Photolysis, oxidation, and hydrolysis of 14C-ethyl prothiofos [O-(2, 4-dichlorophenyl) O-ethyl S-propyl phosphorodithioate]

Article abstract of DOI:10.1080/10426501003657535

The chemical stability of the widely used organophosphorus insecticide 14C-prothiofos was studied. For this study prothiofos insecticide and some of its degradation products have been prepared. The oxon of the parent compound was obtained through the oxidation of the insecticide with various oxidizing agents, where yields depended on the type and nature of oxidizing agent. The effect of ultraviolet light and direct sunlight on prothiofos were investigated. Exposure to direct sunlight caused gradual degradation of prothiofos giving the same products as UV irradiation.The hydrolysis of 14C-prothiofos at pH 5, 7, 8, and 9 in buffered aqueous media at 25C, 40C and 55C was studied. The results indicated that prothiofos was stable in acid medium, but it was hydrolyzed to 2,4-dichlorophenol and O-ethyl S-propyl phosphorodithioate in alkaline medium.The degradation products identified by TLC and GC/MS were prothiofos oxon, O-ethyl S-propyl phosphordithioate, des-propyl thioprothiofos, prothiofos oxon sulfoxide, and one unknown compound in addition to the parent compound. The phenolic compound was identified by GC/MS and by its color.Supplemental materials are available for this article. Go to the publisher's online edition of Phosphorus, Sulfur, and Silicon and the Related Elements to view the free supplemental file. Copyright Taylor & Francis Group, LLC.

Full text of DOI:10.1080/10426501003657535

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Photolysis, Oxidation, and Hydrolysis
of 14C-Ethyl Prothiofos [O-(2, 4-
Dichlorophenyl) O-Ethyl S-Propyl
Phosphorodithioate]
H. Abdel-Gawad a , R. M. Abdel-Hameed a & B. Hegazi a
a Applied Organic Chemistry Department , National Research
Centre , Cairo, Egypt
Published online: 02 Jul 2010.
To cite this article: H. Abdel-Gawad , R. M. Abdel-Hameed & B. Hegazi (2010) Photolysis,
Oxidation, and Hydrolysis of 14C-Ethyl Prothiofos [O-(2, 4-Dichlorophenyl) O-Ethyl S-Propyl
Phosphorodithioate], Phosphorus, Sulfur, and Silicon and the Related Elements, 185:7, 1571-1582,
DOI: 10.1080/10426501003657535
To link to this article: http://dx.doi.org/10.1080/10426501003657535
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Phosphorus, Sulfur, and Silicon, 185:1571–1582, 2010
Copyright ꢀC Taylor & Francis Group, LLC
ISSN: 1042-6507 print / 1563-5325 online
DOI: 10.1080/10426501003657535
PHOTOLYSIS, OXIDATION, AND HYDROLYSIS OF
14C-ETHYL PROTHIOFOS [O-(2, 4-DICHLOROPHENYL)
O-ETHYL S-PROPYL PHOSPHORODITHIOATE]
H. Abdel-Gawad, R. M. Abdel-Hameed, and B. Hegazi
Applied Organic Chemistry Department, National Research Centre, Cairo, Egypt
The chemical stability of the widely used organophosphorus insecticide 14C-prothiofos was
studied. For this study prothiofos insecticide and some of its degradation products have
been prepared. The oxon of the parent compound was obtained through the oxidation of
the insecticide with various oxidizing agents, where yields depended on the type and nature
of oxidizing agent. The effect of ultraviolet light and direct sunlight on prothiofos were
investigated. Exposure to direct sunlight caused gradual degradation of prothiofos giving
the same products as UV irradiation.
The hydrolysis of 14C-prothiofos at pH 5, 7, 8, and 9 in buffered aqueous media at 25◦C,
40◦C and 55◦C was studied. The results indicated that prothiofos was stable in acid medium,
but it was hydrolyzed to 2,4-dichlorophenol and O-ethyl S-propyl phosphorodithioate in
alkaline medium.
The degradation products identified by TLC and GC/MS were prothiofos oxon, O-ethyl
S-propyl phosphordithioate, des-propyl thioprothiofos, prothiofos oxon sulfoxide, and one
unknown compound in addition to the parent compound. The phenolic compound was
identified by GC/MS and by its color.
Supplemental materials are available for this article. Go to the publisher’s online edition
of Phosphorus, Sulfur, and Silicon and the Related Elements to view the free supplemental
file.
Keywords 14C-Prothiofos; degradation products; hydrolysis and oxidation; sunlight; UV light
INTRODUCTION
Organophosphorus pesticides are widely used in modern agriculture as an alternative
to organochlorines for pest control. This common class of compounds represents more than
one-third of the total insecticides used in the world1 and acts via inhibition of the enzyme
cholinesterase, which plays a key role in the insect’s nervous system.2
Among factors affecting the persistence of pesticide in the environment (i.e., oxida-
tion, metabolism in plants and animals, bacteriological degradation, hydrolysis), photolysis
is one of the foremost.3,4 In this regard, an increasing number of articles devoted to the
photodedegradation of pesticides have appeared in the literature in recent years.5–9
Received 9 January 2010; accepted 25 January 2010.
Address correspondence to H. Abdel-Gawad, Applied Organic Chemistry Department, National Research
Centre, Cairo 12622, Egypt. E-mail: abdelgawadhassan@hotmail.com
1571

