Hydrolysis kinetics of fenthion and its metabolites in buffered aqueous media

Article abstract of DOI:10.1021/jf9907539

This study investigates the hydrolysis kinetics of fenthion and its five oxidation metabolites in pH 7 and pH 9 buffered aqueous media at 25, 50, and 65 °C. Five metabolites and three hydrolysis products were synthesized and purified. The reactant and the corresponding hydrolysis products were determined by HPLC. Rate constant and half-life studies revealed that fenthion and its metabolites were relatively stable in neutral media, and their stability decreased as pH increased. The half-lives at 25 °C ranged from 59.0 days for fenthion to 16.5 days for fenoxon sulfone at pH 7, and from 55.5 days for fenthion to 9.50 days for fenoxon sulfone at pH 9; half- lives were greatly reduced at elevated temperatures. The activation energy (E(a)) was found to range from 16.7 to 22.1 kcal/mol for the compounds investigated. The phenol hydrolysis product of fenthion and fenoxon, 3- methyl-4-methylthiophenol was not stable in pH 7 and pH 9 buffered solutions at 50 °C, whereas 3-methyl-4-methylsulfonylphenol and 3-methyl-4- methylsulfinylphenol were relatively stable under the same conditions. At pH 9, the primary hydrolysis mechanisms of fenthion and its oxidation metabolites were combination of hydroxide ion and neutral water molecule attacking on the P atom to form corresponding phenol derivatives. Under neutral conditions, the primary hydrolysis mechanisms of fenthion and its oxidation metabolites were assumed to be the combination of water molecule attacking on the P atom to form phenol derivatives and on the C atom to yield dealkylation products.

Full text of DOI:10.1021/jf9907539

2582
J. Agric. Food Chem. 2000, 48, 2582−2588
Hyd r olysis Kin etics of F en th ion a n d Its Meta bolites in Bu ffer ed
Aqu eou s Med ia
J iping Huang and Scott A. Mabury*
Department of Chemistry, University of Toronto, Toronto, Ontario M5S 3H6, Canada
This study investigates the hydrolysis kinetics of fenthion and its five oxidation metabolites in pH
7 and pH 9 buffered aqueous media at 25, 50, and 65 °C. Five metabolites and three hydrolysis
products were synthesized and purified. The reactant and the corresponding hydrolysis products
were determined by HPLC. Rate constant and half-life studies revealed that fenthion and its
metabolites were relatively stable in neutral media, and their stability decreased as pH increased.
The half-lives at 25 °C ranged from 59.0 days for fenthion to 16.5 days for fenoxon sulfone at pH 7,
and from 55.5 days for fenthion to 9.50 days for fenoxon sulfone at pH 9; half-lives were greatly
reduced at elevated temperatures. The activation energy (Ea) was found to range from 16.7 to 22.1
kcal/mol for the compounds investigated. The phenol hydrolysis product of fenthion and fenoxon,
3-methyl-4-methylthiophenol was not stable in pH 7 and pH 9 buffered solutions at 50 °C, whereas
3-methyl-4-methylsulfonylphenol and 3-methyl-4-methylsulfinylphenol were relatively stable under
the same conditions. At pH 9, the primary hydrolysis mechanisms of fenthion and its oxidation
metabolites were combination of hydroxide ion and neutral water molecule attacking on the P atom
to form corresponding phenol derivatives. Under neutral conditions, the primary hydrolysis
mechanisms of fenthion and its oxidation metabolites were assumed to be the combination of water
molecule attacking on the P atom to form phenol derivatives and on the C atom to yield dealkylation
products.
Keyw or d s: Fenthion; metabolites; hydrolysis; kinetics; half-life
INTRODUCTION
Organophosphorus insecticides are widely used in
agriculture and animal production for the control of
various insects. These compounds generally have higher
acute toxicity than chlorinated insecticides, which is due
to the inhibition of the enzyme cholinesterase, an
essential component of the animal nervous system (Aly
and Badaway, 1982). The persistence of organophos-
phorus insecticides in aquatic environments is affected
by several factors, such as oxidation, photolysis (Oh-
kawa et al., 1974), and biological degradation (Ruzicka
et al., 1967). Chemical hydrolysis of organophosphorus
insecticides was reported to play an important role in
the persistence of these compounds in the environment
(Cowart et al., 1971). In addition to the aquatic environ-
ment, rainfall, atmospheric water vapor, and soil mois-
ture all provide ample opportunity for hydrolysis.
