Photochemical degradation of acifluorfen in aqueous solution

Article abstract of DOI:10.1021/jf0100144

To elucidate the photochemical behavior of diphenyl ether herbicides in superficial waters, the photodegradation of acifluorfen, 5-[2-chloro-4-(trifluoromethyl)phenoxy]-2-nitrobenzoic acid (CAS Registry No. 50594-66-6), was studied in water and acetonitrile. All experiments were carried out under laboratory conditions using a solar simulator (xenon arc) or jacket Pyrex reaction cell equipped with a 125 W high-pressure mercury lamp. The calculated polychromatic quantum efficiencies (Φsolvent) of acifluorfen in different solvents are as follows (units are degraded molecules photon^-1): Φ_water = 10^-4, Φ_acetronitile = 10^-4, Φ_methanol = 10^-4, and Φ_hexane = 10^-2. The results obtained in this work are in good agreement with the literature value of monochromatic quantum yield. HPLC-MS analysis (APCI and ESI in positive and negative modes) was used to identify acifluorfen photoproducts. These results suggest that the photodegradation of acifluorfen proceeds via a number of reaction pathways: (1) decarboxylation, (2) dehalogenation, (3) substitution of chlorine group by hydroxyl or hydrogen groups, and (4) cleavage of ether linkage, giving phenols. Photorearrangement products were studied by other investigators. No such products were observed. In addition, it was found that the trifluoro functional group on acifluorfen was not affected by any transformation, and no products of a nitro group reduction were found.

Full text of DOI:10.1021/jf0100144

J. Agric. Food Chem. 2001, 49, 4795−4800
4795
P h otoch em ica l Degr a d a tion of Aciflu or fen in Aqu eou s Solu tion
†
,†
‡
‡
†
†
E. Vulliet, C. Emmelin,* L. Scrano, S. A. Bufo, J . M. Chovelon, P. Meallier, and
§
M. F. Grenier-Loustalot
L.A.C.E., Universit e´ Lyon I, CNRS, UMR 5634, 43 Boulevard du 11 novembre 1918,
9622 Villeurbanne Cedex, France; Dipartimento Produzione Vegetale, Universit a` della Basilicata,
Via N. Sauro 85, 85100 Potenza, Italy; and S.C.A. (Service Central d’Analyse),
U.S.R.059 CNRS-Echangeur de Solaize, B.P. 22, 69390 Vernaison, France
6
To elucidate the photochemical behavior of diphenyl ether herbicides in superficial waters, the
photodegradation of acifluorfen, 5-[2-chloro-4-(trifluoromethyl)phenoxy]-2-nitrobenzo ¨ı c acid (CAS
Registry No. 50594-66-6), was studied in water and acetonitrile. All experiments were carried out
under laboratory conditions using a solar simulator (xenon arc) or jacket Pyrex reaction cell equipped
with a 125 W high-pressure mercury lamp. The calculated polychromatic quantum efficiencies
-
1
(
Φsolvent) of acifluorfen in different solvents are as follows (units are degraded molecules photon ):
-
4
-4
-4
-2
Φwater ) 10 , Φacetonitrile ) 10 , Φmethanol ) 10 , and Φhexane ) 10 . The results obtained in this
work are in good agreement with the literature value of monochromatic quantum yield. HPLC-MS
analysis (APCI and ESI in positive and negative modes) was used to identify acifluorfen
photoproducts. These results suggest that the photodegradation of acifluorfen proceeds via a number
of reaction pathways: (1) decarboxylation, (2) dehalogenation, (3) substitution of chlorine group by
hydroxyl or hydrogen groups, and (4) cleavage of ether linkage, giving phenols. Photorearrangement
products were studied by other investigators. No such products were observed. In addition, it was
found that the trifluoro functional group on acifluorfen was not affected by any transformation,
and no products of a nitro group reduction were found.
Keyw or d s: Herbicides; diphenyl ether; acifluorfen; photodegradation; HPLC-MS analysis
INTRODUCTION
expected, such as dehalogenation, reduction of the nitro
group, cleavage of the ether linkage, giving phenols,
substitution by solvent, and photo-Claisen-type re-
arrangement, yielding biphenyl compounds.
