than one
heteroatom is present. Genesty et al. (8) also reported that the
2
-
reaction between S4 and 3-bromoquinoline in the presence
of a redox mediator had led to a mixture of products resulting
from SRN1. However, in Lippa et al. (4), the potential role of
radical sulfur anions in the substation reaction of triazines was
neglected and no effort was made to measure changes of free
radical levels during the reaction. The first objective of this study
was therefore to further explore the reaction mechanisms
between polysulfides and three chloro-s-triazine herbicides,
atrazine [2-chloro-4-(ethylamino)-6-iso-propyl-amino-s-triazine],
cyanazine {2-[4-chloro-6-(ethylamino)-s-triazin-2-yl)amino]-2-
methylpropionitrile}, and simazine [2-chloro-4,6-bis(ethylamino)-
s-triazine]. The reaction between nucleophiles and triazine
herbicides was previously studied to understand its potential
significance in natural attenuation of these pesticides in reduced
environments such as marine or salt marsh systems (4). Given
the great efficiency of this reaction, it is likely that this reaction
may be valuable as a remediation strategy for decontaminating
pesticide residues in soil or aquifer phases. The second objective
was therefore to evaluate the feasibility of using the polysulfide-
induced dechlorination reaction for decontaminating herbicide
residues in solid media.
An important group of HOCs, the triazine herbicides, have
been widely used on a great number of important crops in the
United States over the last several decades. Because of their
widespread use as well as their relatively high mobility in the
soil-water phases, triazine herbicides and their metabolites have
been frequently detected in U.S. hydrological systems (3). In a
recently published study, triazine herbicides and other azine
2-
analogues were found to be dechlorinated by polysulfides (Sn
4). The reaction was suggested to be nucleophilic aromatic
)
(
substitution (SNAr), in which the chlorine was replaced by a
sulfur dianion. The SNAr mechanism was deduced from the
second-order nature of the reaction, the products formed, and
reactivity trends (4).
Here, we propose that an alternative mechanism for the
reaction may be radical nucleophilic substitution, or SRN1. It is
*
Corresponding author (E-mail: jgan@mail.ucr.edu).
1
0.1021/jf049505p CCC: $27.50
© 2004 American Chemical Society
Published on Web 10/16/2004

7
052 J. Agric. Food Chem., Vol. 52, No. 23, 2004
Yang et al.
Table 1. Pseudo First-Order Rate Coefficient kobs (h-1) for the
MATERIALS AND METHODS
Chemicals. Standards of atrazine, cyanazine, and simazine (the
chemical purity of each compound was >98%) were obtained from
Chem Service (West Chester, PA). Sodium tetrasulfide crystals (99%)
were purchased from Great Western Inorganics (Arvada, CO). Crystals
of sodium tetrasulfide were purified by rinsing with deoxygenated
toluene to remove excess elemental sulfur that might have formed
during storage and were then dried under nitrogen. Borate buffer (pH
Disappearance of Selected s-Triazine Herbicides (20
µ
M) in Sodium
Polysulfide Solutions at 21
°C
initial polysulfide
concentration
atrazine
<0.0002
cyanazine
<0.0002
0.004
0.010
0.030
0.064
0.149
0.233
simazine
<0.0002
water (control)
1 mM
2 mM
0.003
0.009
0.029
0.061
0.147
0.216
±
±
±
±
±
±
0.001
0.001
0.002
0.003
0.013
0.015
±
±
±
±
±
±
0.001
0.002
0.004
0.005
0.027
0.034
0.005
0.008
0.027
0.059
0.138
0.210
±
±
±
±
±
±
0.001
0.003
0.003
0.004
0.038
0.027
9.5) was obtained from VWR (West Chester, PA). Polysulfide solutions
were prepared by dissolving purified sodium tetrasulfide crystals in
borate buffer. The reagent 1,4-benzoquinone (99%) used in the free
radical inhibition experiment was obtained from Aldrich (Milwaukee,
WI).