Fenthion (O,O-dimethyl O-[4-(methylthio)-m-tolyl]
phosphorothioate (I) (Figure 1) is used as a contact and
stomach insecticide in the control of fruit flies in many
crops (Cabras et al., 1991). Once used extensively,
fenthion was classified by the U.S. EPA as a Restricted
Use Pesticide (RUP) due to its toxicity (Kamrin, 1997).
Residual levels of fenthion have been reported in various
environmental matrices (Wang et al., 1987). Five oxida-
tion metabolites of fenthion have been isolated in
animals and plants and include fenthion sulfoxide (II),
fenthion sulfone (III), fenoxon (IV), fenoxon sulfoxide
(V), and fenoxon sulfone (VI) (Figure 1) (Cabras et al.,
1993). The metabolites showed higher toxicity than the
F igu r e 1. Fenthion (I), its five metabolites (II-VI) and three
hydrolysis products H-I to H-III.
parent compounds: the lethal dose (LD50) was 220 mg/
kg for I, 125 mg/kg for II-IV, 50 mg/kg for V, and 30
mg/kg for VI. The sulfoxide (II) and sulfone (III) were
observed as fenthion photolysis products on orange fruit
surfaces (Minelli et al., 1996). Photoinduced oxidation
of sulfur to the sulfoxide and sulfone has been commonly
observed in water or simulated natural waters; ex-
amples include molinate and thiobencarb (Draper and
Crosby, 1984) and albendazole (Weerasinghe et al.,
1992). Oxidation of the sulfur in phosphorothioates has
also been observed in field water (e.g., fenitrothion to
fenitrooxon), although the oxon derivative is generally
unstable (Lacorte and Barcelo, 1994). Since hydrolysis
is one of the principal detoxification mechanisms for
organophosphorus insecticides, hydrolysis of I-VI leads
to compounds of low toxicity (FAO/WAO, 1971). The
* Corresponding author. Tel: (416) 978-1780. Fax: (416)
978-3596. E-mail: smabury@alchemy.chem.utoronto.ca.
10.1021/jf9907539 CCC: $19.00 © 2000 American Chemical Society
Published on Web 05/03/2000

Hydrolysis Kinetics of Fenthion
J. Agric. Food Chem., Vol. 48, No. 6, 2000 2583
TurboMass mass spectrometer in electron impact (EI, 70 eV)
mode was used for obtaining mass spectra of synthesized
compounds. The UV spectra of each compound was obtained
on a Waters HPLC system containing a diode array detector
(model 996).
II, III, and VI were prepared from fenthion following the
general procedure reported by Cabras et al. (1991) with some
modification.
Fenthion Sulfoxide (II). 0.15 M H2O2/SeO2 in methanol
solution was added dropwise to a solution of fenthion in
methanol at room temperature with a magnetic stirrer. The
reaction mixture was then stirred for 15 min, diluted with a
saturated sodium chloride solution and extracted with chlo-
roform. The chloroform layer was cleaned up and dehydrated
with anhydrous sodium sulfate, and subsequently removed by
rotary evaporation to give the crude fenthion sulfoxide as
yellow oil.
Fenthion Sulfone (III). 0.15 M KMnO4 in methanol was
added to a solution of fenthion in acetic acid at room temper-
ature with a magnetic stirrer. The reaction mixture was then
stirred for 12 h, diluted with 35 mL of HPLC grade water and
extracted with chloroform. The chloroform layer was neutral-
ized with NaHCO3, cleaned up, and dehydrated with anhy-
drous sodium sulfate. After evaporation of solvent, crude III
appeared as a white solid.
chemical structures of the three hydrolysis products (H-
I, H-II, and H-III) are also shown in Figure 1.