Acifluorfen is a diphenyl ether herbicide used in
postemergence for the selective control of most broadleaf
weeds in soybeans and rice (1, 2). Its phytotoxic activity
is observed upon sunlight irradiation (3, 4).
The lifetime of pesticides is mostly governed by biotic
degradation and soil adsorption (5). Soil adsorption is
strongly dependent on its physicochemical properties
such as (1) pH, (2) clay mineral content, (3) soil carbon
content and nature, and (4) quality of humic substances
To identify the largest number of photoproducts and
to elucidate the complete pathway for acifluorfen pho-
todecomposition, all experiments were carried out under
laboratory conditions using a solar simulator and an
immersion apparatus equipped with a high-pressure
mercury arc lamp surrounded by a Pyrex jacket.
The limit of pesticide concentration in drinking water
(6-8). The half-life of acifluorfen in different soils can
-
1
is fixed by European Union directives at 0.1 µg L for
all active matter. As a result, analytical methods used
to identify the photoproducts must reach a limit of
range from 23 days to >112 days (8). As a result, the
half-life of acifluorfen is sufficently long that it may be
washed away toward surface waters (rivers and lakes).
Therefore, hydrolysis and photolysis are potentially
important processes responsible for the destruction of
the herbicide (9, 10). As a result, the study of the
photochemical degradation of acifluorfen is warranted.
In aqueous solution, the rate of the direct photo-
degradation of acifluorfen follows a first-order kinetics
reaction, and up to now, the only identified photoproduct
has been 2-chloro-1-(4-nitrophenoxy)-4-(trifluorometh-
yl)benzene arising from the decarboxylation of acifluo-
rfen (11). However, studies of photolysis of phenyl
ethers, diphenyl ethers, and similar molecules (12-15)
have shown that other numerous reactions can be
-
1
detection of ∼0.02 µg L . Recent progress in direct
coupling of liquid chromatography-mass spectrometry
(HPLC-MS) (16, 17) allowed good sensitivity and iden-
tification of a large number of photoproducts.
MATERIALS AND METHODS
Her bicid e. Acifluorfen, 5-[2-chloro-4-(trifluoromethyl)phe-
noxy]-2-nitrobenzoic acid, CAS Registry No. 50594-66-6,
IUPAC name 5-(2-chloro-R,R,R-trifluoro-p-tolyloxy)-2-nitroben-
zoic acid, C14
7 3 5
H ClF NO , was used: molecular weight, 361.7
-
1
g mol ; vapor pressure, 0.01 mPa (20 °C); solubility in water,
1
2
20 mg L- (23-25 °C); stability, thermal decomposition at
1
35 °C, stable in aqueous solution between pH 3 and 9, UV
decomposition (1).
*
Author to whom correspondence should be addressed
The acifluorfen used for the photodegradation experiments
was obtained from Fluka-Riedel de Ha e¨ n (Sigma-Aldrich
Chimie). A standard from Cil cluzeau info labo (Paris La
defense, France) was used as analytical reference.
Qu a n tu m Efficien cies. The determination of polychro-
matic quantum efficiencies (Φ) was carried out using a solar
[
telephone (33) 04 72 4327 83; fax (33) 04 72 44 81 14; e-mail
Corinne.Emmelin@univ-lyon1.fr].
†
Universit e´ Lyon I.
Universit a` della Basilicata.
S.C.A.
‡
§
1
0.1021/jf0100144 CCC: $20.00 © 2001 American Chemical Society
Published on Web 09/07/2001

4796 J. Agric. Food Chem., Vol. 49, No. 10, 2001
Vulliet et al.
light simulator (Suntest CPS+, Heraeus), equipped with a 1.1
kW xenon arc lamp. The concentrations used ranged from 5
-
5
-4
-1
×
10 to 1.2 × 10 mol L . Incident photonic flux was
measured by chemical actinometry using uranyl oxalate from
Fluka-Riedel de Ha e¨ n (Sigma-Aldrich Chimie). The quantum
efficiencies were calculated using custom application software,
“photon” (18, 19). This software calculated the number of
absorbed photons from the pesticides’ absorption spectrum, the
emission spectrum of the source, and the actinometric results.