5 mM
10 mM
20 mM
30 mM
Kinetics Experiments. A series of kinetics experiments were
conducted in tetrasulfide-borate solution with and without free radical
inhibitors to understand the role of free radicals in the reaction between
the triazine herbicides and the polysulfides. Because tetrasulfide was
expected to be present in borate solution as multiple sulfur dianions,
mixture by electron spin resonance (ESR) spectrometry. ESR analysis
was carried out to evaluate the relative content of free radicals in sodium
tetrasulfide solution in ambient air, in nitrogen, after the addition of
1,4-benzoquinone and after the reaction with the triazine herbicides.
The initial sodium polysulfide concentration was 50 mM, the initial
benzoquinone concentration was 360 µM, and the initial herbicide
concentration was 200 µM. The samples were equilibrated for 24 h
before analysis. Samples for ESR spectroscopy were produced by
placing 0.2 mL of sample solution in a 3 mm (o.d.) × 17.7 cm (length)
quartz tube, which was inserted into the resonator cavity. For analysis,
aliquots of samples were scanned by an ESR spectrometer EMX ER077
2
-
2-
2-
4
including S
2
, S
3
, and S
(5), polysulfide concentrations were
expressed as the nominal concentration of tetrasulfide in this study.
The reaction kinetics between polysulfides and atrazine, simazine, or
cyanazine was first determined without free radical inhibitor but at
varying tetrasulfide concentrations. A stock solution of sodium tetra-
sulfide was prepared in borate buffer at 400 mM. The herbicide stock
solution was prepared at 100 µM in water (atrazine and cyanazine) or
1
:1 (v/v) water-methanol (simazine). The reaction was initiated by
(Bruker, Ettlingen, Germany) equipped with the Bruker WINEPR
mixing stock solutions of the herbicide and sodium tetrasulfide in 25
mL of borate buffer in 40 mL serum bottles. The initial herbicide
concentration was 20 µM, and the initial polysulfide concentrations
were 0, 1, 2, 5, 10, 20, and 30 mM. All treatments were performed in
triplicate. All sample bottles were closed with Teflon-lined rubber septa
and aluminum caps and then incubated at room temperature (21 ( 1
version 4.22 software. The samples were analyzed in the X-band (∼9.33
GHz) frequency range with a TE 102 cavity. All analyses were run
with a 4 G modulation amplitude at 100 kHz modulation frequency
with 8 mW of microwave power (9).
Column Leaching Experiment. A leaching experiment was con-
ducted using packed sand columns to demonstrate the potential
usefulness of polysulfide amendment for decontaminating triazine
herbicides in a solid media. The experiment was carried out with
atrazine and simazine. The sand (90 mesh, P. W. Gillibrand, Simi
Valley, CA) was packed into PVC cylinders 48 cm (length) by 5 cm
°
C). After equilibration for 0.5, 1.0, 1.5, 2.0, 2.75, 3.5, 5.0, 6.75, 9.0,
or 23.0 h, a 0.5 mL aliquot (three replicates) was withdrawn from the
reaction mixture using a 1.0 mL syringe and transferred to a 2 mL
autosampler vial. The samples were kept in a freezer at -21 °C until
analysis by high-pressure liquid chromatography (HPLC).
-
3
(
i.d.) at a bulk density of 1.40 g cm , and the packed columns were
Reaction kinetics was further determined in the presence of a nitrogen
headspace and after the addition of 1,4-benzoquinone to understand
the role of free radicals in the reaction. To eliminate oxygen from the
reaction system, a stream of nitrogen was bubbled through the sodium
tetrasulfide solution in the serum bottle for 30 min before herbicide
addition. A treatment without nitrogen purge was used as the control.
The initial herbicide concentration was 20 µM, and the initial
polysulfide concentration was 10 mM. The spiked samples were kept
at room temperature, and aliquots were removed for analysis of the
remaining herbicide concentration at different time intervals. For the
experiment with 1,4-benzoquinone, the reaction mixture was prepared
in a collapsible plastic glovebox (Fisher) filled with nitrogen. Different
amounts of the 1,4-benzoquinone stock solution (10 mM in acetone)
were added into borate buffer to arrive at initial concentrations of 120,
sealed at both ends with prefabricated caps with a brass inlet and outlet.
