Photodegradation of Dichlorprop and 2-Naphthoxyacetic Acid in Water. Combined GC-MS and GC-FTIR Study

Article abstract of DOI:10.1021/jf960699l

We have examined the photochemical transformations of dichlorprop (1) and 2-naphthoxyacetic acid (2) in aqueous solution, by means of combined GC-MS and GC-FTIR analysis. Photolysis of 1 under oxygen atmosphere led to 2-chlorophenol (5), 2,4-dichlorophenol (6), 4-chlorophenol (7), 2,4-dichlorophenyl acetate (8), the lactone of 2-(4-chloro-2-hydroxyphenoxy)propionic acid (9), and 2-(2-chlorophenoxy)propionic acid (10). Irradiation under argon atmosphere led again to 5, 6, 7, and 10 together with 2,4-dichlorophenyl ethyl ether (11). Photolysis of 2 under aerobic conditions gave β-naphthol (12), together with minor amounts of 2-hydroxy-1-naphthaldehyde (13) and naphtho [2,1-b]furan-2(1H)-one (14). Under argon atmosphere only 12 and 14 were detected. Therefore, the most general processes were photolytic cleavage of the aryl-halogen bond (route i) and the aryloxy-carbon bond (route ii). Similar photodegradation pathways had been previously observed for 2,4-D and 4-CPA and were confirmed in this work. The formation of 8, 11, and 13 must occur via cleavage of the carbon-carbon bond a to the carboxy group (route iii). Formation of this type of photoproducts in phenoxyalkanoic acid pesticides is unprecedented. Its structure was further assessed by alternative synthesis.

Full text of DOI:10.1021/jf960699l

1916
J. Agric. Food Chem. 1997, 45, 1916−1919
P h otod egr a d a tion of Dich lor p r op a n d 2-Na p h th oxya cetic Acid in
Wa ter . Com bin ed GC-MS a n d GC-F TIR Stu d y
Maria J ose´ Climent and Miguel A. Miranda*
Departamento de Qu´ımica-Instituto de Tecnolog´ıa Qu´ımica, Universidad Polite´cnica de Valencia,
Camino de Vera s/n Apartado 22012, E-46071 Valencia, Spain
We have examined the photochemical transformations of dichlorprop (1) and 2-naphthoxyacetic
acid (2) in aqueous solution, by means of combined GC-MS and GC-FTIR analysis. Photolysis of
1 under oxygen atmosphere led to 2-chlorophenol (5), 2,4-dichlorophenol (6), 4-chlorophenol (7),
2,4-dichlorophenyl acetate (8), the lactone of 2-(4-chloro-2-hydroxyphenoxy)propionic acid (9), and
2-(2-chlorophenoxy)propionic acid (10). Irradiation under argon atmosphere led again to 5, 6, 7,
and 10 together with 2,4-dichlorophenyl ethyl ether (11). Photolysis of 2 under aerobic conditions
gave â-naphthol (12), together with minor amounts of 2-hydroxy-1-naphthaldehyde (13) and naphtho-
[2,1-b]furan-2(1H)-one (14). Under argon atmosphere only 12 and 14 were detected. Therefore,
the most general processes were photolytic cleavage of the aryl-halogen bond (route i) and the
aryloxy-carbon bond (route ii). Similar photodegradation pathways had been previously observed
for 2,4-D and 4-CPA and were confirmed in this work. The formation of 8, 11, and 13 must occur
via cleavage of the carbon-carbon bond R to the carboxy group (route iii). Formation of this type
of photoproducts in phenoxyalkanoic acid pesticides is unprecedented. Its structure was further
assessed by alternative synthesis.
Keyw or d s: Dichlorprop; 2-naphthoxyacetic acid; 2,4-D; 4-CPA; GC-MS; GC-FTIR; photodegra-
dation; photodecarboxylation; photodehalogenation; photorearrangement
INTRODUCTION
EXPERIMENTAL PROCEDURES
The phenoxyalkanoic acids and their esters have been
extensively used as herbicides especially for control of
annual and perennial weeds. In agriculture, they are
employed for protection of large crops. The most com-
mon technique involves either foliar application or
topical application on soil by spraying of aqueous
formulations of pesticides (Hassall, 1969).
