Photolysis experiments on phosmet, an organophosphorus insecticide

Article abstract of DOI:10.1021/jf034253y

The organophosphorus insecticide phosmet is used in plant protection as well as against parasites on animals. Phosmet showed numerous photoinduced reaction pathways, which first were studied in the presence of model environments for animal fur lipids (e.g

Full text of DOI:10.1021/jf034253y

5990 J. Agric. Food Chem. 2003, 51, 5990−5995
Photolysis Experiments on Phosmet, an Organophosphorus
Insecticide
KATRIN SINDERHAUF AND WOLFGANG SCHWACK*
Institut fu¨r Lebensmittelchemie, Universita¨t Hohenheim, Garbenstrasse 28,
D-70593 Stuttgart, Germany
The organophosphorus insecticide phosmet is used in plant protection as well as against parasites
on animals. Phosmet showed numerous photoinduced reaction pathways, which first were studied
in the presence of model environments for animal fur lipids (e.g., wool wax). The model solvents for
saturated and unsaturated lipids were cyclohexane and cyclohexene, whereas methanol and
2-propanol were used as models for primary and secondary alcohol moieties of lipids. The measured
degradation rates over an irradiation period of 7 h in all solvents used were very similar (49-55%).
The obtained photoproducts generally included phthalimide, N-hydroxymethylphthalimide, and
N-methoxymethylphthalimide. Furthermore, depending on the solvent used, additional degradation
products were detectable as N-isopropoxy- and N-methylphthalimide in the presence of 2-propanol
and cyclohexene, respectively. However, in the presence of cyclohexene, despite the similar turnover,
distinctly lower concentrations of photoproducts were found, indicating further still unknown degradation
pathways. Irradiations in methanol with increasing percentages of water led to higher degradation
rates; however, the products were found to be the same. Irradiation experiments with pure phosmet
on silica TLC plates and glass surfaces resulted in degradation rates of 19 and 32%, respectively,
after 6 h. The results obtained clearly demonstrate for the first time that the photoinduced degradation
of phosmet is strongly dependent on the chemical environment, affecting less the turnover than the
photoproducts formed. The results additionally demonstrate the need to investigate the degradation
behavior of phosmet on wool and in the presence of wool wax.
KEYWORDS: Phosmet; photodegradation; organophosphorus insecticide
INTRODUCTION
Phosmet [phosphorodithioic acid, S-[(1,3-dihydro-1,3-dioxo-
2H-isoindol-2-yl)methyl] O,O-dimethyl ester, 1, Figure 1] is a
nonsystemic organophosphorus insecticide belonging to the
subclass of dithiophosphates. It is commonly applied on plants
(e.g., cotton, fruit, and potatoes) as well as on animals [e.g.,
cattle, pigs, sheep, and dogs (through dog collars)] in com-
mercially available formulations such as Imidan, Prolate, or GX-
118.
To the best of our knowledge, no studies on the photo-
degradation of pesticides on animals have been published yet,
in particular not for phosmet. Some experiments on photo-
degradation rates of organophosphorus insecticides in wool have
been undertaken, but no degradation products were identified
(1). The same authors also mentioned from earlier studies that
insecticides dissipated from sheep more rapidly during summer
as compared to winter conditions, which was presumably
attributed to sunlight and summer heat (2).
Figure 1. Structure of phosmet (1).
to the lipid environment of animal hair or sheep wool, pesticides
can be photodegraded in a quite different manner as compared
to that in an aqueous environment, also yielding degradation
products different from metabolites usually found during in vivo
or in vitro studies. Additionally, bimolecular reaction with lipids’
constituents may result in photoaddition products so far
unknown, which cannot be detected, for example, in wool wax,
by standard analytical methods of residue analysis.
Only a few photodegradation experiments have been per-
formed on phosmet to date. In 1974, Tanabe et al. (3) irradiated
solutions of phosmet in diethyl ether. As main degradation
products, they identified N-methylphthalimide and N-meth-
oxymethylphthalimide among numerous unidentified products.
Further on they focused on clarifying the photoproducts resulting
from irradiation of the first photolysis product N-methylphthal-
imide. Vaintraub et al. (4) and Weintraub et al. (5) described
photodegradation experiments on silica plates and on apples.