The packed columns were saturated with water, and a pulse of 20 mL
of acetone-water solution (1:9, v/v) containing 1.6 mg of atrazine or
simazine was injected into the column from the top end. In one
treatment, 20 mL of solution containing 80 mg of sodium tetrasulfide
was injected into the column from the top end, and in another treatment,
20 mL of water was similarly injected. The columns were kept at room
temperature for 7 days, and then, water was applied to the top end of
-
1
the column at 1.0 mL min . The leachate was collected from the
bottom end using a Bio-RAD model 2110 fraction collector (Bio-RAD,
Richmond, CA). The collected leachate samples were analyzed for
atrazine and simazine by HPLC under the conditions given above.
2
40, and 360 µM. The initial polysulfide concentration was 20 mM,
RESULTS AND DISCUSSION
and the initial herbicide concentration was 20 µM. A control treatment
without 1,4-benzoquinone amendment was similarly prepared. The
samples were equilibrated at room temperature, and aliquots of the
reaction mixture were removed for analysis after different time intervals
following the initiation of the reaction.
To analyze atrazine, simazine, or cyanazine in the reaction media,
a 30 µL aliquot of the aqueous sample was injected into an Agilent-
Effect of Free Radical Inhibitors on Reaction Kinetics.
The reaction of atrazine, cyanazine, and simazine with polysul-
fides was followed under different conditions to understand the
role of free radicals in the reaction. The reaction rates were
first measured between polysulfides and herbicides at different
initial tetrasulfide concentrations under ambient conditions
(room temperature and ambient air headspace). The disappear-
ance of the herbicide parent compound over time was fitted to
a first-order decay model to estimate the rate coefficient kobs
1
(
100 series HPLC. Herbicides were eluted on a 250 mm × 4.0 mm
i.d.) reverse phase column (Hypersil ODS, 5 µm, Agilent, Wilmington,
DE) and detected using a variable multiwavelength UV detector at 230
10 nm for all compounds. The mobile phase was made of acetonitrile
(
(
Table 1). The fit was generally good, with the correlation
and water that was acidified to pH 3 with phosphoric acid. The
percentage of acetonitrile was 60% for atrazine, 55% for cyanazine,
and 50% for simazine. The flow rate of the mobile phase was 0.9 mL
2
coefficient r g 0.95. In untreated water, all chloro-s-triazine
herbicides were found to be stable with no noticeable degrada-
tion over 72 h. However, the disappearance of all chloro-s-
triazine herbicides was accelerated in the sodium tetrasulfide
solution, and the rate of disappearance was proportional to the
initial polysulfide concentration, as shown in Figure 1 for
atrazine. At an initial polysulfide concentration of 10 mM, the
-
1
min , and the temperature of the column was maintained at 35 °C.
The concentration of herbicide was calculated by external calibration
with standards.
Spectroscopic Analysis. To understand the role of free radicals in
the reaction, spectroscopic analysis was performed using the reaction

Reaction of Triazine Herbicides with Polysulfides
J. Agric. Food Chem., Vol. 52, No. 23, 2004 7053
Table 2. Pseudo First-Order Rate Coefficient kobs (h-1) for the
Disappearance of Selected s-Triazine Herbicides in Sodium Polysulfide
Solutions under Ambient (Control Treatment), Nitrogen-Purged, and
Benzoquinone-Amended Conditions
polysulfide
treatment
control
nitrogen purged
control
(mM)
atrazine
cyanazine
simazine
10
10
20
20
20
20
0.061
±
±
±
±
±
±
0.003 0.064
±
±
±
±
±
±
0.005
0.016
0.027
0.013
0.0108 0.106
0.007 0.068
0.059
±
±
±
±
±
±
0.004
0.214
0. 147
0.124
0.106
0.070
0.024 0.224
0.013 0.149
0.009 0.126
0.006 0.107
0.004 0.075
0.204
0.138
0.123
0.009
0.038
0.007
0.004
0.005
120
240
360
µ
µ
µ
M benzoquinone
M benzoquinone
M benzoquinone
Figure 1. Dissipation of atrazine (20
µM) in sodium tetrasulfide solutions
at room temperature (21 C). Legends indicate the initial sodium
±
1 °
tetrasulfide concentrations used. Symbols are measured data points, and
curves are fitted lines by using a simplified second-order kinetics model
as defined in eq 1.