Sunlight-induced degradation is one of the factors
controlling the fate and persistence of pesticides and
other chemicals in the environment (Lohmann and
Petrak, 1989; Parlar, 1990; Klopffer, 1992). Also, pho-
tolysis is involved in the photoactivation and photocon-
trolled release of a number of bioactive molecules
including some insecticides, fungicides, and herbicides.
Hence, identification of the photoproducts allows us to
establish the involved photolytic pathways, thus provid-
ing valuable information on possible ways of protecting
the environment.
In this context, we have examined the photochemical
transformations of two phenoxyalkanoic acid derivatives
dichlorprop (1) and 2-naphthoxyacetic acid (2) in aque-
ous solution, by means of combined GC-MS and GC-
FTIR analysis of the resulting reaction mixtures. In
order to simulate solar light, the output from a medium
pressure mercury lamp was filtered through pyrex (λ
> 290 nm). For comparison, the analogous 2,4-dichlo-
rophenoxyacetic acid (2,4-D, 3) and 4-chlorophenoxy-
acetic acid (4-CPA, 4) have been included in the study,
as their photochemical behavior has been well estab-
lished by several research groups (Crosby and Tutass,
1966; Crosby and Wong, 1973; Smith, 1989).
Ch em ica ls. 2-(2,4-Dichlorophenoxy)propionic acid (1),
2-naphthoxyacetic acid (2) 2,4-dichlorophenoxyacetic acid (3),
and 4-chlorophenoxyacetic acid (4) were purchased from
Aldrich (Madrid, Spain). All the solvents used were of analyti-
cal reagent grade and were redistilled before use. Water was
obtained with a Milli-Q water purification system (Millipore).
Ap p a r a tu s a n d Ch r om a togr a p h y. In order to identify
the photoproducts, the reaction mixtures were analyzed by GC,
GC-MS, and GC-FTIR. GC were obtained with a Hewlett-
Packard 5890 with an FID fitted with a capillary column (HP-
5, 25 m × 0.32 mm × 0.52 mm).
GC-MS spectra were obtained with a Hewlett-Packard
spectrometer (HP 5988 A, 70 eV, electron impact ionization)
provided with a capillary column (SPB-5, 30 m × 0.25 mm ×
0.25 mm). GC-MS conditions: initial temperature 60 °C for
3 min, rate 30 °C/min up to 300 °C for 10 min. Injector port
temperature was 250 °C. Helium was the carrier gas (1.5 mL/
min). The ratios m/ z and their relative abundances in
percentages (in brackets) are given only for the most signifi-
cant peaks (down to 7% of the main signal).
IR spectra were obtained with a GC-FTIR instrument
(Hewlett-Packard 5890 GC, coupled with a 5965 A FT-IR
detector and provided with a capillary column like GC-MS).
The GC-FTIR conditions: temperature program 100-300 °C,
rate 30 °C/min. Wavenumber absorptions (cm-1) are given
only for the hydroxyl or carbonyl bands.
P r oced u r es. Irradiation Experiments. Irradiation of four
pesticides (ca. 4.2 × 10-3 M) was carried out in aqueous
solution at room temperature, under air atmosphere and
magnetic stirring. Irradiation took place in an immersion well
photoreactor for 7 h, through a Pyrex sleeve, using a 125 W
medium-pressure mercury lamp as light source.
Parallel experiments were carried out under an argon
atmosphere. For that purpose, the samples were placed in
Pyrex test tubes and sealed with septa. Then, argon was
bubbled through the solutions for 10 min prior to irradiation,
using a long syringe for inlet and a short one for outlet. The
irradiated solutions were acidified with 4 N H2SO4 and
extracted twice with dichloromethane. The organic phase was
dried over anhydrous Na2SO4, and the solvent was evaporated
at reduced pressure (14 Torr) at room temperature. On the
While we have confirmed these results for 2,4-D and
4-CPA, we have obtained chemical evidence for the
involvement of new reaction pathways in the case of
dichlorprop and 2-naphthoxyacetic acid.
* Corresponding author (tel +34 96 387 7343; fax +34
96 387 7349).