After application on animal skin or hair, phosmet is directly
exposed to sunlight during the grazing of sheep or cows. Due
* Author to whom correspondence should be addressed (telephone
+49711 4593979; fax +49711 4594096; e-mail wschwack@uni-
hohenheim.de).
10.1021/jf034253y CCC: $25.00 © 2003 American Chemical Society
Published on Web 08/22/2003

Photolysis Experiments on Phosmet
J. Agric. Food Chem., Vol. 51, No. 20, 2003 5991
(ESI+), m/z 192 ([M + H]+), 209 ([M + NH4]+), 160 ([M - MeOH]+);
1H NMR (DMSO) δ 3.27 (3H, s, CH3), 4.92 (2H, s, CH2), 7.94-7.86
(4H, m, Ar-H); 13C NMR (DMSO) δ 56.73 (s, CH3), 68.66 (s, CH2),
The objective of our study is the determination of the
photodegradation and reaction pathways of phosmet, depending
on the respective environments in different model solvents
simulating animal skin fat (e.g., wool wax). Like many former
studies (6-10) demonstrated, the photochemistry of pesticides
depends in the first instance on the functional chemistry of the
environment. As wool grease is a constituent element (10-25%
w/w) of the sheared greasy wool and is a complex mixture of
polyesters and monoesters of high molecular weight, free fatty
acids, and alcohols (11, 12), in the first step several simple
models were utilized for UV irradiation experiments, resembling
common structure moieties of wool wax. Therefore, cyclohexane
and cyclohexene, which are also parts of the skeleton of sterols,
were generally used as models for saturated and unsaturated
constituents, respectively, of animal skin or hair lipids (e.g.,
sheep wool wax). Methanol and 2-propanol were models for
primary and secondary alcohol groups as they occur in free fatty
alcohols, hydroxy fatty acids, and sterols. These simple models,
following models for plant cuticle lipids introduced by Schwack
(7-10), have proved to be the best practical way to study
possible reaction pathways and product formation, prior to
moving on to the more complex natural system. In our previous
studies (8) we additionally showed that the photoproducts,
derived through these simple model experiments, could as well
be found on plant surfaces, which also confirms the utilization
of model systems in the first step. Furthermore, the resulting
products can effectively be used as analytical standards (13)
for later studies in natural environments.
123.75, 131.64, 135.12 (s, Ar-C), 167.96 (s, CdO); IR (ATR) (cm-1
)
2998, 2959, 2932, 2843, 1774, 1709, 1605, 1481, 1467, 1448, 1436,
1412, 1344, 1325, 1287, 1229, 1186, 1162, 1083, 985, 960, 912, 889,
852, 798, 722, 711, 693] and to published data [mp ) 121-122 °C
(ethanol) (16), mp 120-121 °C (17)].
Phosmet-oxon was synthesized from isolated phosmet by use of
bromine solution (methanol, 200 mM). A solution of phosmet (20 mM)
in methanol (1.5 mL) was treated with 1.5 mL of bromine solution
and agitated for 2 min in the closed vessel. The excess of bromine was
reduced with an aqueous solution of ascorbic acid. The reaction mixture
was treated with saturated saline and extracted twice with diethyl ether.
Extracts of five reaction mixtures were combined, the solvent was
evaporated under reduced pressure, and the residue was dissolved in
methanol and subjected to preparative HPLC. Fractions were collected,
methanol was evaporated, and residual water phase was lyophilized
(yield ) 35% of theory). Spectroscopic data of the obtained colorless
crystals agreed with the proposed structure [MS (ESI+), m/z 302 ([M
1
+ H]+), 319 ([M + NH4]+), 340 ([M + K]+); H NMR (DMSO) δ
3.67 (6H, d, CH3), 4.97 (2H, d, CH2), 7.95-7.87 (4H, m, Ar-H); 13
C
NMR (DMSO) δ 36.89 (d, CH2), 54.75 (d, CH3), 123.62, 131.41, 135.04
(s, Ar-C), 166.23 (s, CdO); IR (ATR) (cm-1) 3041, 2960, 1777, 1719,
1610, 1465, 1423, 1382, 1307, 1286, 1244, 1189, 1160, 1085, 1012,
922, 836, 770, 685; mp ) 68 °C].