T1/2 of s-triazine herbicides was reduced to <12 h. When the
initial polysulfide concentration was increased to 30 mM, the
T1/2 values of atrazine, cyanazine, and simazine decreased to
∼
3 h. Under these conditions, polysulfides enhanced the
dissipation of each herbicide by several orders of magnitude.
Because SRN1 is a second-order reaction as an SN2 reaction
Figure 2. ESR spectra of 50 mM sodium tetrasulfide solution following
different treatments measured at 298 K. Legends indicate different
treatments or conditions.
(
10), the reaction between the polysulfides and the triazine
herbicides may be characterized using second-order kinetics (2).
When the initial herbicide concentration C0 is much smaller
than the initial polysulfide concentration X0, the kinetics can
be approximated by the following first-order model:
inhibitory influence of free radical scavengers suggests that free
radicals may have contributed to the transformation reaction of
s-triazine herbicides by polysulfides.
Changes in Free Radical Levels during Reaction. Analysis
of changes in free radical concentrations in the polysulfide
solution under the various experimental conditions provided
additional evidence for the involvement of free radicals in the
reaction. While there are a number of methods for the detection
of free radicals, the most specific method is ESR (13). Typical
ESR spectra are given in Figure 2 for sodium tetrasulfide
solution under ambient atmosphere, following nitrogen purge,
after the addition of 1,4-benzoquinone and after the reaction
with atrazine. The recorded ESR spectra of polysulfide solutions
following various treatments were fitted to a Lorentzian line
shape:
-
µX0t
C ) C e
(1)
t
0
where µ is the second-order rate coefficient (M- s-1), t is the
elapsed time, and Ct is herbicide concentration at time t. The
second-order rate coefficient µ was estimated through regression
by fitting the measured data to eq 1. The mean µ values were
1
-
3
2
-3
2
1
.98 × 10 (r ) 0.99), 1.90 × 10 (r ) 0.99), and 2.14 ×
-3 -1 2
s-1 (r ) 0.99) for atrazine, simazine, and cyanazine,
10
M
respectively. These values were in general agreement with
observations by Lippa and Roberts (4), who reported a µ value
-
3
-1 -1
-3
-1
of 6.90 × 10
M
s
for cyanazine and 5.64 × 10
M
-
1
s
for atrazine at a temperature (25 °C) slightly higher than
0
-1
that used in this study (21 °C).
3
2Y(H - H )(∆H )
pp
The effect of molecular oxygen and benzoquinone, two
common radical inhibitors, was evaluated in the reaction
between sodium tetrasulfide and herbicides (11, 12). When
oxygen was removed from the reaction media after purging with
nitrogen, the reaction rate between polysulfides and each
herbicide increased significantly (R ) 0.05; Table 2). For
f(H) )
(2)
0
-1 2 2
{
3 + [2(H - H )(∆H ) ] }
pp
0
where H is the center of the signal, ∆Hpp is the peak-to-peak
line width, and Y is the maximum of the derivative of the signal.
The fit between the experimental data and the f(H) value was
performed using a nonlinear least-squares method. The area A
of the ESR signal was deduced from the parameters ∆Hpp and
Y:
-
1
instance, the estimated kobs for atrazine in nitrogen (0.214 h )
-
1
was more than two times greater than that in air (0.061 h ). A
similar enhancement was observed also for cyanazine and
simazine after nitrogen purge. These results suggest that
molecular oxygen in the ambient air may have scavenged free
radicals in the polysulfide solution, thus reducing the reaction
rate. The reaction was inhibited after the addition of another
free radical inhibitor, 1,4-benzoquinone, and the inhibition
became more pronounced at higher benzoquinone concentrations
2
A )
x
4/3πY(∆Hpp)
(3)
Table 3 gives the calculated results for the ESR spectra. It
is evident that after reaction with the triazine herbicides, the
ESR signal as area A for the reaction mixture significantly
decreased. Under similar conditions, the addition of 1,4-
benzuqionone also resulted in a much smaller response, while
(
Table 2). Under the test conditions, the addition of benzo-
quinone at 360 µM reduced the reaction by >50%. The overall

7054 J. Agric. Food Chem., Vol. 52, No. 23, 2004
Yang et al.