S0021-8561(96)00699-1 CCC: $14.00
© 1997 American Chemical Society

Photodegradation of Dichlorprop and 2-Naphthoxyacetic Acid
J. Agric. Food Chem., Vol. 45, No. 5, 1997 1917
Ta ble 1. GC-MS a n d GC-F TIR Da ta of th e P h otop r od u cts
photoproduct
MW
method
retention time (min)
spectral data
5
6
7
8
128
162
128
204
GC-MS (m/ z, %)
GC-MS (m/ z, %)
GC-MS (m/ z, %)
GC-MS (m/ z, %)
4.40
6.13
6.24
7.24
6.89
7.80
7.57
8.31
7.17
130(32), 128(100), 100(12)
166(11), 164(68), 162(100), 126(20), 100(11)
130(34), 128(100), 100(24)
206(6), 204(15), 162(100), 133(15), 43(37)
1795 (CO)
200(9), 198(27), 127(18), 155(100), 144(7)
1810 (CO)
GC-FTIR (cm-1
)
9
198
GC-MS (m/ z, %)
GC-FTIR (cm-1
)
10
11
200
190
GC-MS (m/ z, %)
GC-MS (m/ z, %)
202(37), 200(100), 155(50), 128(25)
194(10), 192(66), 190(100), 162(38), 133(8)
12
13
144
172
GC-MS (m/ z, %)
GC-MS (m/ z, %)
8.15
8.57
9.21
9.25
10.05
144(100), 116(27), 115(94)
172(30), 144(54), 126(12), 116(25), 115(100)
1655 (CO)
184(100), 156(68), 128(73), 102(11)
1836(CO)
GC-FTIR (cm-1
)
14
15
17
184
184
134
GC-MS (m/ z, %)
GC-FTIR (cm-1
)
GC-MS (m/ z, %)
GC-FTIR (cm-1
7.00
7.88
186(12), 184(40), 156(20), 155(100)
1816 (CO)
)
GC-MS (m/ z, %)
7.13
6.50
7.48
7.59
7.38
134(100), 122(8), 106(44), 105(29), 78(10)
1838 (CO)
152(75), 128(5), 107(100), 77(10)
170(27), 168(81), 142(26), 140(85), 112(100)
1842(CO)
GC-FTIR (cm-1
)
18
19
152
168
GC-MS (m/ z, %)
GC-MS (m/ z, %)
GC-FTIR (cm-1
)
Sch em e 1. P h otod egr a d a tion P a th w a ys of 2-(2,4-Dich lor op h en oxy)p r op ion ic Acid (1), 2,4-Dich lor op h en oxya cetic
Acid (3), a n d 4-Ch lor op h en oxya cetic Acid (4) In volvin g Clea va ge of th e Ca r bon -Ch lor in e (P r ocess i) a n d
Oxygen -Ca r bon (P r ocess ii) Bon d s
basis of the weight of the dry residues, the mass balance was
always higher than 95%.
different response factors. They are given in % of the starting
amount of parent compound.
Identification of the Photoproducts. In all experiments,
analysis of the photomixtures was done by GC-MS and GC-
FTIR. The structures of the known photoproducts were
established by comparison of their spectral properties with
those of authentic samples purchased from commercial sources.
Their yields were established by GC taking into account the
RESULTS AND DISCUSSION
The photoproduct retention times, MS fragmentation
patterns, and IR spectral data are shown in Table 1.
Photolysis of 1 (Schemes 1 and 2) under oxygen
atmosphere led to six different photoproducts identified

1918 J. Agric. Food Chem., Vol. 45, No. 5, 1997
Climent and Miranda
Sch em e 2. P h otod eca r boxyla tion Rou tes of 2-(2,4-Dich lor op h en oxy)p r op ion ic Acid (1)
Sch em e 3. P h otod egr a d a tion of 2-Na p h th oxya cetic Acid (2)
as the previously known 2-chlorophenol (5, 6%), 2,4-
dichlorophenol (6, 19%), 4-chlorophenol (7, 6%), 2,4-
dichlorophenyl acetate (8, 6%) (Sithole et al., 1986), the
lactone of 2-(4-chloro-2-hydroxyphenoxy)propionic acid
(9, 6%) (Cavill and Ford, 1954), and 2-(2-chlorophenoxy)-
propionic acid (10, 13%). Irradiation under argon
atmosphere (Schemes 1 and 2) led again to 5 (10%), 6
(26%), 7 (14%), and 10 (7%) together with the photode-
carboxylation product 2,4-dichorophenyl ethyl ether (11,
5%) (Fast et al., 1989). Hence, the total percent pho-
tolysis was 56% under air and 62% under argon.