General Techniques. High-performance liquid chromatography
(HPLC) analyses of phosmet and photolysis products were carried out
on an HP1100 system consisting of an autosampler, a gradient pump,
and a diode array detector (DAD) module (Hewlett-Packard, Wald-
bronn, Germany). For data acquisition and processing HP ChemStation
software (rev. A.06.01) was used. DAD was performed at wavelengths
of 220, 250, and 320 nm, each with spectral bandwidth (SBW) 8 nm,
ref 500 nm (SBW 100 nm). Separation was performed at 25 °C with
a reversed phase analytical column (5 µm Eurospher 100-C18, 250 ×
3 mm, Knauer, Berlin, Germany) including a precolumn (Eurospher 5
µm C18, 5 × 3 mm, Knauer), using a gradient system consisting of 20
mM phosphate buffer (pH 4.0) and methanol (injection volume ) 10
µL, flow rate ) 0.5 mL/min): 0/50, 1/80, 12/100, 20/100, 28/50, and
31/50 (minute/percent methanol).
Preparative HPLC was carried out on a system consisting of two
pumps (Kronlab, Sinsheim, Germany), an A0293 variable-wavelength
monitor (Knauer), a C-R3A integrator (Shimadzu, Duisburg, Germany),
and a Kronlab HPLC column (guard column, 50 × 20 mm; column,
250 × 20 mm; Nucleosil RP 18, 7 µm); a gradient system consisting
of ammonium formate buffer (10 mM, pH 4.0) and methanol as eluents
(flow rate ) 20 mL/min), with gradient as above, was used. Pumps
were controlled by Prepcon software.
High-performance liquid chromatography-mass spectrometry (LC-
MS) analyses were performed on an HPLC system (HP1100 as
described above), coupled to a VG platform II quadrupole mass
spectrometer (Micromass, Manchester, U.K.) equipped with an elec-
trospray interface (ESI). For data acquisition and processing, MassLynx
3.2 software was used. The same gradient system as in HPLC analysis
was used except the mobile phase, which consisted of 10 mM
ammonium formate buffer (pH 4.0) and methanol. All other parameters
remained as mentioned before. MS parameters: ESI+; source tem-
perature, 120 °C; capillary, 3.5 kV; HV lens, 0.5 kV; cone ramp, 20-
60 V. For LC analysis, the MS was operated in full-scan mode (m/z
80-1200).
MATERIALS AND METHODS
Materials. All solvents used were of analytical grade or distilled
prior to use. Cyclohexene (Merck, Darmstadt, Germany) was distilled
over 10% of P2O5 (Roth, Karlsruhe, Germany) before use.
Water was purified by a Milli-Q 185 plus water purification system
(Millipore Corp., Bedford, MA).
N-Methylphthalimide (Sigma-Aldrich, Steinheim, Germany), N-
hydroxymethylphthalimide, and phthalimide (Fluka, Deisenhofen,
Germany) were obtained as standards in analytical quality. Silica gel,
63-200 µm (Merck) was stirred with 10% of nitric acid (65%) for 30
min, washed until neutral, and dried for 15 h at 130 °C before use for
column chromatography.
Phosmet was extracted from Imidan [52% Phosmet (w/w), Siegfried
Agro, Zofingen, Switzerland) with acetone in a Soxhlet apparatus and,
after evaporation under reduced pressure, recrystallized from methanol
twice before use. Spectroscopic properties [MS (ESI+), m/z 318 ([M
1
+ H]+); H NMR (DMSO) δ 3.67 (6H, d, CH3), 4.96 (2H, d, CH2),
7.95-7.86 (4H, m, Ar-H); 13C NMR (DMSO) δ 54.32 (d, CH3), 39.05
(d, CH2), 123.80, 131.55, 135.23 (s, Ar-C), 166.39 (s, CdO); IR (ATR)
(cm-1) 3481, 3025, 2995, 2949, 2867, 1780, 1719, 1612, 1463, 1452,
1406, 1376, 1304, 1279, 1188, 1082, 1072, 914, 831, 819, 797, 722,
692, 676] and melting point of the obtained colorless crystals
corresponded to the expected structure and data given in refs 14 (melting
point) and 15 (IR spectra). The purity of the recrystallized phosmet
was determined by HPLC to be >99% (UV, 220 nm). The log ꢀ221nm
(methanol) calculated (4.69) matched the determined log ꢀ221nm
(methanol) of the commercial standard substance (Pestanal, Sigma-
Aldrich).