Table 3. Parameters of the ESR Absorption Signal in Sodium
Tetrasulfide Borate Solution (pH 9.5) at 298 K Following Various
Treatments
Na
3309
1.38
2
S
4
atrazine
−
Na
2
S
4
cyanazine
−
Na
S
2 4
simazine
−
Na
104
0.07)
2 4
S
H0 (G)
Y
3316
3319
3312
×
105
1.53
×
104
1.52
113.8
7.1 (
×
104
1.46
112.9
6.8 (
×
∆
H
×
pp (G) 102.5
114.3
8
A (
10 ) 52.6 (
±
0.16)
7.3 (±
0.09)
±
0.08)
±
the purge with nitrogen resulted in a larger response, when
compared to the reference sample that contained sodium
tetrasulfide under an ambient atmosphere. These results suggest
that after the substitution reaction the concentration of free
radicals in the polysulfide solution decreased, and the effect
was similar to that of free radical scavengers such as oxygen
and 1,4-benzoquinone.
Reaction Mechanism. The above evaluation suggests that
the reaction between polysulfides and triazine herbicides likely
involved free radicals. A free radical chain reaction typically
includes initiation, propagation, and termination steps (14).
Oxygen or 1,4-benzoquinone can effectively scavenge free
radicals, terminating the reaction chain (15). The polymerization
reaction between sodium sulfide and chloroaromatic compounds
was characterized in a previous study, where it was observed
that if oxygen or hydroquinone was present, the reaction slowed
(
16). The authors interpreted the reaction as a SRN1 reaction.
2-
Levillain et al. (9) studied Sn by ESR spectroscopy and found
that the signal of ESR significantly decreased with a decreasing
•
-
S3 concentration in the solution. Therefore, it may be
•
-
speculated that S3 is likely the most important free radical in
polysulfides for the SRN1 reaction with the triazine herbicides.
Many SRN1 substitutions require an external stimulation
involving the injection of a catalytic amount of electrons by
dissolving metals in liquid ammonia or from an electrode (17).
However, there are several examples where the reaction occurs
without external stimulation, i.e., via the “thermal” SRN1
mechanism (18). The reaction of triazine herbicides with
polysulfides may occur through the thermal SRN1 mechanism.
In the initiation step, an ET from the Sn2 may take place. This
ET can follow a concerted dissociative step to directly afford a
Figure 3. Effect of sodium tetrasulfide amendment on leaching of atrazine
and simazine through sand columns. (A) Herbicide concentrations in
leachate and (B) cumulative fractions of herbicide leached through the
packed column.
step (22). Because the SN and ET mechanisms have a common
link (23), it is possible that the reaction between polysulfides
and triazine herbicides may occur simultaneously via the SNAr
and SRN1 pathways.
-
-
triazine radical and Cl (19). The triazine radical can react with
the Sn2 to give another intermediate radical (PhX ), which
-
•-
Decontamination of Herbicide Residues in Sand. The rapid
reaction and detoxification under varying environmental condi-
tions makes the polysulfide-induced dechlorination reaction a
versatile option for decontaminating these herbicides in envi-
ronmental media. We tested the use of this approach for
removing herbicide residues from sand as a simulation of sandy
aquifer remediation. The amendment of polysulfides into the
sand columns significantly reduced the herbicide concentrations
in the leachate as well as the cumulative fraction of herbicide
leached through the column (Figure 3). The peak concentrations
of atrazine and simazine in the leachate were detected at 53.4
and 55.4 µM, respectively. In the polysulfide-amended columns,
the maximum concentration was only 7.1 µM for atrazine and
1.6 µM for simazine. Cumulatively, about 79% of the spiked
atrazine and 85% of the spiked simazine were recovered in the
leachate in the nonamended columns. In contrast, only 7.1% of
the spiked atrazine and 2.7% of the spiked simazine broke
through the column after the amendment of sodium tetrasulfide.