Kinetic studies of photodegradation (<10% of degradation)
gave the number of photodegraded molecules during the same
time.
F igu r e 1. Absorption spectrum of acifluorfen in aqueous
P h otop h ysica l P r op er ties. UV-visible absorption spectra
were recorded with a Uvikon 930 spectrophotometer (Kontron
Instruments). Fluorescence and phosphorescence spectra were
obtained with an SFM 25 spectrofluorometer (Kontron Instru-
ments). The phosphorescence spectrum was recorded, in liquid
nitrogen temperature, in ethyl ether/2-methylbutane/ethanol
solution.
(EPA), respectively 5:5:2 in volume.
P h otoch em ica l P r oced u r es. For kinetics studies and
photoproduct identification, the experiments were carried out
under laboratory conditions with a solar light simulator
(
Suntest CPS+, Heraeus) equipped with a 1.1 kW xenon arc
-4
lamp. The concentration of irradiated solutions was ∼10 mol
-
1
L
in pure water (MilliQ-50 18 MΩ). Most experiments were
carried out under atmospheric conditions at 20 °C. The role
of oxygen has been studied in acetonitrile with a jacketed
Pyrex immersion apparatus equipped with a high-pressure
mercury arc lamp (125 W).
F igu r e 2. Photodegradation of acifluorfen in differents sol-
vents (Suntest).
Ta ble 1. Qu a n tu m Efficien cy (Φ) of Aciflu or fen in
Differ en t Solven ts
P h otop r od u ct Id en tifica tion . Six samples of 15 mL were
irradiated in the Suntest. At regular intervals a sample was
collected, stocked, and then gathered with the others in one
solution to observe the highest variety of photoproducts in the
same HPLC-MS analysis. The obtained sample was precon-
centrated by three successive liquid-liquid extractions with
ethyl acetate (1:1, v/v). The organic phase was evaporated
under vacuum at 30 °C, and the dry residue was then dissolved
in 1 mL of methanol.
solvent
hexane
methanol
water
Φ (molecules/photon)
-2
≈10
-4
3.6 × 10
-4
1.8 × 10
-4
acetonitrile
1.2 × 10
solar light spectrum slightly overlaps the first absorp-
tion band, meaning that a direct photodegradation is
possible in natural conditions. To estimate the photo-
chemical stability of molecules, the quantum yield (for
monochromatic light) or the quantum efficiency (for
polychromatic light) is a useful parameter. The quan-
tum efficiencies of acifluorfen in water, acetonitrile, and
methanol (Table 1) using the “photon” software have
been found to be close to 10 degraded molecules per
absorbed photon. The results obtained in this study are
in good agreement with those obtained by other inves-
tigators (20) using monochromatic radiation. The quan-
Kin etics Exp er im en ts. Aqueous solutions (225 mL) of
acifluorfen were irradiated in the Suntest. Products were
irradiated for 140 h, and every 10 h samples were collected
and analyzed directly (without preconcentration).
1
H NMR An a lysis. Acifluorfen, from Riedel de Ha e¨ n and
from Cluzeau, was dissolved in a mixture of CDCl
3
and DMSO
(80:20) and analyzed using a Bruker Avance 200 spectrometer.
-
4
HP LC Con d ition s. For the kinetic studies a Hypersil BDS
18 column (5 µm, 125 × 4 mm) was used. An isocratic elution
C
(66:34, % v/v) was carried out using a mobile phase of
3 4
acetonitrile/water (with H PO buffer to reach pH 3). The flow
-1
rate was 1 mL min , injection volume 10 µL, and wavelength
-2
tum efficiency obtained in hexane (Φ ≈ 10 degraded
of detection 254 nm.
molecules per absorbed photon) was higher. The pho-
todegradation is faster in hexane (Figure 2) despite a
hypsochromic shift of the first absorption band (in
P h otop r od u ct Id en tifica tion . An HPLC-MS apparatus
[Hewlett-Packard (HP) 1100 series lc-msd] equipped with a
precolumn and a Hypersil HP Elite column (5 µm, 150 × 3
mm) was used (column was thermostated at 40 °C). The
injection volume was 2 µL. An elution gradient was carried
-
1
-1
hexane λmax ) 274 nm with ꢀ274 ) 4990 L mol cm ).