Ultraviolet (UV) spectra were measured with a Perkin-Elmer
Lambda2 instrument (U¨ berlingen, Germany) or an HP1100 DAD
module, respectively.
1H and 13C nuclear magnetic resonance (NMR) spectra were recorded
on a Unity Inova 300 (Varian, Darmstadt, Germany), at 298 K at 300
and 75 MHz (nominal frequency), respectively, in dimethyl-d6 sulfoxide
(DMSO-d6). Chemical shifts are given in δ (parts per million) relative
to tetramethylsilane (TMS).
Syntheses. N-Methoxymethylphthalimide was synthesized by em-
ploying a modified procedure according to ref 16 starting from
N-hydroxymethylphthalimide and absoluted methanol. N-Hydroxy-
methylphthalimide (1.5 mmol, 260 mg) was dissolved in 6 mL of
absoluted methanol, and the reaction was catalyzed by 0.3 mL of
sulfuric acid (98%). After 15 h at 70 °C, the mixture was neutralized
with sodium hydrogen carbonate and extracted twice with diethyl ether.
After evaporation of the solvent under reduced pressure, the crystal
needles were recrystallized from ethanol (yield ) 73% of theory).
Spectroscopic properties corresponded to the proposed structure [MS
Infrared attenuated total reflection (IR-ATR) spectra were recorded
on a Avatar 320 E.S.P. Fourier transform IR spectrometer (FTIR)
(Nicolet, Offenbach, Germany).

5992 J. Agric. Food Chem., Vol. 51, No. 20, 2003
Sinderhauf and Schwack
Melting points were determined on a digital melting point apparatus
model 8100 (Electrothermal, Southend-on-Sea, U.K.) and are not
corrected.
Irradiation Experiments. In a typical experiment, 3.1 mmol/L (1.00
g/L) of phosmet was dissolved in the respective solvent and degassed
with nitrogen to prevent solvent oxidation, especially in the case of
cyclohexene. Subsequently, the solution was irradiated for up to 7 h
while stirring in water-cooled 40 mL quartz cuvettes (60 × 35 mm)
with Teflon caps. Irradiation source was a metal halogen lamp (SOL
500, Dr. K. Ho¨nle GmbH, Martinsried, Germany; 120000 lx, 900 W/m3)
with a UV filter (WG 295, Schott Glaswerke, Mainz, Germany).
Samples of 0.5 mL were taken, diluted with methanol, and used for
HPLC analysis.
Irradiation on surfaces and in films was performed by employing a
suntest CPS+ (Heraeus-Industrietechnik, Kleinostheim, Germany;
xenon lamp, UV filters and air cooling, irradiance at 250 W/m2, standard
black temperature of 35 °C). For irradiation experiments on glass
surfaces, Petri dishes (diameter ) 9 cm, without top) and, as silica gel
surfaces, TLC plates (silica gel 60, 20 × 20 cm F254 Merck, cut to 5 ×
5 cm, cleaned up with methanol once before use) were used. Per sample,
1 mg of phosmet, dissolved in 0.1 mL of methanol, was placed on
both the glass surface and the silica gel surface. The solvent was
evaporated, and the samples were irradiated for up to 48 h. For sample
preparation, Petri dishes were rinsed with 25 mL of methanol; the silica
gel coating was scraped off and extracted with methanol.
Figure 2. Photodegradation rates of phosmet in four different model
solvents, percent of initial phosmet concentration [methanol (a, 9);
2-propanol (b, 1); cyclohexane (c, 2); cyclohexene (d, b)].
RESULTS
Irradiation Experiments in Solutions. Phosmet was readily
degraded by UV irradiation (>300 nm) in the four model
solvents used (methanol, 2-propanol, cyclohexane, and cyclo-
hexene, 1.00 g/L phosmet) as shown in Figure 2.
