Photochemical transformation of acifluorfen under laboratory and natural conditions

Article abstract of DOI:10.1002/ps.299

Acifluorfen was irradiated in pure water at various excitation wavelengths and pH values. Numerous photoproducts were obtained which were identified by [¹H]NMR and /or HPLC-MS/MS. The main reaction pathways were photo-decarboxylation, photo-cleavage of the ether bonding with formation of phenolic compounds, photo-dechlorination and photo-Claisen type rearrangement. Decarboxylation was observed in acidic and neutral media whereas cleavage of the ether bonding dominated in basic media. The photo-Claisen type rearrangement only occurred on excitation at short wavelengths. The quantum yield of photolysis was significantly lower at 313nm (6.1 × 10^-5) than at 254nm (2.0 × 10^-3). The photoreactivity of acifluorfen was then studied in conditions approaching environmental conditions. Acifluorfen was dissolved in pure water, in water containing humic substances or in a natural water, and exposed to solar light in June at Clermont-Ferrand (latitude 46°N). In pure water, the half-life was estimated at 10 days and photo-decarboxylation accounted for 30% of the conversion. The presence of humic substances (10mg litre^-1) did not affect the rate of photo-transformation. However, the half-life of acifluorfen dissolved in the natural water was only 6.8 days.

Full text of DOI:10.1002/ps.299

Pest Management Science
Pest Manag Sci 57:372±379 ꢀ2001)
Photochemical transformation of acifluorfen
under laboratory and natural conditions
Delphine Vialaton,1 Daniela Baglio,2 Ana Paya-Perez3 and Claire Richard1*
1
´
´
`
Laboratoire de Photochimie Moleculaire et Macromoleculaire, UMR CNRS No. 6505, F-63177 Aubiere Cedex, France
2EC Environment Institute, JRC, TP 290, I-21020 Ispra (VA), Italy
3EC Institute for Health and Consumer Protection (IHCP), JRC, TP 290, I-21020 Ispra (VA), Italy
Abstract: Aci¯uorfen was irradiated in pure water at various excitation wavelengths and pH values.
Numerous photoproducts were obtained which were identi®ed by [1H]NMR and /or HPLC±MS/MS.
The main reaction pathways were photo-decarboxylation, photo-cleavage of the ether bonding with
formation of phenolic compounds, photo-dechlorination and photo-Claisen type rearrangement.
Decarboxylation was observed in acidic and neutral media whereas cleavage of the ether bonding
dominated in basic media. The photo-Claisen type rearrangement only occurred on excitation at short
wavelengths. The quantum yield of photolysis was signi®cantly lower at 313nm /6.1Â10À5) than at
254nm /2.0Â10À3). The photoreactivity of aci¯uorfen was then studied in conditions approaching
environmental conditions. Aci¯uorfen was dissolved in pure water, in water containing humic
substances or in a natural water, and exposed to solar light in June at Clermont-Ferrand /latitude
46°N). In pure water, the half-life was estimated at 10 days and photo-decarboxylation accounted for
30% of the conversion. The presence of humic substances /10mg litreÀ1) did not affect the rate of
photo-transformation. However, the half-life of aci¯uorfen dissolved in the natural water was only 6.8
days.
# 2001 Society of Chemical Industry
Keywords: aci¯uorfen; photolysis; water; mechanism; solar light
1
INTRODUCTION
Aci¯uorfen ꢀ5-ꢀ2-chloro-a,a,a-tri¯uoro-p-tolyloxy)-2-
nitrobenzoic acid) is a pollutant ®guring on the priority
list 91/414/EEC given by EU member states to the
European Chemicals Bureau ꢀJRC). This nitrodi-
phenyl ether derivative is solar-light absorbing ꢀFig
1, lmax =297nm, ε297 =8200 ꢀÆ100) MÀ1cmÀ1 at pH
7) and photolysis therefore constitutes a possible way
of abiotic degradation. Literature data on the photo-
chemical behaviour of this compound in water are
scarce. From a quantitative point of view, no data on
quantum yields of photolysis or half-life times are
available. From an analytical study point of view, it has
previously been reported that decarboxylation is the
only photo-transformation pathway.1 If we take into
account results given in the literature for related
compounds, such a speci®c photo-reactivity seems
surprising. For example, aqueous nitrofen ꢀ2,4-di-
chlorophenyl 4-nitrophenyl ether) was shown to
undergo photo-cleavage of the ether linkage with
formation of substituted phenols.2 Dehalogenation,
hydroxylation and reduction of the nitro group were
also reported on irradiation of oxy¯uorfen ꢀ2-chloro-
a,a,a-tri¯uoro-p-tolyl 3-ethoxy-4-nitrophenyl ether) in
organic solvents.3
Figure 1. UV spectra in water: (—) acifluorfen at pH 7, (-Á-Á-) acifluorfen at
pH 2, (ÁÁÁ) 5-methoxy-2-nitrobenzoic acid at pH 2.
We therefore undertook a detailed study of the
mechanism of photo-transformation of aci¯uorfen in
water. For this purpose, we irradiated aci¯uorfen in
monochromatic light at 254nm and at 313nm. The
in¯uence of pH on the reaction was also studied. The
photo-transformations of 5-methoxy-2-nitrobenzoic
acid and 2-nitrobenzoic acid were investigated for
´
´
`
* Correspondence to: Claire Richard, Laboratoire de Photochimie Moleculaire et Macromoleculaire, UMR CNRS No 6505, F-63177 Aubiere
Cedex, France
E-mail: Claire.RICHARD@univ-bpclermont.fr
Contract/grant sponsor: EC-JRC Environment Institute; contract/grant number: 14089-1998-06 F1EI ISPFR
(Received 24 January 2000; revised version received 7 September 2000; accepted 5 December 2000)
# 2001 Society of Chemical Industry. Pest Manag Sci 1526±498X/2001/$30.00
372