1572
H. ABDEL-GAWAD ET AL.
Previous studies have focused on the hydrolysis of organophosphorus pesticides
and revealed that hydrolysis is pH- and temperature-dependent.10,11 In the environment,
hydrolysis of organic chemicals occurs in dilute solution. Under these conditions, water
is present in large excess, and the concentration of water is essentially constant during
hydrolysis. Hence the kinetics of hydrolysis is pseudo first-order at a fixed pH.
Phosphorothioate sulfoxides are proposed as the toxicologically relevant bioac-
tivation products of many phosphorothioate pesticides, but they have not been char-
acterized from biological systems.12 The oxidative bioactivation of phosphorothioates
may reflect their conversion to more potent phosphorylating agents upon treatment with
peracids.13
The present work has been carried out to investigate the following:
(i) Effect of sunlight, ultraviolet radiation, and various pH levels on the stability of
prothiofos
(ii) The effect of different oxidizing agents on the stability of prothiofos insecticide
RESULTS AND DISCUSSION
Photolysis of Prothiofos
Effect of sunlight. The percentage loss of 14C-labeled prothiofos after exposure to
direct sunlight for one week was 73% then increased to 100% at the end of the experiment
period (Figure 1). The parent compound almost disappears after one month exposure to
sunlight; when 14C-labeled prothiofos was kept in darkness, the insecticide suffered no
change after one month. The half-life time period T1/2 for sunlight reaction is 2.81 days.
These finding are similar to the data obtained by El-Sayed et al.14 in their studies
about the persistence and nonbiological transformation of selecron, cyolane, and reldan
insecticides as affected by sunlight. Thin layer chromatography showed the degradation
products of 14C-labeled prothiofos (63 µg) after exposure to direct sunlight at different
time intervals and were identified as the parent compound, its oxon, and one unknown
compound (Table I).
These findings are similar to the data obtained by GC/MS after one month exposure
to direct sunlight [prothiofos oxon (II), RT 36.64 minutes and its fragments at m/z 328, 294,
266, 224, 161, 142, and 114]. Iwao et al.15 found that exposure to sunlight caused gradual
70
60
50
40
30
20
10
0
0
10
20
30
40
time
Figure 1 Effect of sunlight on prothiofos insecticide after different times.