Unexpectedly, in each of the solvents used degradation rates
were found to be very similar at ∼50% after 7 h of irradiation
(methanol, 49.1 ( 1.5%; 2-propanol, 51.3 ( 0.7%; cyclohexane,
53.7 ( 1.1%; cyclohexene, 54.9 ( 0.3%). Consequently, the
half-life period of phosmet averaged to ∼7 h. Blank values,
measured from samples of the same concentrations as irradiated
samples but kept in the dark (same temperature, same period
of time), showed no degradation at all.
Even though the degradation rates of phosmet in the model
solutions used were very similar, the compositions of photo-
degradation products were varied (Scheme 1).
During irradiation of phosmet in methanol, the photodegra-
dation products phthalimide (2), N-hydroxymethylphthalimide
(3), and N-methoxymethylphthalimide (4) were obtained. Those
products were also obtained from hydrolysis experiments (data
not shown); 2 (18, 19) and 3 (19) have been described as
hydrolysis products before.
Isolation of Photoproduct 5. The unknown substance formed during
irradiation of phosmet in 2-propanol was isolated as follows. Phosmet
(300 mg) was dissolved in 300 mL of 2-propanol and irradiated in
aliquots of 25 mL for 15 h each. Photolysis mixtures were combined
and evaporated to dryness, and the unknown photoproduct was isolated
by silica gel column chromatography (eluent light petroleum/diethyl
ether 50:50; the first fraction contained the photoproduct) and subse-
quent preparative HPLC. Methanol was removed in the vacuum, and
the remaining aqueous solution was lyophilized, yielding 5 mg of
colorless crystals. Purity control was carried out by HPLC analysis
(220 nm). The spectroscopic properties agreed with the proposed
structure of N-isopropoxymethylphthalimide [1H NMR (DMSO) δ 1.22
(6H, d, CH3) 3.81-3.67 (1H, sep, CH), 5.00 (2H, s, CH2), 7.89-7.98
(4H, m, Ar-H); 13C NMR (DMSO) δ 123.78, 131.67, 135.18 (s, Ar-
C), 167.94 (s, CdO), 69.95 (s, CH2), 65.12 (s, CH), 22.36 (s, CH3);
MS ESI, m/z 220 ([M + H]+), 160 ([M - 2-propanol]+), 237 ([M +
NH4]+), 258 ([M + K]+); IR (ATR) (cm-1) 2955, 1777, 1721, 1587,
1438, 1410, 1350, 1327, 1208, 1175, 1077, 991, 729].
Quantification of Phosmet and Photolysis Products. Quantification
was performed using external standardization at 220 nm as all measured
substances have absorption maxima near 220 nm. Calibration curves
were created for phosmet, N-methylphthalimide, N-methoxymeth-
ylphthalimide, N-hydroxymethylphthalimide, phosmet-oxon, and
phthalimide. N-Isopropoxymethylphthalimide was quantified by one-
point calibration.
During irradiation in 2-propanol, the photolysis products 2-4
also appeared, and additionally one more peak was detected at
higher retention times than the other photoproducts but at lower
retention time than the parent phosmet. The UV spectrum
Scheme 1. Photoproducts Observed during Irradiation Experiments of Phosmet in Different Solvents [Phosmet (1), Phthalimide (2), N-Hydroxy-
methylphthalimide (3), N-Methoxymethylphthalimide (4), N-Isopropoxymethylphthalimide (5), and N-Methylphthalimide (6)]

Photolysis Experiments on Phosmet
J. Agric. Food Chem., Vol. 51, No. 20, 2003 5993
Figure 3. UV spectra (methanol) of (a) phosmet and (b) N-isopropoxy-
methylphthalimide (DAD spectra).
Figure 5. Photodegradation rates of phosmet in methanol with different
percentages of water [methanol (a, 9); methanol/10% (b, 1); methanol/
20% (c, 2); methanol/30% (d, b)].