A free radical mechanism is often suspected when the
photodegradation is favored in nonpolar solvents. So,
for a better understanding of the acifluorfen photo-
chemical degradation mechanism, some experiments
-
4
out with methanol/water (+10 M ammonium formate) from
1
00 to 0% water in 50 min. The flow rate was set at 0.4 mL
-
1
min . The MS detection was performed using two related
techniques: electrospray ionization (ESI), in positive and
negative modes, and atmospheric pressure chemical ionization
-4
were carried out in the Suntest (Cacifluorfen ) 10 mol
-1
L
(
in acetonitrile) with a radical initiator, 2,2′-azobis-
(APCI), using nitrogen as atomizing gas. Ionization conditions
-3
-1
2-methylpropionitrile) (AIBN) (CAIBN ) 10 mol L ).
of ESI analysis were as follows: capillary potential, 300 V; N
2
-
1
Table 2 shows that the half-life of irradiated acifluorfen
was reduced in the presence of AIBN. On the other
hand, the introduction of a well-known inhibitor of
flow, 13 L min ; pressure, 55 psig. Ionization conditions of
APCI were as follows: capillary potential, 1750 V; N flow, 6
2
-
1
L min ; vaporization temperature, 185 °C.
-3
-1
radicals, 1,4-benzoquinone (Cbenzoquinone ) 10 mol L ),
increased the half-life from 107.3 to 1861.5 h. Of course,
this reduction of the photodegradation rate may be a
result of other phenomena such as a screen effect or a
triplet quencher activity. This is consistent with recently
published results (22) that put in evidence the triplet
state involvement in the acifluorfen photodegradation.
To investigate if 1,4-benzoquinone can act as a triplet
RESULTS AND DISCUSSION
P h ot op h ysica l P r op er t ies of Aciflu or fen . Aci-
fluorfen in aqueous solution shows two maxima in the
UV-visible absorption spectrum (Figure 1), the first one
-
1
-1
at λ ) 296 nm (ꢀ296 ) 8000 L mol cm ) and the second
-
1
-1
one at λ ) 197 nm (ꢀ197 ) 32400 L mol cm ). The

Photochemical Degradation of Acifluorfen in Aqueous Solution
J. Agric. Food Chem., Vol. 49, No. 10, 2001 4797
Ta ble 2. Su n test Kin etic Da ta of Aciflu or fen
P h otod egr a d a tion (Ca ciflu or fen ) 10-4 m ol L in
-1
Aceton itr ile) in th e P r esen ce of Ra d ica l In itia tor (AIBN)
-
3
-1
a n d Ra d ica l In h ibitor (Ben zoqu in on e) (C ) 10 m ol L
)
molar ratio
added
substance
(pesticide/added
substance)
t0.5 (h)
k (h-1)
1
1:10
1:10
:0
107.3
32.9
1861.5
0.0065
0.0210
0.00037
AIBN
benzoquinone
Ta ble 3. En er gy of th e F ir st Excited Sin glet (S1) a n d
Tr ip let Sta tes (T1) of Aciflu or fen in Aceton itr ile
S1
T1
λmax (nm)
E (kJ mol-1)
297
410
402.9
291.8
∆
quencher, the energies of the first excited singlet and
triplet states were determined from the absorption,
fluorescence (in acetonitrile), and phosphorescence spec-
tra (in EPA). Results are summarized in Table 3. The
energy levels determined for acifluorfen (Es1 ) 410 kJ
-
1
-1
mol , ET1 ) 291 kJ mol ) show that 1,4-benzoquinone
cannot act as a triplet quencher (ET1,benzoquinone ) 287
-
1
kJ mol ) because its triplet state energy it not low
enough. Effectively, to have an efficient triplet-triplet
transfer, the difference between the two levels must be
-
1
at least 15 kJ mol (22). On the other hand, the light
screen effect cannot be neglected because 1,4-
benzoquinone absorbs wavelengths >280 nm (23)
(
ꢀmax,benzoquinone ) 1047 at 290 nm) and the ratio of
F igu r e 3. HPLC-UV chromatograms of acifluorfen (a) from
pesticide to 1,4-benzoquinone is in favor of 1,4-benzo-
quinone. The results using AIBN are in favor of a free
radical mechanism, but recent work (20) has shown that
the photodegradation kinetics were pH-dependent. This
result is consistent with a heterolytic mechanism. These
results are not incompatible. Like Vialaton and col-
leagues (20), we think that the first excited triplet state
may undergo either a homolytic or a heterolytic pho-
tolysis, giving competitive nucleophilic substitution and
homolytic decarboxylation or substitution.