14C-ETHYL PROTHIOFOS
1573
Table I Degradation products of 14C-labeled prothiofos (63 µg) after exposure to direct sunlight at different
time intervals
Concentration
Rf values in various
solvent system
Week
2 Weeks
3 Weeks
1 Month
∗∗
∗∗
∗∗
∗∗
Degradation products
1
2
3
4
(µg)∗
%
(µg)∗
%
(µg)∗
%
(µg)∗
%
Prothiofos (I)
Prothiofos oxon (II)
Unknown
0.56 0.47 0.21 0.80 16.7 26.5
0.01 0.06 0.00 0.52 11.2 17.8
2
7.4
4.2
3.2
11.7
6.6
0.4
3.7
3
0.7
6
5
0
2
0.4
0
3.2
0.6
0.85 0.45 0.65 0.49
1.7
2.6
System 1: n-hexane:ethyl acetate, 99.5:0.5 (v/v).
System 2: n-heptane:ethylacetate, 99:1 (v/v).
System 3: n-heptane pure.
System 4: n-heptane:chloroform:methanol, 9:4:1 (v/v).
∗Data are the average of two replicates.
∗∗Percent remaining of prothiofos and its degradation products.
degradation of prothiofos, giving the same products (oxon, despropylthio prothiofos, and
2,4-dichlorophenol).
Effect of UV light. Photodecomposition is only one of the various transformation
processes determining the fate of pesticides in the environment. It may occur in water and
air as well as on target organisms and on soil surface. Useful data on the photolysis of
pesticides in water are available.16 We focused our investigation on alkaline medium (pH
= 8), which is similar to the pH of canal water.17 Photodegradation of organophosphorus
insecticides depended on the exposure time to ultraviolet rays. The percent loss of 14C-
labeled prothiofos increases as the time of exposure increases and reaches 99% after 6 h of
exposure (Table II). The half-life time period was calculated from the slope of the straight
line, which represented the logogrammatic relation between the concentration of prothiofos
and reaction time; we found that T1/2 of prothiofos is 1.93 h.
TLC and GC/MS analysis of extracts of photodegradation products showed seven
degradation products in addition to the parent compound (Scheme 1; see also Table S1 in the
Supplemental Materials, available online) and were identified as prothiofos oxon, O-ethyl
phosphorothioate, O-ethyl S-propyl phosphorothioate, O-ethyl phosphoric acid, prothiofos
oxon sulfoxide, O-ethyl S-propyl phosphorodithioate, and one unknown compound, which
was found in the organic layer, and has Rf values (0.85, 0.45, 0.65, and 0.49) in the following
solvent systems: n-hexane:ethyl acetate, 99.5:0.5; n-heptane:ethyl acetate, 99:1; n-heptane
only, and n-heptane:chloroform:methanol 9:4:1, respectively.
Oxidation of Prothiofos
Table III shows the effect of different oxidizing agents on 14C-labeled prothiofos.
Nitric acid oxidized 64% of the insecticide after 2 h at 25◦C. When the reaction was
refluxed for 3 h using hydrogen peroxide, the yield of oxygen analogue was 39%. When
dimethylsulfoxide and/or potassium permanganate were used as oxidizing agents, the yield
of oxon was 36% and 23%, respectively.
The behavior of P = S organophosphorus compounds towards different oxidizing
agents to give P = O products has been studied.18 Prothiofos oxon (II), O-ethyl S-propyl
phosphorodithiote (V), O-ethyl S-propyl phosphoric acid, and prothiofos oxon sulfoxide
were detected by the thin layer chromatographic technique. In all cases, these results are

1574
H. ABDEL-GAWAD ET AL.
Table II Degradation products of 14C-labeled prothiofos (252 µg) after exposure to UV light
Concentration
Rf values in systems
1 h
2 h
4 h
6 h
(µg)∗
∗∗
∗∗
∗∗
∗∗
Degradation products
1
2
3
4
(µg)∗
%
(µg)∗
%
(µg)∗
%
%
1
Prothiofos (I)
Prothiofos oxon (II)
0.56 0.47 0.21 0.80 160 63.4 120 47.6 60 23.8
2.3
1.6
3.2
0.01 0.06 0.00 0.52
41 16.3 11.6
4.6
5.9 32
2.4
1
0
1.3
O-Ethyl phosphorothioate 0.21 0.52 0.45 0.60
(VII)
O-Ethyl S-propyl
phosphorothioate (IX)
O-Ethyl phosphoric acid
(VIII)
Prothiofos oxon sulfoxide 0.96 0.81 0.46 0.42
(X)
O-Ethyl S-propyl
phosphorodithioate (V)
Unknown
9.6
3.8 15
6.2 22
0.0 25
0
0.43 0.41 0.32 0.85 15.7
8.7
0
8
0
7.1
3.5
0.8
0.6
0
0.77 0.65 0.60 0.89
6.4
0
9.9 20
4.2 18
0
0
0
0
0
0
0
0
0
10.6
0.7
0
8.7
0.68 0.65 0.35 0.83
0
1.8
2
0.85 0.45 0.65 0.49
0
0
0
1.6
∗Data are the average of two replicates.
System 1: n-hexane:ethyl acetate, 99.5:0.5 (v/v).
System 2: n-heptane:ethylacetate, 99:1 (v/v).
System 3: n-heptane pure.
System 4: n-heptane:chloroform:methanol, 9:4:1 (v/v).
∗∗Percent remaining of prothiofos and its degradation product.
in agreement with those obtained from GC/MS as listed in Table II. These results are
in accordance with those reported by Fakhr et al.18 in their studies about the stability of
organophosphorus insecticides actellic and malathion. In addition to oxidation, hydroly-
sis reactions occur and lead to the formation of O-ethyl S-propyl phosphorodithiote (V),
O-ethyl S-propyl phosphoric acid, and prothiofos oxon sulfoxide. Prothiofos was oxi-
dized by peracids, giving not only oxon but also other degradation products. Segall and
Casida19 and Miyamoto20 found that profenofos insecticide underwent oxidation with m-
chloroperbenzoic acid to give 86% of profenofos sulfone. Also, oxidation of diazinon by
aqeuoes chlorine leads to the formation of diazoxon.21
Phosphorothioatesulfoxides areproposedas thetoxicologicallyrelevant bioactivation
products of many phosphorothioate pesticides, but they have not been characterized from
biological systems.12 The oxidative bioactivation of phosphorothioates may reflect their
conversion to more potent phosphorylating agents upon treatment with peracids.13
Hydrolysis of Prothiofos
Surprisingly, there are only a few data on the reactivity of organophosphorus com-
pounds in aquatic systems despite their prevalence in the environment. Hydrolysis path-
ways, for which acids, bases, or neutral species serve as catalysts, have been proposed for
phosphoric and thiophosphoric trimesters.11
We found that prothiofos is not as resistant to hydrolysis. Under basic conditions,
it hydrolyzed more rapidly than under acidic conditions. The hydrolysis reaction and
pathways are given in Scheme S1 (Supplemental Materials).