Figure 4. Photoproduct distribution of phosmet turnover after 7 h of
irradiation in different solvents: 2 (white bars); 3 (slashed bars); 4 (cross-
hatched bars).
showed absorption bands typically for the phthalimide chro-
mophore (Figure 3). After isolation from the reaction mixture,
the proposed structure of N-isopropoxymethylphthalimide (5)
could be verified by MS and NMR measurements. 5 was formed
in 2-propanol up to 6.45 mol % after 7 h of irradiation (mole
percent of phosmet turnover).
The irradiation experiments in cyclohexane showed the same
products found in methanol. During irradiation of phosmet in
cyclohexene, 2-4 were also obtained, but the concentrations
of all three components were significantly lower as compared
to the other solvents. Additionally, small amounts of N-
methylphthalimide (6) were identified, and many yet unidenti-
fied products were found during RP-HPLC analysis at later
retention times than the parent phosmet. Furthermore, the sum
of derived photoproducts was substantially lower than the sum
of photoproducts in the other solvents, which denotes that
phosmet must have partly reacted in a different way. Formation
of phosmet-oxon (7) during irradiation was near or below the
limit of detection in all four solvents.
Figure 4 shows the distribution of the three main photolysis
products (2-4) of phosmet in the different solvents after 7 h of
irradiation. The amount of 2 occurring in cyclohexene during
irradiation was very low in comparison to the amounts measured
in the other model solutions. The highest amount of 2 was
obtained in methanol (21.9 ( 1.5 mol %).
Likewise, the formation of 3 in cyclohexene was also very
low, whereas the highest yield of 3 could be measured in
2-propanol (27.1 ( 1.1%). 4 was observed in amounts of 11.0-
Figure 6. Dependency of water percentage of methanol and formation
of the photoproducts 2 (a, 9), 3 (c, b), and 4 (b, 1) after an irradiation
time of 7 h.
20.8% in all model solutions, with lowest values in cyclohexene
and highest in methanol.
As the observed reaction products are also formed in
hydrolysis reactions, further irradiation experiments in methanol
were carried out with various percentages of water [10, 20, and
30% (v/v)]. Figure 5 shows the photoinduced degradation as a
function of the solvent composition. Once again, degradation
was not detectable in un-irradiated samples.
Up to a percentage of 10% water, no increased degradation
was detectable, but starting with 20% water, an increased
degradation took place, and, at a water percentage of 30%,
merely 26% of the initial phosmet was left after 7 h. Simulta-
neously, the formation of 2 was strongly increased with higher
water percentages (Figure 6). The impact of water on the
formation of 3 showed a small decrease between 0 and 10%,
but had no more effect between 10 and 30% of water. The
occurrence of 4 seems to be independent of the presence of
water as in all cases with up to 30% of water, from 9.0 to 10.2
mol % of the initial phosmet was obtained.
Irradiation Experiments on Surfaces. Some irradiation
experiments on surfaces were carried out to study the photo-
chemistry of the pure nondissolved active agent in interaction
with air contact (oxygen).

5994 J. Agric. Food Chem., Vol. 51, No. 20, 2003
Sinderhauf and Schwack
Scheme 2. Proposed Mechanisms of the Formation of Photoproducts of Phosmet during Irradiation
Glass surfaces were chosen as a relatively inert material
(droplets on glass surface); silica gel plates offer a surface for
good interaction possibilities between phosmet and air.
The product spectrum after irradiation on silica gel plates
for up to 48 h (19% of degradation after 6 h, 73% after 48 h)
included 2, 3, 4, and 7. Whereas products 2, 3, and 7 have
already been identified in earlier studies on silica gel plates (5,
20), 4 could additionally be detected in this study. After 6 h,
photoproducts 2, 4, and 7 were measured in amounts of 66, 22,
and 4 mol %, respectively. After 48 h, 59, 2, 34, and 4 mol %
of 2, 3, 4, and 7, respectively, were obtained (mole percent of
phosmet turnover).