Photolysis of 2 (Scheme 3) under aerobic conditions
gave â-naphthol (12, 73%) as the major photoproduct,
together with minor amounts of 2-hydroxy-1-naphthal-
dehyde (13, 3%) and naphtho[2,1-b]furan-2(1H)-one (14,
4%) (Kito et al., 1991). Upon irradiation under argon
atmosphere only 12 (24%) and 14 (8%) were detected.
Hence, the total percent photolysis was 80% under air
and 32% under argon.
and 15 (9%). Hence, the total percent photolysis was
94% under air and 51% under argon.
Finally, photolysis of 4 (Scheme 1) under oxygen
atmosphere led to five different products: phenol (16,
38%), 7 (28%), 2-(3H)-benzofuranone (17, 7%) (Binkley
and Oakes, 1974), phenoxyacetic acid (18, 3%), and
5-chloro-2-(3H)-benzofuranone (19, 12%) (Vallejos and
Christidis, 1993). From the photolysis under argon
atmosphere, the major photoproducts detected were
phenols 16 (51%) and 7 (43%) together with minor
amounts of 19 (4%). Hence, the total percent photolysis
was 88% under air and 98% under argon.
When aqueous solutions of 2-(2,4-dichlorophenoxy)-
propionic acid (1) were irradiated, the most general
processes were photolytic cleavage of the aryl-halogen
bond (route i) and the aryloxy-carbon bond (route ii)
(see Schemes 1 and 2). Similar photodegradation
pathways had been previously observed for 2,4-D and
4-CPA and were confirmed in this work (Crosby and
Tutass, 1966; Crosby and Wong, 1973; Smith, 1989).
When irradiation of 3 (Scheme 1) was carried out
under oxygen, only four photoproducts were detected
and identified: the phenol derivatives 5 (5%), 6 (58%),
and 7 (26%) and the lactone of 4-chloro-2-hydroxyphen-
oxyacetic acid (15, 5%) (Brown and McCall, 1955; Zepp
et al., 1975; Cavill and Ford, 1954). Under argon
atmosphere, the products were 5 (4%), 6 (30%), 7 (8%),
The formation of 2,4-dichlorophenyl acetate (8) and
2,4-dichlorophenyl ethyl ether (11) must occur via
cleavage of the carbon-carbon bond R to the carboxy
group to give radicals (I) (route iii). Trapping of I by
oxygen would lead ultimately to the isolated product 8
through the intermediacy of hydroperoxy radicals (II).

Photodegradation of Dichlorprop and 2-Naphthoxyacetic Acid
J. Agric. Food Chem., Vol. 45, No. 5, 1997 1919
In the absence of oxygen, radical I would lead to the
ethyl ether 11.
FID, flame ionization detector; MS, mass spectrometry;
FTIR, infrared spectrophotometry.
Lactones 9 and 15 are the thermal cyclization prod-
ucts of hydroxyacids 9a and 15a (Brown and McCall,
1955; Cavill and Ford, 1954). Likewise, the benzofura-
nones 17 and 19 arise from hydroxy acid precursors 17a
and 19a . It is worth mentioning that, in addition to
the widely used GC-MS techniques, GC-FTIR provides
a very useful tool for the analysis of the reaction
mixtures resulting from photolysis of phenoxyalkanoic
acid pesticides. Thus, phenols exhibit a sharp hydroxy
band at 3581 cm-1, and phenoxyalkanoic acids give rise
to typical hydroxy and carbonyl bands at ν > 3500 and
ca. 1800 cm-1 respectively (data not given in Table 1).
More interestingly, the unexpected acetate 8 displayed
a CdO absorption at 1795 cm-1, and the lactones were
also recognized through their carbonyl bands. The ν
values were higher for CdO groups within five-
membered rings (ca. 1840 cm-1 in 17 and 19) than for
the six-membered analogues (ca. 1810 cm-1 in 9 and
15).
LITERATURE CITED
Binkley, R. W.; Oakes, T. R. Photochemical reactions of methyl
phenoxyacetates. J . Org. Chem. 1974, 39, 83-86.
Brown, J . P.; McCall, E. B. Some chlorinated hydroxyphen-
oxyacetic acids. J . Chem. Soc. 1955, 3681-3687.