14C-ETHYL PROTHIOFOS
1575
Cl
O
P
OC2H5
Cl
O
SC3H7
O
(X)
UV oxidation
Cl
Cl
O
S
P
OC2H5
UV oxidation
OC2H5
SC3H7
Cl
O
Cl
O
P
SC3H7
(II)
prothiofos
(I)
hydrolysis
hydrolysis
O
P
OC2H5
S
HO
OC2H5
SC3H7
UV oxidation
SC3H7
HO
P
(IX)
(V)
depropylation and
elimination of sulfur
depropylation and
elimination of sulfur
O
P
S
OC2H5
OH
OC2H5
UV oxidation
HO
HO
P
OH
(VIII)
(VII)
Scheme 1 Some degradation products resulting from irradiation of 14C-labeled prothiofos in buffered aqueous
solution at pH = 8 at 254 nm.
Under acidic conditions, upon hydrolysis prothiofos undergoes dealkylation, which
results in the formation of O-(2,4-dichlorophenyl) O-ethyl phosphorothioate. In an alkaline
medium, hydrolysis of prothiofos results in the formation of 2,4-dichlorophenol and O-ethyl
S-propyl phosphorodithioate.
All hydrolysis products were analyzed by TLC on silica gel plates using different
solvent systems. The Rf values and concentration of identified 14C-labeled compounds from
hydrolysis are shown in Table IV. The phenolic compounds were non-labeled and could be
detected qualitatively by GC/MS or spraying the plate with palladium chloride reagent and
exposure to I2 vapor.
Hydrolysis data will generally be important in assessing the fate of organic chem-
icals that have hydrolysable functional groups, e.g., esters, peroxides, and halogenated
compounds. Besides substitution, halogen substances can also undergo elimination reac-
tions in water to produce dechlorinated products.22 This study indicates that hydrolysis can
significantly affect the fate of prothiofos in the environment.