Riedel de Ha e¨ n and (b) from Cluzeau in aqueous solution
(detection λ ) 254 nm).
Ta ble 4. HP LC-MS An a lysis of Aciflu or fen (fr om Ried el
d e Ha e1 n ): Reten tion Tim es (tR), Molecu la r P ea k s (M -
H)(, a n d F r a gm en ts Detected w ith Electr osp r a y
Ion iza tion (Nega tive Mod e)
(M - H)( (amu)
compd
tR (min)
fragments (amu)
acifluorfen
isomer
35.1
37.2
360
360
316; 195
316; 249; 195
An a lysis of th e In itia l Com p ou n d . HPLC analysis
of the standard from Cluzeau (Figure 3) showed one
well-defined peak, whereas the chromatogram of the
acifluorfen (Riedel de Ha e¨ n) used for the photodegra-
dation experiments pointed out the presence of a
secondary product (from chromatographic area ∼15%).
On the basis of the molecular peaks and fragments
detected, we can suggest different structures for the
photoproducts detected by HPLC-MS. The single struc-
ture confirmed, at the moment, is the 1,4-nitrophenol
(
product E). Product E and the 1,4-nitrophenol from
Acros gave the same result with HPLC and HPLC-MS.
The structure proposed for product C is in agreement
with previously reported results using GC-MS analysis
1
Additional H NMR and HPLC-MS analyses (Table 4)
have suggested a positional acifluorfen isomer, which
could be 5-[2-chloro-5-(trifluoromethyl)phenoxy]-2-ni-
trobenzoic acid obtained as a byproduct during acifluo-
rfen synthesis (2). The samples used for the photo-
degradation experiments contained ∼15% of 5-[2-chloro-
(23, 24). The cleavage of the ether linkage of the
herbicide molecule gives products A and B (13, 14). The
complementary fragments of these molecules [2-chloro-
4
-(trifluoromethyl)phenol and 3-(trifluoromethyl)chlo-
robenzene], which should have been detected at 196 and
80 amu, respectively, were not observed. There are
5
-(trifluoromethyl)phenoxy]-2-nitrobenzoic acid, which
can be photodegraded during the herbicide irradiation.
This presence could not be neglected, and consequences
on the previous pathways can be expected.
1
three possible explanations: (1) analytical, these parts
of the molecule are too polar and they are eluted in the
HPLC dead volume or they are not detected by MS
ionization; (2) sample preparation, their solubility in
ethyl acetate is not sufficient to be extracted during the
preconcentration; (3) photochemical, they are quickly
photodegraded.