These preliminary observations suggest that the reaction may
be used for removing herbicide residues in contaminated
environmental media. Although not tested in this study, potential
treatments may be also feasible for treating herbicide-containing
wastewater from manufacturing processes or rinsing of pesticide
by ET to the substrate forms the intermediates needed to
•
-
continue the propagation cycle. PhX , a short-lived species,
has been demonstrated to exist in SRN1 processes by electro-
chemical investigation (20). On the whole, the process affords
a nucleophilic substitution, in which a chlorine is replaced by
a sulfur dianion. It must be noted that both SRN1 and SNAr obey
the second-order kinetics and that both reactions lead to the
formation of the same products. Therefore, analysis of reaction
kinetics and products alone, as used in Lippa et al. (4), may
not preclude the potential occurrence of SRN1.
Although evidences presented above suggest a pathway from
free radical reaction between polysulfides and triazine herbi-
cides, SNAr and SRN1 may exist at the same time. Many of the
features of the SN/ET dichotomy have been analyzed, and the
theoretical aspects have been reviewed (21, 22). In fact, because
SN and ET mechanisms both involve activation, which is
associated with the single electron movement, they have a
common link. Sometimes, the SN reaction is even called “a
single electron shift” mechanism, which means that the single
ET occurs in a single step, which also involves the bond
coupling between the nucleophile and the substrate, while in
the ET mechanism, the single ET precedes the bond coupling

Reaction of Triazine Herbicides with Polysulfides
J. Agric. Food Chem., Vol. 52, No. 23, 2004 7055
containers, for decontaminating spills, and for cleaning up
contaminated sites (e.g., at dealerships). However, more research
is needed for further developing these applications. For instance,
the role of oxygen needs to be better understood under field
conditions as it lowers the reactivity of polysulfides, as
demonstrated in this study. On the other hand, the use of
polysulfides under reduced conditions may lead to the formation
of undesirable byproducts such as hydrogen sulfide. The
environment compatibility of this reaction should be also
evaluated with consideration of the potential formation and
adverse effects of various reduced sulfur species.
(12) Shiraishi, Y.; Kojima, S.; Tomita, H.; Ohsuka, H.; Kawamura,
T.; Toshima, N.; Hirai, H. Selective carboxylation of benzoic
acid using cyclodextrin as mediator. Polym. J. 1996, 28, 619-
6
26.
(13) Hanns, F. Structure of free radicals by ESR spectroscopy. In
Free Radicals; Kochi, J., Eds.; Wiley Interscience: New York,
1
973; Vol. 2, pp 435-491.
(
14) Motherwell, W.; Crich D. Free Radical Chain Reactions in
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10-14.
(15) Michael, B.; Jerry, M. March’s AdVanced Organic Chemistry:
Reactions, Mechanisms, and Structure, 5th ed.; Wiley Publica-
tion: New York, 2001; pp 894-899.
(
16) Annenkova, V.; Antonik, L.; Vakulskaia, T.; Voronkov, M. The
interaction between sodium sulfide and chloroaromatic com-
pounds in the n-methyl-2-pyrrolidone medium. Doklady Aka-
demii Nauk SSSR 1986, 286, 1400-1403.
ACKNOWLEDGMENT
We thank Dan Borchardt, Department of Chemistry, University
of California, Riverside, for help with the experiment of ESR
spectra.
(
17) Saveant, J. Mechanisms and reactivity in electron-transfer-
induced aromatic nucleophilic-substitutionsRecent advances.
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Received for review March 25, 2004. Revised manuscript received
August 20, 2004. Accepted September 3, 2004. This study was supported
by USDA-NRI Award 2001-35102-10861.
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JF049505P