1576
H. ABDEL-GAWAD ET AL.
Table III Rf values and concentrations of 14C-labeled prothiofos and its degradation products after oxidation
Concentration
Hydrogen
peroxide permanganate sulfoxide
(3h) reflux (6h) reflux (4h) reflux
Potassium
Dimethyl
Nitric acid
(2h) 25◦C
Rf values in systems
∗∗
∗∗
∗∗
∗∗
%
Degradation products
1
2
3
4
(µg)∗
%
(µg)∗
%
(µg)∗
%
(µg)∗
Prothiofos oxon (II)
O-Ethyl S-propyl
phosphorodithiote (V)
O-ethyl S-pro O-Ethyl
S-propyl phosphoric
acph phosphorothioate
(IX)
0.01 0.06
0
0.52 162.3 64.3 97.3 38.6 90.26 35.8 56.7
22.5
19.7
0.68 0.65 0.35 0.83 67.3 26.7 95.8 38
45.4 18
49.9
0.43 0.41 0.32 0.85
0
0
0
0
50.4 20
84.2
33.4
Prothiofos oxon
sulfoxide (X)
0.96 0.81 0.46 0.42 22.4 8.9 58.7 23.3 66.8 26.5 61.5
24.4
∗Data are the average of two replicates.
System 1: n-hexane:ethyl acetate, 99.5:0.5 (v/v).
System 2: n-heptane:ethylacetate, 99:1 (v/v).
System 3: n-heptane pure.
System 4: n-heptane:chloroform:methanol, 9:4:1 (v/v).
Effect of pH on the Fate of Prothiofos Insecticide
Chemical hydrolysis of the organophosphorus insecticide prothiofos at 25◦C, 40◦C,
and 55◦C was studied using buffered solution at pH values of 5, 7, 8, and 9, which are usual
conditions in natural water. The data presented in Figure 2 show that in acidic medium (pH
5, 25◦C) 52% of prothiofos was degraded after 24 h, while at pH 7 about 73% of prothiofos
was decomposed after the same time. At pH 9, the loss of prothiofos was 82%. From this
Table IV Concentration and Rf values of 14C-labeled prothiofos and some of its hydrolysis products
Hydrolysis reagents
Hydrochloric acid
Aqueous Chloroform
Sodium hydroxide
Aqueous Chloroform
Rf values in various
solvent system
∗∗
∗∗
∗∗
∗∗
%
Degradation products
1
2
3
4
µg∗
%
µg∗
%
µg∗
%
µg∗
Prothiofos (I)
0.56 0.47 0.21 0.80
0.68 0.65 0.35 0.83
0
0
0
0
167.2 66.0
0
0
63.8 25
O-Ethyl S-propyl
phosphorodithiote(V)
Des-propylthio
prothiofos (IV)
0
0
0
0
133.3 53
0
0
0
0
0.54 0.49 0.30 0.78 32.2 13
0
0
System 1: n-hexane:ethyl acetate, 99.5:0.5 (v/v).
System 2: n-heptane:ethylacetate, 99:1 (v/v).
System 3: n-heptane pure.
System 4: n-heptane:chloroform:methanol, 9:4:1 (v/v).
∗Data are the average of two replicates.
∗∗Percent remaining of prothiofos and its degradation products.

14C-ETHYL PROTHIOFOS
1577
Cl
Cl
S
P
O
P
O
O
Cl
O
Cl
O
S
S
PROTHIOFOS OXON (II)
PROTHIOFOS (I)
Cl
Cl
S
S
O
OH
Cl
O
P
Cl
O
P
OH
S
DESPROPYLTHIO PROTHIOFOS ( IV)
DESETHYL PROTHIOFOS (III)
Cl
S
O
Cl
OH
HO
P
S
2,4-DICHLOROPHENOL
O-ETHYL-S-PROPYL
PHOSPHORODITHIOATE
(VI)
(V)
Figure 2 Prothiofos and its main degradation products.
data it becomes evident that the stability of the compound decreases as the pH increases in
the following order: pH 5 < pH 7 < pH 8 < pH 9. The same trend was also observed at
40◦C and 55◦C. These data are in agreement with those reported by Aly and Badawy2 in
their studies on the hydrolysis of organophosphorus insecticides (profenofos, bromophos,
and fenitrothion) in aqueous medium.
The effect of pH on the hydrolysis of prothiofos is shown in several figures and
tables available in the Supplemental Materials. Throughout the hydrolysis of prothiofos
at 25◦C, there was an excellent linear relationship between log(a/(a–x)) (concentration of
prothiofos) and t (time). This indicated that the hydrolysis of prothiofos is of the first order.
The linear regression equations are as follows:
d[C]
dt
−
= kapp[C]
kapp = (2.303/t) log (a/a − x)
The half times of degradation (t1/2) at 25◦C were 22.9, 12.6, 10.5, and 9.6 h at pH 5, 7, 8,
and 9, respectively (Figure S1, Supplemental Materials); the half times of degradation (t1/2
)
at 40◦C were 18, 10.28, 10, and 7.8 h, respectively (Figure S2). At the same pH values, the
half times of degradation (t1/2) at 55◦C were 14.5, 9, 7.8, and 6.5 h, respectively (Figure S3).
Effect of Temperature on the Fate of 14C-Labeled Prothiofos Insecticide
Four groups of experiments were carried out to study the effect of temperature on
the hydrolysis of prothiofos under different buffer solutions (pH = 5, 7, 8, and 9). In all
cases, the hydrolysis rate of prothiofos increased with temperature (see Figures S4–S7,
Supplemental Materials). The linear regression equations are as follows:
d[C]
−
= kapp[C]
dt
kapp = (2.303/t) log (a/a − x)