In the negative detection mode (Table 5) two different
peaks (tR ) 28.1 and 30.3 min) were observed with the
same m/z ratio (376 amu). These two products with the
same mass and almost the same retention times have
been found to be two isomers of position (G1 and G2)
coming from the presence of 5-[2-chloro-5-(trifluoro-
Id en tifica tion of th e P h otop r od u cts. An aqueous
solution of acifluorfen is stable in the dark but gives a
lot of photoproducts under irradiation. After irradiation
with a solar simulator, Suntest (λ g 290 nm), the
preconcentrated solutions were analyzed using HPLC-
MS (ESI analysis, in positive and negative modes). All
results are summarized in Table 5. Figure 4 shows a
characteristic chromatogram of acifluorfen solution
gathering several samples collected at different times
of irradiation. The APCI results are consistent with the
ESI results.

4798 J. Agric. Food Chem., Vol. 49, No. 10, 2001
Vulliet et al.
F igu r e 4. HPLC-UV chromatogram of aqueous irradiated solution of acifluorfen (detection λ ) 254 nm).
+
Ta ble 5. Reten tion Tim es (tR), Molecu la r P ea k s (M + H) , a n d F r a gm en ts Detected w ith Electr osp r a y Ion iza tion
(
P ositive a n d Nega tive Mod es)
positive mode
negative mode
+
(M - H)- (amu)
compd
tR (min)
(M + H) (amu)
fragments (amu)
fragments (amu)
316; 195
+
acifluorfen
isomer
35.1
37.2
3.9
362
360
184
201
334
318
140
384 (M + Na)
316; 249; 195
360
A
B
C
D
E
not detected
8.7
42.7
43.8
19.1
20.6
28.1
30.3
39
332
+
340 (M + Na) ; 300
not detected
307; 249; 227; 195
+
162 (M + Na)
138
342
376
376
348
298
F
G1
G2
I
not detected
332; 285; 231; 193
332; 313; 255
332; 249
J
40
249; 113
irradiation and have been evaluated at 1.1 10- mol L-1
5
Sch em e 1. F or m a tion of P h otop r od u ct G
2
(
taking ꢀ close to the acifluorfen one). After this delay,
the quantity of product D decreased, a result that is
consistent with its photodegradation. Products A and
E appear after product D; their concentrations increase
to reach a maximum after 100 h of irradiation (Figure
methyl)phenoxy]-2-nitrobenzoic acid in acifluorfen
(
Scheme 1).
Kin etic Stu d ies. After 140 h under simulated light
Suntest), 75% of acifluorfen was converted, whereas
(
9
9.6-100% was recovered after the same period in
darkness. The evolution of some characteristic photo-
products (A, D, E, G1, and I) was analyzed using HPLC.
Figures 5-7 show chromatographic areas versus ir-
radiation time. In water the major photoproduct comes
from the decarboxylation (D) of the acifluorfen. The
maximum concentrations are obtained after 30 h of
F igu r e 6. Evolution of photoproduct A and 1,4-nitrophenol
(E) during Suntest irradiation.
F igu r e 5. Evolution of photoproduct D from the decarboxy-
lation of acifluorfen.
1
F igu r e 7. Evolution of photoproducts G and I during Suntest
irradiation.

Photochemical Degradation of Acifluorfen in Aqueous Solution
J. Agric. Food Chem., Vol. 49, No. 10, 2001 4799
Ta ble 6. Kin etic P a r a m eter s of Aciflu or fen UV
P h otod egr a d a tion w ith or w ith ou t Oxygen (k Is th e
P seu d o-Ra te Con sta n t Ca lcu la ted for F ir st-Or d er
Kin etics)
Sch em e 3. P h otod egr a d a tion P a th w a y of Aciflu or fen :
Hom olytic Su bstitu tion s
atmosphere
t1/2 (h)
k (h-1)
air
argon
6.6
21.4
0.1044
0.0323
Sch em e 2. P h otod egr a d a tion P a th w a y of Aciflu or fen
Deca r boxyla tion (P r od u ct D) P r op osed by P u sin o a n d
Gessa (11): Hom olytic Mech a n ism
Sch em e 4. P h otod egr a d a tion P a th w a y of Aciflu or fen :
Heter olytic Su bstitu tion s
4
). There is no delay between the appearance of products
E (1,4-nitrophenol) and A (4-hydroxy-2-nitrobenzoic
acid), suggesting that the formation of product E is not
only due to the photodegradation of product A but comes
also from the photolysis of the ether function of product
D. The decrease of product E, observed after 100 h,
shows that 1,4-nitrophenol is not a final residue and
that other reactions occur, such as a hydroxylation or
an opening of the aromatic ring. However, even after
1
50 h of irradiation and beyond, the quantity of these
third-generation photoproducts is under the threshold
of HPLC-MS detection, and no product have been
identified.