During irradiation studies on glass surfaces, a higher degrada-
tion rate than on silica gel plates was found (32% after 6 h;
95% after 48 h). During these experiments very few degradation
products were obtained. Only small amounts of 7 were
determined; the photoproducts 2-4 could not be detected.
intramolecular rearrangements from split-off thiophosphate
groups (3), presumably starting from an exiplex. Cleavage of
the sulfur-carbon bond, followed by hydrogen transfer, led to
6, whereas a hydroxyl transfer in terms of photohydrolysis
through traces of water yielded 3. However, during the
methanol/water experiments, 3 was reduced slightly rather than
increased, and phthalimide (2) became the main product.
Assuming that 3 will nevertheless be the first and main product
during methanol/water irradiations, UV radiation will also be
mainly absorbed by 3, enforcing further degradation reactions
which can easily be explained by a six-membered ring McLaf-
ferty-type rearrangement with scission of formaldehyde (Scheme
2). A separate irradiation experiment with 3 in cyclohexane
resulted in 2 as the only photoproduct, which was not formed
in nonexposure experiments, whereas Khan (23) described 2
as a hydrolysis product of 3 under alkaline conditions.
During irradiation in cyclohexene, N-methylphthalimide (6),
which is reported to be one of the main photolysis products in
diethyl ether along with N-methoxymethylphthalimide (4) (3),
could be identified as a minor photolysis product in concentra-
tions of up to 2.8 mol % (after 6 h of irradiation, mole percent
of initial phosmet). 6 could not be identified in any other of the
solvents used. As the sum of identified photoproducts in
cyclohexene was substantially lower as determined in the other
solvents used, and many yet not identified products were found
at later retention times during RP-HPLC, phosmet must have
reacted in a different way as well. In addition to extensive
photodecomposition, photoaddition reactions with cyclohexene
can be assumed from our own experiences. Products of the well-
known olefin/phthalimide photochemistry (24) were also ob-
served during earlier irradiations of the fungicide folpet and
other N-substituted phthalimides (25) in the presence of
cyclohexene or methyl oleate.
DISCUSSION
Unexpectedly, the degradation rates of phosmet in the
presence of the four different solvents used were determined to
be nearly independent of the chemical environment. Former
studies with different pesticides such as parathion (8) or folpet
(21) showed an increased degradation in the presence of olefins
as compared to alkanes or alcohols. However, in contrast to
phosmet, these pesticides have chlorine atoms or nitro groups
as part of the molecular structure, which are known for fast
and effective reactions under irradiation conditions, for example,
dechlorination and nitroreduction (7, 22).
Concerning the mechanisms of photoproduct formation, bond
cleavages around the phosphorus dithioate group are generally
favored, being partly influenced by the solvent environment.
Thus, the photoproduct 5 has to stem from intramolecular
reactions between phosmet and the isopropanol solvent (pho-
tosolvolysis) (Scheme 2), being the only possible source for
isopropoxy moieties, and, consequently, was not found in the
other solvents. During irradiation in methanol, the same
solvolysis mechanism can be discussed for the formation of 4
(Scheme 2). However, in the presence of cyclohexane, cyclo-
hexene, or 2-propanol, 4 can only be understood as a result of
Whereas the nitrogen-purged solvents were not absolutely
free of oxygen, the irradiation experiments on surfaces clearly
showed only a slight influence of oxygen on phosmet photo-
degradation. The typical photooxidation-derived oxon derivative
(7) was found only in very small amounts. Product 3 was found
to be the major product in solutions, but only a minor constituent
after surface experiments, whereas 2 was determined to be the

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J. Agric. Food Chem., Vol. 51, No. 20, 2003 5995
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main product. Evaporation, which was partly observed during
irradiations on glass surfaces only, was not taken into account.
Recapitulating, in model environments providing different
functional groups phosmet was readily photodegraded, yielding
phthalimide derivatives mostly with the loss of the dithiophos-
phorus group. Additionally many not yet identified photoprod-
ucts were formed in cyclohexene, which are supposed to be
photoadditionon products. With the understanding of general
photodegradation principles, in vitro and in vivo irradiation
experiments in the presence of wool wax/skin lipids have to be
performed and will be undertaken in the next step.
ACKNOWLEDGMENT
We thank S. Mika (Institute of Chemistry, University of
Hohenheim) for recording the NMR spectra.
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Received for review March 14, 2003. Revised manuscript received July
29, 2003. Accepted August 1, 2003.
JF034253Y