1578
H. ABDEL-GAWAD ET AL.
Where: [C] = concentration.
(a) = the original concentration of prothiofos.
(x) = the amount of hydrolytic products.
(t) = time of hydrolysis.
The apparent rate constant kapp is usually assumed to be the sum of three terms: ka
[H+], kn, and kb [HO−], respectively, corresponding to acid, neutral, and basic pathways.23
Hence, the plot of the logarithm of the normalized concentration as a function of the reaction
time gives straight lines, the slope of which identifies –kapp
.
The temperature has a great effect on the hydrolysis of prothiofos. At pH 5, 7, 8, and
9, the rate of hydrolysis at 55◦C is higher than at other temperatures. From the experimental
results, we can see that prothiofos hydrolyzes in water, but the hydrolysis rate constant
is different at different pH values and at different temperatures. At pH 5 and 25◦C, the
half time of prothiofos is 22.5 h, i.e., prothiofos is relatively stable in water under these
conditions. At pH 9 and 55◦C, the half time of prothiofos is 6.57 h, i.e., the compound is
relatively unstable in water under these conditions.
CONCLUSION
In conclusion, this paper provides useful knowledge on the degradation of the
organophosphorus insecticide prothiofos in the environment; the results of the investi-
gations comprise the reactions of prothiofos with various oxidizing reagents; the hydrolysis
as well as the photolysis is described. The yield of oxon depends on the type of oxidizing
agent. The effect of ultraviolet light and direct sunlight on prothiofos was investigated.
Exposure to direct sunlight caused gradual degradation of prothiofos, giving the same
products as UV irradiation. Prothiofos was relatively stable in acidic but less stable in al-
kaline medium; stability of prothiofos in alkaline medium (pH 8–9), similar to canal water,
decreases with increasing temperature.
MATERIALS AND METHODS
Chemicals
14C-prothiofos labeled at the carbon atom of the ethyl group was prepared in one-pot
reaction according to Abdel-Gawad et al.24 The synthesized 14C-prothiofos had specific
activity (0.02 mCi/g) (0.74 MBq/g), and the radiometric purity was greater than 98%
(Scheme 2).
Non-labeled prothiofos and some of its degradation products were synthesized for
comparison purposes.24 Chemical structures and abbreviations of the insecticide prothiofos
and some of its degradation products are shown in Figure 1.
Spectral Analysis
1
The H NMR spectra were obtained with a Jeol-EX (270 MHz) spectrometer with
tetramethylsilane (TMS) as internal standard for samples dissolved in CDCl3. The EI–MS
(70 eV) spectra were obtained with a Jeol JMS-AX500 mass spectrometer. The IR spec-
tra were recorded in potassium bromide rolled (KBr) on Nexus 670 FTIR spectrometer
(Nicolet) at the National Research Centre, Cairo, Egypt.