Several studies concerning the persistence of fenthion
in the aquatic environment have been carried out. It is
relatively stable under acidic conditions, and moderately
stable in alkaline conditions, with the following half-
lives at 22 °C: 223 days at pH 4, 200 days at pH 7, and
151 days at pH 9 (Tomlin, 1994). Fenthion degrades
faster in river water (Eichelberger and Lichtenberg,
1971) than in the presence of sediment (difference in
half-life was from 7 to 23 days), biodegradation being a
relevant process (Walker et al., 1988). Fenthion de-
grades much faster under sunlight conditions than in
darkness (Lartigues and Garrigues, 1995), with the
following half-lives at 22 °C: 42 days in darkness and
2 days under sunlight for river water at pH 7.3; 26 days
in darkness and 5 days under sunlight for seawater at
pH 8.1. The half-lives of fenthion and its photooxidation
product, fenthion sulfoxide, in estuarine waters were
reported to be 4.6 and 6.9 days, respectively (Lacorte
et al., 1995).
We hypothesized photodegradation of fenthion in
natural waters would lead to similar degradation prod-
ucts (Huang and Mabury, 2000a) which are more toxic
than the initial insecticide. Fenthion provides an excel-
lent structural template in order to observe the influ-
ence of oxidation on hydrolysis rates. We further
hypothesized the oxidized metabolites would be more
susceptible to hydrolysis and thus yield a faster detoxi-
fication. In this study, we investigated the kinetics of
hydrolysis of fenthion and its five metabolites at two
pH values (pH 7 and pH 9) and three temperatures; we
also identified and measured the rate of appearance of
hydrolysis products in order to determine the reaction
mechanism more accurately. This research work is of
particular environmental significance since the pH
range of most natural waters is from pH 7 to pH 9
(Huang and Mabury, 2000b). The objective of this
investigation was to enhance the knowledge of the fate
of fenthion and its metabolites in aqueous solutions and
the hydrolysis mechanisms at different pH values.
Fenoxon sulfone (VI) was obtained by procedures similar to
those used for fenthion sulfone: 0.20 M KMnO4 in methanol
was added to a solution of fenthion in acetic acid. The reaction
mixture was then stirred for 16 h.
Intermediate to the synthesis of fenoxon (II), 3-methyl
4-methylthiophenol (H-I), was prepared by reaction of m-cresol
with methyl disulfide (Ho et al., 1987). The concentrated
sulfuric acid was added to a mixture of m-cresol and methyl
disulfide, and the reaction mixture was stirred for 6 h in an
ice bath. The solution was diluted with HPLC grade water and
extracted two times with chloroform. The organic layers were
neutralized with NaHCO3, washed with water, and dehydrated
with anhydrous sodium sulfate. Under reduced pressure of 10
Torr, the organic solvent and the remaining m-cresol were
removed to give crude 3-methyl-4-methylthiophenol. The
chemical structure was confirmed by GC-MS with m/z 154 (M+,
100), 139 (65), 107 (100), 95 (32), and 77 (23), 1H NMR δΗ 2.33
(S, 3H), 2.40 (S, 3H), 5.56 (S, 1H), 6.69 (d, 2H), and 7.15 (d,
1H), and UV spectra with a specific peak at 250 nm.
Fenoxon (IV) was prepared by reaction of 3-methyl-4-
methylthiophenol (MMTP) with dimethyl phosphorochloride
in acetone in the presence of sodium carbonate following the
general procedure reported by Green et al. (Green et al., 1987).
MMTP was put in acetone, and Na2CO3 and dimethyl phos-
phorochloride were added. The reaction mixture was then
refluxed at 60-70 °C water bath for 5 h. After acetone was
removed, the solution was diluted and extracted with chloro-
form. The organic layer was washed with water and dehy-
drated with anhydrous sodium sulfate. A rotary evaporator
was used to obtain crude IV liquid.
MATERIALS AND METHODS
Ch em ica ls. Fenthion (98%) was obtained from Chem
Service (West Chester, PA). Sodium phosphate dibasic (ACS
grade) and potassium phosphate (ACS grade) were obtained
from ACP Chemicals, Inc. (Montreal, Quebec). Disodium
tetraborate (Borax, ACS grade) was purchased from BDH Inc.
(Toronto, ON). Potassium permanganate, m-cresol (99%),
hydrogen peroxide (30%), sodium sulfate anhydrous (ACS
grade), and silica gel (70-230 mesh, 60 Å) were obtained from
Aldrich Chemical Co. (Oakville, ON). Selenium dioxide (99%)
was obtained from BPH Chemicals Ltd. (Poole, England).