Photochemicaltransformation of aci¯uorfen
comparison. In a second step, we evaluated the photo-
degradability of aci¯uorfen in conditions approaching
natural conditions. With this aim, the compound was
dissolved in pure water, in water containing humic
substances or in natural water, and exposed to solar
light.
The eluent was water±orthophosphoric acid ꢀ0.1%) ‡
acetonitrile in the following proportions: 60% ‡ 40%
from 0 to 5min, then a regular increase of acetonitrile
to 90% reached at 10min.
HPLC±MS analyses were performed with a Thermo
Separation Products series gradient pump P400
equipped with a thermostated ꢀ20°C) 25cmÂ4.6mm
ID column packed with 5-mm Alltima C18 reversed-
phase material ꢀAlltech). A mixture of ꢀA) Milli-Q-
water acidi®ed with 1ml litreÀ1 of acetic acid and ꢀB)
methanol was used as mobile phase at a constant ¯ow
rate of 0.8ml minÀ1. The optimised gradient pro-
gramme was: isocratic condition of 50% A‡50% B for
5min, then a linear gradient to 10% A‡90% B in
10min, which was the mobile phase until the end of
the analysis ꢀ20min). The outlet of the HPLC column
was split ꢀ1:3). The MS consisted of a Finnigan MAT
LCQ mass spectrometer equipped with a pneumati-
cally assisted electrospray ꢀESI) interface operating in
negative mode. The MS parameter optimisation was
2
EXPERIMENTAL
2.1 Materials
Aci¯uorfen ꢀpurity 98%) was purchased from Riedel
De HaeÈn and used as received. 5-Methoxy-2-nitro-
benzoic acid, 2-nitrobenzoic acid and 4-nitrophenol
were obtained from Aldrich. Water was puri®ed with a
MilliQ-Millipore device. Humic substances were
purchased from Aldrich.
2.2 Preparation of solutions prior to irradiation
Analytical studies at 254 and 313nm were performed
with aci¯uorfen at a concentration of 2Â10À4 M. In
these conditions optical densities are high, but the
detection of photo-products is made easier. The pH of
the solutions was adjusted using perchloric acid,
phosphate buffer ꢀpH 7) or sodium hydroxide. For
quantum yield measurements, we used solutions
having optical densities of 1 at 254nm ꢀirradiation in
cylindrical reactor) and 0.1 at 313nm ꢀirradiation in
parallel beam). For solar irradiation, we used aqueous
solutions of aci¯uorfen ꢀ1.4Â10À5 M). The optical
density of solutions containing aci¯uorfen in pure
water was about 0.09 at 297nm. Humic substances
were added at a level of 10mg litreÀ1. Mixtures of
aci¯uorfen and humic substances were buffered at pH
7 with phosphate buffer. The optical density of the
mixture was 0.33 at 297nm. Aci¯uorfen was also
dissolved in a natural water sampled in the `barrage de
Villerest' ꢀRoanne, France) and stored at 4°C before
use. The pH of this water was 8.5 and the DOC was
7.2mg C litreÀ1. The optical density of the solution of
aci¯uorfen ꢀ1.4Â10À5 M) in the natural water was
0.28 at 297nm.