Compound (G1), from the hydroxylation of acifluorfen,
appeared at the beginning of the irradiation. It was
detected at the end of the 20th hour (Figure 7), and its
quantity increased regularly during the following 120
h and then decreased. Concerning product I, maximal
concentration is reached during the first 20 h and then
gradually decreases, which can be explained by a
competition of several pathways. Products from the
photolysis of product I ether function have not been
detected.
Role of Oxygen . The role of oxygen was tested in
acetonitrile using a jacketed Pyrex immersion apparatus
equipped with a high-pressure mercury arc lamp. Argon
was used to saturate the solution. Under inert atmo-
sphere the kinetic rate of photolysis clearly decreased
-
1
-1
from 0.1044 h , in air conditions, to 0.0323 h (Table
). These results are in opposition with other published
2). Afterward, the phenyl radical can couple with the
hydrogen atom formed by the reaction of the hydrated
electron with water.
6
results. For Pusino and Gessa (11), in distilled water
the half-time of photolysis decreased under inert condi-
tions, and for Vialaton et al. (20) no oxygen influence
was observed. As a result, to date, the role of oxygen in
acifluorfen photodegradation is unclear, particularly
with respect to a solvent or wavelength dependence of
the photodegradation of acifluorfen.
e- + H O f H + OH
•
-
aq
2
This reaction gives two other possibilities of reaction:
•
•
H Abstraction. The formation of free radical H can
explain the appearance of products E, 1,4-nitrophenol,
and J (Scheme 3).
P h otod egr a d a tion P a th w a y. The irradiation of
acifluorfen gives numerous photoproducts. Our results
suggest several competitive pathways.
Photodecarboxylation. The principal photoproduct
obtained was decarboxylated product D. Pusino and
Gessa (11) suggested that the mechanism for this
reaction involved the formation of a phenyl radical by
loss of a hydrated electron and carbon dioxide (Scheme
-
Nucleophilic Substitution by OH . The presence of
hydroxyl groups (Scheme 4) can explain the hydrolysis
of different molecules (acifluorfen giving products G1,
F, and C or hydroxylation of product A giving product
B). Moreover, the results of Vialaton et al. (20) have put
in evidence an increase of the scission of the ether
linkage with the hydroxyl ion concentration. Therefore,

4800 J. Agric. Food Chem., Vol. 49, No. 10, 2001
Vulliet et al.
cleavage of the aromatic ether linkage is probably a
photonucleophilic displacement, as was suggested in
nitrofen photodegradation (25), as well as in the pho-
tosubstitution of Cl by a hydroxyl group.
Con clu sion . Acifluorfen in aqueous solution absorbs
light around 300 nm. Therefore, when it is exposed to
sunlight, it can be photodegraded. We have identified
(9) Kl o¨ pffer, W. Sci. Total Environ. 1992, 123/ 124, 145-
1
59.
(
10) Scheunert, I. Physical and physico-chemical processes
governing the residue behaviour of pesticides in ter-
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1
1 photoproducts. The identification of numerous pho-
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(
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The photoproducts formed, in their turn, can be
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(
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Photorearrangement products observed by other in-
vestigators were not identified due to a less energetic
irradiation (λ > 290 nm). In addition, it was found that
the trifluoro functional group on acifluorfen was not
affected by any transformation. On the other hand, the
chloro functional group on the aromatic ring undergoes
photodehalogenation. No product of nitro group reduc-
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ACKNOWLEDGMENT
We are grateful to Olivier Paiss e´ (S.C.A.) for HPLC-
MS analyses and helpful suggestions in the spectra
interpretations.
(
(
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