Sulfuric acid (ACS grade) was purchased from J .T. Baker Inc.
(Phillipsburg, NJ ). Methyl disulfide (98%) was obtained from
MC/B (Norwood, OH). Acetonitrile (HPLC grade), water
(HPLC grade), methanol (HPLC grade), hexanes (ACS grade),
ethyl acetate (ACS grade), and acetic acid (99.7%) were
purchased from Caledon (Georgetown, ON). Other metabolites
and hydrolysis products were synthesized from the following
procedures.
Syn th esis of Com p ou n d s. The metabolites of fenthion
(II-VI), H-I, H-II, and H-III were synthesized through the
following procedures. These compounds were then purified
through silica gel column chromatography with hexane and
ethyl acetate at varying ratio as solvent. The chemical
structure of each compound was confirmed by GC-MS. A
Perkin-Elmer Autosystem XL GC with a MDN-5 column (30
m × 0.25 mm, 0.25 µm film thickness) coupled to a PE
Fenoxon sulfoxide (V) was obtained by the same procedures
as those used for fenthion sulfoxide reported by Cabras et al.
(1991).
The other two hydrolysis products of its metabolites, 3-meth-
yl-4-methylsulfinylphenol (H-II) and 3-methyl-4-methylsulfo-
nylphenol (H-III), were prepared by hydrolysis of fenthion
sulfoxide and fenoxon sulfone under strong basic conditions.
Fenthion sulfoxide or fenoxon sulfone was added in 1 N NaOH
ethanol solution (ethanol:H2O ) 1:1). The reaction mixture was
agitated under magnetic stirring for 8 h in 50-60 °C water
bath, and then 2.5 N HCl was added to neutralize the mixture.
The mixture was diluted with saturated sodium chloride and
extracted with ethyl acetate. The organic layer was washed
with water and dehydrated with anhydrous sodium sulfate.
The rotary evaporator was used to turn crude H-II and H-III
into crystals. The chemical structure of H-II was identified
by GC-MS with m/z 186 (M+, 100), 171 (95), 123 (39), 107 (56),
and 77 (40) and UV spectra with a specific peak at 240 nm.
The chemical structure of H-III was identified by GC-MS with

2584 J. Agric. Food Chem., Vol. 48, No. 6, 2000
Huang and Mabury
Ta ble 1. HP LC An a lysis Con d ition s
mobile phase
retention time
(min)
analysis compound
fenthion (I), (H-I)
(ACN:H2O) λ (nm)
75:25
65:35
35:65
30:70
55:45
40:60
65:35
30:70
250 I, 7.5; H-I, 3.9
250 IV, 5.7; H-I, 4.8
245 II, 18; H-II, 3.6
245 V, 6.2; H-II, 4.2
225 III, 8.3; H-III, 3.7
225 VI, 5.6; H-III, 4.5
250 H-I, 4.8
fenoxon (IV), (H-I)
fenthion sulfoxide (II), (H-II)
fenoxon sulfoxide (V), (H-II)
fenthion sulfone (III), (H-III)
fenoxon sulfone (VI), (H-III)
H-I
H-II; H-III
240 H-II, 4.2; H-III, 5.7
m/z 170 (M+, 35), 155 (87), 154 (100), 139 (42), 107 (23), and
109 (24), and UV spectra with a specific peak at 240 nm.
Exp er im en ta l P r oced u r es. Buffer solutions were utilized
for determination of hydrolysis kinetics in order to ensure
consistent pH over the course of the experiment given the
production of weak acids following cleavage of the ester bond;
our primary aim was elucidation of reaction kinetics and
mechanism which required strict control of solution conditions.
Individual solutions were prepared to pH 7 (0.025 M KH2PO4
and 0.025 M Na2HPO4) or pH 9 (0.01 M Borax), which were
adjusted with 0.1 M HCl and 0.1 M NaOH at different
temperatures of 25, 50, or 65 °C; the water used was filter
sterilized (0.45 µm) to preclude microbial degradation. Indi-
vidual solutions of I-VI, 50-100 µM in duplicate, at pH 7 and
pH 9 buffers were prepared in 15 mL screw-top test tubes
which were wrapped with foil to preclude photolysis. The test
tubes were placed into a water bath at the specified temper-
atures. Samples at appropriate time intervals were taken for
analysis of the organophosphate and corresponding hydrolysis
product; each time point was determined in triplicate.