14C-ETHYL PROTHIOFOS
1579
S
Cl
Cl
(C2H5)3N
Cl
H
O
PSCl3
+
Cl
S
P
Cl
C
O
- HCl
Cl
H
7SH
3
Cl
(C2H5)3N
S
Cl
SC H
- HCl
Cl
P
O
3
7
*
C2H5OH
*
O
C
Cl
P
2H 5
O
Cl
(C2H5)3N
C H
S
3
7
Prothiofos
Scheme 2 Pathway of 14C-prothiofos synthesis.
Preparation of O-(2, 4-Dichlorophenyl) O-Ethyl S-Propyl
Phosphorothioate (Prothiofos Oxon) (II)
This compound was prepared by adding equimolar amounts of 2,4-dichlorophenol
(0.01 mol, 1.62 g) and triethylamine (0.03 mol, 4.2 mL) in dry benzene (20 mL) to a cooled
(5–10◦C) solution of phosphorus oxychloride (0.01 mol, 0.9 mL) in dry benzene (20 mL)
for 15 min. The reaction mixture was stirred for 4 h, followed by dropwise addition of propyl
mercaptane (0.01 mol, 0.9 mL) in dry benzene (5 mL) in an ice bath at 5–10◦C. Then the
temperature was raised to 70◦C for 4 h, followed by dropwise addition of absolute ethyl
alcohol (0.01 mol, 0.6 mL) in dry benzene (5 mL). The reaction mixture was stirred at 25◦C
for 20 h and filtered. The crude oil was purified on silica gel column using hexane:diethyl
ether (1:1) for elution and isolated in 45% yield. Thin layer chromatography on silica gel
plate showed one spot of Rf 0.01 in solvent system n-hexane:ethyl acetate 99.5:0.5. IR
(KBr) νmax/cm−1: 1220 (P O), 1559, 1475 (C C ring stretch), 2922 (C H aliphatic),
1037 (POC-).1H NMR (CDCl3/TMS): δ = 0.99 (t, J = 7.0 Hz, 3H, CH3CH), 1.76 (m,
2H, C CH2 C), 2.93 (m, 2H, S CH2 ), 1.27 (t, J = 7.11 Hz, 3H, CH3 CH2), 4.4 (m,
2H, CH2 CH3), 7.2–7.5 (m, 3H, arom-H). EI-MS (m/z): 329 (M+), 331 (M+ +2), 333
(M+ +4), 294, 252, 162, 164, 166, 98, 63.
Photolysis of Prothiofos
Effect of direct sunlight. In sealed tubes, 63 µg of pure 14C-prothiofos was
exposed to direct sunlight during May 2006 for 1, 2, 3, and 4 weeks. The samples were
qualitatively and quantitatively analyzed.
Effect of ultraviolet rays. Prothiofos (252 µg, 0.0007 mmol) in 5 mL of acetone
and 20 mL of buffer solution at pH 8 was applied in a Pyrex Petri dish and was exposed to
short wave length ultraviolet rays (λ = 254 nm) at a distance of 10 cm for period of 1, 2,
4, and 6 h. Experiments were carried out at 28◦C. The insecticide products were extracted
three times with chloroform; the aqueous and organic layers were analyzed by TLC and
GC-MS.
Oxidation of Prothiofos
Solutions of definite concentrations of prothiofos were subjected to different oxidiz-
ing agents such as concentrated nitric acid, hydrogen peroxide, dimethyl sulfoxide, and
potassium permanganate.

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H. ABDEL-GAWAD ET AL.
With conc. nitric acid. Nitric acid [2.5 mL (0.059 mol, d = 1.49)] was added
dropwise to 252 µg (0.0007 mmol) of the cooled 14C- labeled insecticide at 5◦C, and the
mixture was stirred at room temperature for 2 h. After removal of nitrogen dioxide gas in
vacuum, the mixture was poured into ice water and extracted with diethyl ether. The ether
extract was washed with 10% cold sodium bicarbonate solution and then with cold water
until neutral. The ether layer was evaporated under vacuum, and oxidation products were
determined by thin layer chromatography.
With hydrogen peroxide. To a solution of 252 µg (0.0007 mmol) of the 14C-
labeled insecticide in 2 mL of glacial acetic acid, 4 mL of 30% hydrogen peroxide were
added, and the mixture was refluxed for 3 h. The reaction mixture was poured on ice.
After extraction with diethyl ether and washing of the extract with 10% sodium bicarbonate
solution and water, the products were qualitatively and quantitatively analyzed.
With dimethyl sulfoxide. A mixture of 252 µg (0.0007 mmol) of 14C-labeled
prothiofos in excess of dimethyl sulfoxide and 0.1 mL of concentrated sulfuric acid was
refluxed for 6 h. After removal of the separated sulfur and concentration in vacuum,
the oxidation products were determined by thin-layer chromatography (TLC) and liquid
scintillation counting (LSC).
With potassium permanganate. A solution of 252 µg (0.0007 mmole) 14C-
prothiofos in 20 mL of acetone was mixed with 15 mL solution of sodium carbonate in
the least amount of water, and powdered potassium permanganate (1.25 g, 0.008 mol) was
added. The mixture was refluxed for 2 h, filtered from precipitated manganese dioxide, and
evaporated to give the oxidation products, which were determined by TLC and LSC.
Hydrolysis of Prothiofos
For determining the hydrolysis products of the insecticide prothiofos, a solution of
252 µg 14C-labeled prothiofos in 10 mL acetone was heated to reflux for 1/2 h with 20 mL
of 1N HCl or 1N NaOH solution. The aqueous and organic layers were subjected to LSC.
The hydrolyzed products were identified by thin layer chromatography.
Preparation of the Buffer Solutions
pH 5.00: Solutions of 238 mL of 0.1 M NaOH and 500 mL of 0.1 M monobasic potassium
phthalate (C6H4C2O4HK) were mixed and then diluted to 1000 mL.
pH 7.00: Solutions of 297 mL of 0.1 M NaOH and 500 mL of 0.1 M potassium dihydrogen
phosphate (KH2PO4) were mixed and then diluted to 1000 mL.
pH 8.00: Solutions of 125 mL of 0.1 M potassium dihydrogen phosphate (KH2PO4) and
116.5 mL of 0.1 M NaOH were mixed and then diluted to 250 mL.
pH 9.00: Solutions of 125 mL of 0.025 M sodium borate (Na2B4O7) and 11.5 mL of 0.1
M HCl were mixed and then diluted to 250 mL.
All of the pH values of the buffer solutions were measured by a pH meter.
Effect of pH on the Hydrolysis of Prothiofos
Four samples of 20 mL of various buffer solutions (pH 5, 7, 8, and 9) in 30 mL
screw-top test tubes were rapped with foil to preclude photolysis and were fortified with
252 µg of 14C-labeled prothiofos in 10 mL of acetone by vigorous shaking. These samples