Cavill, G. W.; Ford, D. L. The chemistry of plant-growth
regulators. Part I. 2,4-Dichloro-6-hydroxyphenoxyacetic acid
and related compounds. J . Chem. Soc. 1954, 565.
Crosby, D. G.; Tutass, H. O. Photodecomposition of 2,4-
dichlorophenoxyacetic acid. J . Agric. Food Chem. 1966, 14,
596-599.
Crosby, D. G.; Wong, A. S. Photodecomposition of p-chloro-
phenoxyacetic acid. J . Agric. Food Chem. 1973, 21, 1049-
1052.
Fast, D. M.; Reddy, V. V.; Ashley, D. L.; Hollet, J . S.
Application of analytical standards in method develop-
ment: Selected chlorinated phenols and herbicides. J . Assoc.
Off. Anal. Chem. 1989, 72, 378-83.
Hassall, K. A. World Crop Protection; CRC Press: Cleveland,
OH, 1969; Vol. 2, p 220.
The photodegradation of 2-naphthoxyacetic acid was
also examined. When irradiation was carried out under
aerobic conditions, a mixture of three products (12-14)
resulted. Under argon only 12 and 14 were obtained
(Scheme 3). In general, the photodegradation pathways
were similar to those outlined in the case of the
analogue pesticides 1-3. Thus, cleavage of the aryl-
oxy-carbon bond (route ii) predominates, to give naph-
thol or the rearranged hydroxy acid 14a , which is
detected as the corresponding lactone 14 (ν ) 1836
cm-1). Carbon-carbon cleavage (route iii) must also
occur to some extent. Although formate IV is not
detected, the isomeric 2-hydroxy-1-naphthaldehyde (13)
was clearly identified by its MS and FTIR spectra (see
Table 1).
J imenez, M. C.; Miranda, M. A.; Tormos, R. Formation of
dichloromethyl phenyl ethers as major products in the
photo-Reimer-Tiemann reaction without base. Tetrahedron
1995, 51, 5825-5830.
Kito, T.; Yoshinaga, K.; Yamaye, M.; Mizobe, H. Base-catalyzed
alkylation of 2-naphthol with glyoxal. J . Org. Chem. 1991,
56, 3336-3339.
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chemicals in the environment: A critical assessment of the
state of the art. Sci. Total Environ. 1992, 123, 145.
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release of bioactive materials. Crit. Rev. Ther. Drug Carrier
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Horspool, W. M., Song, P. S., Eds.; CRC Press: Boca Raton,
FL, 1995; p 570.
Formation of this type of photoproducts in phenoxy-
alkanoic acid pesticides is unprecedented. Its structure
was further assessed by alternative synthesis, consisting
in the irradiation of â-naphthol in aerated chloroform
(J imenez et al., 1995). All the spectra of the synthesized
standard were coincident with those of the product
resulting from irradiation of 2. From the mechanistic
point of view, the photochemical transformation of
formate IV into hydroxyaldehyde 13 can be explained
as a photo-Fries rearrangement, which is well docu-
mented in the literature (Miranda and Garcia, 1992;
Miranda, 1995).
In summary, the phenoxyalkanoic acid derivatives
dichlorprop and 2-naphthoxyacetic acid are photolabile
and undergo cleavage of carbon-halogen, carbon-
oxygen, and/or carbon-carbon bonds. Most of their
photodegradation pathways are similar to those of 2,4-D
and 4-CPA, which have been well documented in the
scientific literature. However, other processes such as
non-oxidative decarboxylation (to give 11) or the forma-
tion of formyl phenols (e.g., 13) are unprecedented in
the field of pesticide photochemistry.
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of Functional Groups. Suppl. B. The Chemistry of Acid
Derivatives; Patai, S., Ed.; J ohn Wiley and Sons: Chichester,
1992; Vol. 2, p 1271.
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benzofuranone and 5-chloro-2-hydroxyphenylacetic acid. Fr.
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Received for review September 13, 1996. Accepted February
4, 1997.X
J F960699L
ABBREVIATIONS USED
2,4-D (3), 2,4-dichlorophenoxyacetic acid; 4-CPA (4),
4-chlorophenoxyacetic acid; GC, gas chromatography;
X Abstract published in Advance ACS Abstracts, March
15, 1997.