F igu r e 2. 2. Hydrolysis rate of I-III at 50 °C and pH 7 and
pH 9 (a) and IV-VI at 50 °C and pH 7 and pH 9 (b); N ) 2.
An a lysis by HP LC. I-VI and the primary hydrolysis
products (H-I to H-III) concentration were determined by
direct injection in an HPLC. A complete Waters HPLC system
was employed with a Waters 600 pump and controller and a
Waters 486 tunable absorbance detector. Separations were
performed using a reverse-phase Econosil C18 column (Alltech)
25 cm long with an inner diameter of 4.6 mm and 5 µm particle
size. Concentrations (50-100 µM) were chosen so that aqueous
samples were analyzed directly, without preconcentration or
sample purification. The isocratic conditions were carried out
with acetonitrile-water in various ratios at a flow rate of 1.0
mL/min; HPLC analysis conditions including mobile phase,
detection wavelength, and retention time are shown in Table
1. Calibration was performed daily using external standards.
Ta ble 2. Hyd r olysis Ra te Con sta n t k a n d Ha lf-life t1/2 a t
Differ en t Tem p er a tu r esa
k (day-1) t1/2 (days) k (day-1) t1/2 (days)
compound at pH 7 at pH 7 at pH 9 at pH 9
temp
25 °C
fenthion
0.0117 59.0
0.0125 55.5
fenthion sulfoxide 0.0169
fenthion sulfone 0.0278
41.0 0.0200 34.7
24.9
46.5
21.3
16.5
0.0461 15.0
0.0213 32.5
0.0425 16.3
0.0730 9.50
fenoxon
0.0149
fenoxon sulfoxide 0.0326
fenoxon sulfone 0.0420
50 °C
fenthion sulfoxide 0.181
fenthion sulfone 0.257
fenoxon 0.146
fenoxon sulfoxide 0.314
fenoxon sulfone 0.446
fenthion
0.143
4.86
0.144
3.59
2.11
2.80
1.25
0.55
0.583
0.95
0.48
0.90
0.26
0.11
4.80
1.19
3.83
2.69
4.74
2.55
1.55
0.545
1.26
0.64
1.15
0.68
0.38
0.193
0.331
0.247
0.557
1.250
1.27
0.725
1.43
0.766
2.66
RESULTS AND DISCUSSION
65 °C
fenthion
fenthion sulfoxide 0.550
fenthion sulfone 1.08
Effect of p H on Hyd r olysis. Chemical hydrolysis
of fenthion and its metabolites was investigated using
buffered solutions at pH 7 and pH 9, a typical range of
natural waters. All plots of ln(C/C0) (C0 is the reactant
concentration at T ) 0, C is the reactant concentration
at various times) versus time were linear at pH 7 and
pH 9, indicating that the reaction tends to be pseudo
first order as indicated in Figure 2. The hydrolysis rate
constant and half-life (Table 2) indicate the relative
stability of fenthion and its metabolites in a neutral
medium. The rate constant were reproducible with a
relative error less than 2%. The stability of these
compounds decreased as the pH increased from pH 7
to pH 9, and the hydrolysis rate constants were higher
at pH 9 than that at pH 7 for fenthion and each of the
metabolites investigated. At 50 °C, the hydrolysis half-
life varied from 4.86 days for fenthion to 1.55 days for
fenoxon sulfone at pH 7, and from 4.80 days for fenthion
to 0.55 day for fenoxon sulfone at pH 9. Chemical
hydrolysis of fenthion and its metabolites at 25 and 65
fenoxon
0.602
fenoxon sulfoxide 1.02
fenoxon sulfone 1.84
6.30
a
The data of measured rate constants were reproducible with
relative error less than 2%.