14C-ETHYL PROTHIOFOS
1581
were then kept at fixed temperatures (25◦C, 40◦C, and 55◦C). At different time intervals, 3
mL duplicate samples were taken and extracted three times with chloroform. The organic
and aqueous layer extracts were radioassayed.
Effect of Temperature on the Hydrolysis of Prothiofos
The effect of temperature on the hydrolysis of prothiofos at fixed pHs (pH 5, 7, 8, and
9) was studied at 25◦C, 40◦C, and 55◦C. The container containing 20 mL of various buffer
solutions in 30 mL screw-top test tubes was fortified with 252 µg of 14C-labeled prothiofos
in acetone by vigorous shaking and kept at certain temperatures in the thermostat. At
different time intervals (6, 18, and 24 h), 3 mL duplicate samples were taken and extracted
three times with chloroform. The organic and aqueous layer extracts were radioassayed.
Chromatography
Extracts of organic and aqueous layer were investigated by thin-layer chromatography
(TLC) using silica gel 60 F254 thin-layer chromatoplates (20–20 cm, 0.25 mm thickness,
Merck). The following solvent systems were used for development:
System 1: n-hexane:ethyl acetate 99.5:0.5 (v/v)
System 2: n-heptane:ethylacetate 99:1 (v/v)
System 3: n-heptane pure
System 4: n-heptane:chloroform:methanol 9:4:1 (v/v)
Prepared degradation products were run alongside as references, and spots were
detected under ultraviolet light at 254 nm and made visible by spraying the plates with a
freshly prepared Hanes–Isherwood reagent25 or by subjecting to I2 vapor, after preliminary
spray with PdCl2 solution.26,27 To detect the phenolic compounds, the plates were developed
in the above systems and sprayed with ferric chloride–potassium ferricyanide solution,
where blue spots on yellow background appeared.28
Radioassay
Thin layer chromatographic technique was used to identify the degradation products.
After development, each plate was divided into 1 cm zones which were separately scrapped,
extracted with 2 mL of methanol, and then counted for radioactivity in a liquid scintillation
counter.
Radioactivity was determined by mixing each zone with 2 mL ethanol and 10 ml of
Packard Emulsifier Scintillator cocktail and analyzed by LSC with Packard Model TRI-
CARB 2300 TR. The background level of radioactivity in LSC averaged 30 dpm and was
subtracted from the values of the measured samples.
GC/MS Analysis
GC/MS analysis was used for the identification of the parent compound and its
degradation products in this study. It was carried out with a GC/MS Finnigan Mat SSQ
7000, EI 70 eV. Operating conditions included the following: a capillary column DB-5,
30 m × 0.25 mm I.D. [(5%-phenyl)methylpolysiloxane]. The analysis was carried out at

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H. ABDEL-GAWAD ET AL.
a programmed temperature: initial temperature 50◦C for zero min, then increasing at rate
of 5◦C /min until 300◦C (kept for 5 min). The injector temperature was set at 250◦C,
and the detector temperature at 280◦C. Helium was used as a carrier gas at 1 mL/min,
injected volume was 2 µL, injection mode was splitless. The compounds were identified
by matching their MS with those recorded in the MS library (Wiley) and comparison with
those of reference compounds.
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