°C was also investigated using the same buffered
solutions. As a result, the hydrolysis rate constant and
half-life indicate the same relative stability in a neutral
medium, which is shown in Table 2. Hydrolysis pro-
ceeded at higher rates under alkaline conditions, sug-
gesting that the reaction was more effectively catalyzed
by hydroxide ions than by neutral water molecules or
hydronium ions; this was more apparent for the oxon
derivatives. However, the slight difference of rate
constants for fenthion at pH 7 and 9 indicated no
significant influence of pH on the hydrolysis rate of
fenthion over this pH range. The relatively rapid

Hydrolysis Kinetics of Fenthion
J. Agric. Food Chem., Vol. 48, No. 6, 2000 2585
Ta ble 3. Activa tion En er gy Ea , F r equ en cy F a ctor A, a n d
Activa tion En tr op y ∆Sq
compound
at pH 7
fenthion
fenthion sulfoxide
fenthion sulfone
fenoxon
fenoxon sulfoxide
fenoxon sulfone
at pH 9
Ea (kcal/mol) A (s-1
)
∆Sq (cal/(mol K))
19.2
17.4
18.6
18.4
17.3
18.8
1.65 × 107
1.31 × 106
1.28 × 107
5.05 × 106
1.74 × 106
2.97 × 107
-27.7
-32.7
-28.3
-29.9
-32.1
-26.4
fenthion
19.2
17.9
16.7
18.1
20.6
22.1
1.61 × 107
3.00 × 106
1.33 × 106
4.11 × 106
5.52 × 108
1.40 × 1010
-27.9
-31.2
-32.6
-30.4
-20.6
-11.7
fenthion sulfoxide
fenthion sulfone
fenoxon
fenoxon sulfoxide
fenoxon sulfone
fenthion oxidation derivatives perhaps explains the lack
of literature citations for these compounds in field
waters.
Effect of Tem p er a tu r e on Hyd r olysis. At 25 °C,
the half-life of fenthion is 59 days at pH 7 and 55.5 days
at pH 9, which is relatively shorter in comparison with
half-lives of fenthion reported at 22 °C: 223 days at pH
4, 200 days at pH 7, and 151 days at pH 9 (Tomlin, 1994)
and half-lives of fenthion at 23.5 °C: 101.8 days at pH
7 and 101.7 days at pH 9 (Wang et al., 1989). The slight
difference of half-lives of fenthion at pH 7 and pH 9
reported by Wang et al. (1989) is consistent with our
results. The half-lives of fenthion as 189 days at 6 °C
and 71 days at 22 °C in darkness for Mill-Q water at
pH 6.1 was reported by Lartiges and Garrigues (Lar-
tigues and Garrigues, 1995). The half-life of fenthion
in buffered aqueous media is much longer than in
environmental water systems, such as half-lives of 26
days at 22 °C in darkness, 5 days under sunlight
condition for seawater at pH 8.1 (Lartigues and Garri-
gues, 1995); persistence of up to 4 weeks in river water
(Eichelberger and Lichtenberg, 1971); half-life of 4.6
days in estuarine waters (Lacorte et al., 1995). Other
reaction pathways to degrade organophosphate pesti-
cides in natural waters such as oxidation, photolysis,
and microbial degradation in addition to hydrolysis
would contribute to increase the overall transformation
rate of fenthion.
F igu r e 3. At 50 °C, the percentage of fenthion and its
metabolites converted into H-I to H-III at the first time point.
Fenoxon sulfone and fenoxon sulfoxide at pH 9 have
the highest rate constants in comparison with other
metabolites at different temperatures. However, their
activation energies were also highest among the com-
pounds investigated with 22.1 and 20.6 kcal/mol, re-
spectively. So, the highest rate constants were due to
the frequency factors A. The frequency factor (A) of V
and VI at pH 9 were 5.52 × 108 and 1.40 × 1010 s-1
respectively; these values were larger than the fre-
quency factors for the other compounds. The relatively
higher frequency factor is due to the entropy effect based
on the activated complex theory (Ruff and Csizmadia,
1994). To prove this point, the activation entropies (∆S‡)
were calculated using the equation of Ruff and Csiz-
madia (1994); all values are shown in Table 3 (the
relative error of activation entropies was less than 5%).
The entropy of activation for fenthion were -27.7 and
-27.9 cal/(mol K) at pH 7 and 9, which is higher than
that of II and III and due to the hydrogen bonding on
the O atom of the sulfinyl and sulfonyl group with water
at the activated complex. As a result, the entropy of
activation of II and III is decreased in comparison with
I.
Using the data shown in Table 2, the activation
energy can be derived from the Arrhenius equation:
On the other hand, the entropy of activation for
fenoxon and V and VI at pH 9 was apparently decreased
from -30.4 cal/(mol K) for fenoxon to -20.6 and -11.7
cal/(mol K) for V and VI, respectively. This large
difference presumably comes from the electrostatic force
between the P atom and hydroxide ion. The strong
electronegative O atom connecting to the P atom would
make the phosphorus of IV, V, and VI more positive
than that of I, II, and III when the S atom replaces the
O atom. In addition, the strong electron-withdrawing
group of sulfonyl and sulfinyl on V and VI result in the
P atom being even more positive than that of IV.
Therefore, the P atom on V and VI, with a partial
positive charge, can form an ion pair with hydroxide ion
through electrostatic force, which greatly decreases the
entropy of reactants. Then, the entropy of activation ∆Sq
between the entropy of the activated complex and
reactants would increase; ∆Sq results were consistent
with the frequency factor (A). This entropy effect may
explain why VI and V at pH 9 with the highest
activation energy (Ea) also have the highest rate con-
stants. At pH 7, the same entropy effect was not
K ) Ae-E /RT
(1)
a
where k (time-1) is the rate constant at temperature T,
A is the frequency factor (time-1), Ea is the activation
energy, and R is the gas constant. The activation energy
(Ea) and frequency factor (A) were calculated using the
Arrhenius equation. Ea and A may be determined with
an accuracy of about 95% since the relative error in the
rate constant was less than 2% (Mabey and Mill, 1978).
The range of Ea was from 17.3 kcal/mol for fenoxon
sulfoxide to 19.2 kcal/mol for fenthion at pH 7 and from
16.7 kcal/mol for fenthion sulfone to 22.1 kcal/mol for
fenoxon sulfone at pH 9. These Ea values were consis-
tent with the activation energies for a range of organo-
phosphate pesticide hydrolysis, ranging from 14 to 22
kcal/mol (Freed et al., 1979). Although pH determines
the hydrolytic mechanism, its effect on Ea is less clear
since Ea depends both on the structure of the molecule
and on the mechanism. The magnitude of the activation
energy provides an indication of the sensitivity of the
reaction to temperature change.

2586 J. Agric. Food Chem., Vol. 48, No. 6, 2000
Huang and Mabury
pH 7, which indicates that there possibly exist different
mechanisms at different pHs. At 50 °C, there are 88%
and 62% of the hydrolysis products for II and III at pH
9, and only 34% and 2% at pH 7. There are 88% and
96% of the hydrolysis products for V and VI at pH 9,
and only 34% and 61% of the hydrolysis products at pH
7. Even in neutral medium, H-III is still the primary
product through water molecules-catalyzed hydrolysis
of fenoxon sulfone. At pH 9, there are 33% H-I and 49%
H-I for fenthion and fenoxon at 50 °C, and only 8% and
2% at pH 7.
It was necessary to determine the stability of three
hydrolysis products at pH 7 and pH 9. Figure 4 shows
the stability of three hydrolysis products at 50 °C. It
can be seen that H-III is stable at pH 7 as well as at
pH 9, and H-II is stable only at pH 7. Also, H-II is
relatively stable at pH 9 with a first-order rate constant
of 0.0634 day-1. H-I is not stable at pH 7 or at pH 9,
with a first-order rate constant of 0.232 day-1 and 2.59
day-1, respectively, which are higher than the hydroly-
sis rate constant of fenthion and fenoxon at similar pH.
This may explain in part why the percentage of H-I is
lower than that of the other two products.
Hyd r olysis Mech a n ism . Fenthion and its metabo-
lites are thiophosphate or phosphate esters. The hy-
drolysis of such compounds occurs through reaction with
a nucleophile by nucleophilic substitution (SN2), both
at the phosphorus atom with an alcohol moiety or a
phenol derivative as the leaving group and at the carbon
bound to the oxygen of an alcohol moiety with the
diester as the leaving group (Schwarzenbach et al.,
1993). The reaction rate of the SN2 reaction depends on
the electrophilic ability of the central atom and on the
presence of a good leaving group. The hydrolysis mech-
anisms of fenthion and its metabolites are fundamen-
F igu r e 4. Hydrolysis rate of H-I, H-II, and H-III at 50 °C.
apparent for IV, V, and VI which we believe is due to
the lack of an ion pair formed with the neutral water
molecule.
Sta bility of th e Hyd r olysis P r od u cts. From their
chemical structures in Figure 1, H-I, H-II, and H-III
are the hydrolysis product of I and IV, II and V, and
III and VI, respectively. These products result from
hydrolysis of the parent compound via hydroxide ion or
water attack at the P atom and can be determined
simultaneously following direct injection into the HPLC.
The percentage of the reactant converted into the
hydrolysis product at 50 °C is shown in Figure 3;
percentage (%) conversion was calculated at the second
time point. The percentage is higher at pH 9 than at
F igu r e 5. Proposed primary hydrolysis pathway for I to VI at pH 7 and pH 9.

Hydrolysis Kinetics of Fenthion
J. Agric. Food Chem., Vol. 48, No. 6, 2000 2587
tally different at pH 7 and pH 9. At pH 9, hydrolysis of
these compounds results in the formation of phenol
derivatives and dialkylphosphoric acid because OH- is
about 108 times stronger than H2O as a nucleophile
toward P atom (Barnard et al., 1961). However, the
neutral water molecule can also act as a nucleophile
toward the P atom at pH 7, which is shown clearly on
the Figure 3. The formation of the phenol derivative is
presumably due to the combined attack of OH- ion and
H2O at the phosphorus atom, which results in cleavage
of the P-O aryl bond. Since H-I is not stable at pH 9,
the SN2 reaction occurring at the P atom is still possibly
the primary mechanism for fenoxon and fenthion even
if the percentage of product was lower than that of other
metabolites. The proposed reaction mechanism is shown
in Figure 5. This mechanism is similar to that reported
for parathion which showed both dearylation and dealkyl-
ation important at different pH values (Weber, 1976).
The percentages of phenol derivative products of
fenoxon (IV) and its oxidized compounds (V, VI) are
higher than those of fenthion and its oxidized com-
pounds (II, III) because of the increased electrophilicity
of the P atom when an O atom replaces an S atom. On
the other hand, H-III is a better leaving group than H-II
or H-I because the negative charge on the phenolic
oxygen atom after the P-O bond cleavage can delocalize
on the two oxygen atoms on the sulfur atom. Similarly,
H-II is a better leaving group than H-I. Even at pH 7,
there is 62% of H-III for VI due to the good leaving
group (50 °C), which indicates that the SN2 reaction
occurring at the phosphorus atom with H2O as nucleo-
phile is still the primary mechanism of hydrolysis
(Figure 5).
Under neutral conditions, the amount of phenol
derivative products is lower than that under alkaline
conditions because water molecules are weaker nucleo-
philes than hydroxide ions. The primary neutral reac-
tion mechanism is presumably through dealkylation of
organophosphate pesticides, which is associated with
the reaction of a water molecule at the alkyl carbon
atom, followed by cleavage of the C-O bond to form the
corresponding alcohol (Trucklik and Kovacicova, 1977);
we were unable to determine the products due to lack
of standards for the resulting aryl monoalkyl phosphoric
acid derivative. The dealkylation of organophosphate
insecticides has previously been observed in buffered
distilled water (Greenhalgh et al., 1980) and in natural
water systems (Maguire and Hole, 1980). We propose
the primary hydrolysis mechanism of fenthion, fenoxon,
and fenthion sulfone at pH 7 is an SN2 reaction
occurring at the carbon bound to the oxygen to form
methanol (Figure 5). If a good leaving group is present,
the neutral reaction may proceed by both reaction
mechanisms, that is, C-O as well as P-O cleavage
(Schwarzenbach et al., 1993). At 50 °C, there are still
34% of H-II in II and 34% of H-II in V at pH 7. Both
reaction mechanisms therefore exist under neutral
conditions for II and V (Figure 5).
products are generally nontoxic and indicate the im-
portance of the role sunlight plays in the natural
cleansing of field waters. This investigation can be used
to enhance the predictive capabilities of determining
persistence of related insecticides by including a more
accurate model of hydrolysis to the large knowledge base
available for predicting volatilization and sorption.
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2588 J. Agric. Food Chem., Vol. 48, No. 6, 2000
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