performed using the infusion technique at 3ml minÀ1
.
The mass selected in this operation was m/z 360,
corresponding to [MÀH]À. The following optimised
values were obtained: capillary temperature 265°C;
source voltage 4.20kV; source current 100mA; capil-
lary voltage 24V; sheath gas ¯ow ꢀnitrogen) 80ml
minÀ1; auxiliary gas ¯ow ꢀhelium) 20ml minÀ1. The
MS analyses were carried out in full-scan mode
scanning the range from m/z 60 to m/z 450 in 1.1s.
The MS/MS analyses were performed in product ion
mode using a CID technique inside the ion trap. The
collision energy was set to 20% for all compounds.
[1H]NMR spectra were recorded on a AC-400
Bruker apparatus.
2.5 Preparation and identification of
photoproducts
Photoproducts were identi®ed on the basis of HPLC±
MS/MS analyses ꢀTable 1) and of [1H]NMR spectra
in the following cases.
2.5.1 5-Hydroxy-2-nitrobenzoic acid,
2.3 Irradiation
5-Methoxy-2-nitrobenzoic acid ꢀ0.5mM) and pyridi-
nium hydrochloride ꢀ3mM) were heated for 3h at
150°C. Compound 1 was recovered with dichloro-
methane. [1H]NMR ꢀdeuterochloroform), d ppm:
7.87 ꢀd, 1H, J=9.1Hz), 7.09 ꢀd, 1H, J=2.5Hz),
6.96 ꢀdd, 1H, J=9.1 and 2.6Hz).
Samples were irradiated at 254nm in a quartz
cylindrical reactor and in a device equipped with six
germicidal lamps. The chemical actinometer was
uranyl oxalate. Irradiations at 313nm were performed
using a high-pressure mercury lamp equipped with a
Bausch and Lomb monochromator. Potassium iron
ꢀIII) oxalate was used as a chemical actinometer.
Solutions were exposed to solar light at Clermont-
Ferrand ꢀlatitude 46°N) between 2 and 8 June 1999 in
a Pyrex reactor ꢀinternal diameter 0.7cm) closed with
a septum.
2.5.2 2,2'-bisꢀp-Hydroxybenzoic acid),
Basic solutions ꢀpH 12) of aci¯uorfen ꢀ2.1Â10À4 M)
were irradiated at 254nm for 30min. After irradiation,
the solution was acidi®ed to pH 1. A ®rst extraction
with dichloromethane allowed the separation of
aci¯uorfen from polar photoproducts that remained
in the aqueous phase. A second extraction with diethyl
ether allowed the recovery of a mixture of 1, 4-
nitrophenol and 4. The chemical structure of 4 was
determined on the basis of [1H]NMR spectrum and
HPLC±MS/MS analysis ꢀFig 2). [1H]NMR ꢀdeutero-
2.4 Analyses
The irradiated samples were analysed by analytical
HPLC using a Waters apparatus equipped with a
photodiode array detector ꢀmodel 996) using a
reverse-phase Spherisorb S5 ODS2 column ꢀWaters).
Pest Manag Sci 57:372±379 ꢀ2001)
373

D Vialaton et al
Table 1. Photoproducts of acifluorfen
Compound
Structure
m/z and main fragments in HPLC±MS-MS
Aci¯uorfen
360 ꢀMÀ1), 362;
316 ꢀMÀ1ÀCO2),
280 ꢀMÀ1ÀCO2 ÀHCl)
1
182 ꢀMÀ1);
138 ꢀMÀ1ÀCO2)
2
3
138 ꢀMÀ1)
196 ꢀM), 198 ꢀin GC±MS)
4
274 ꢀM);
255 ꢀMÀ1ÀH2O),
229 ꢀMÀ1ÀCO2),
211 ꢀMÀ1ÀCO2 ÀH2O)
185 ꢀMÀ1À2ÂCO2)
5
6
316 ꢀMÀ1), 318;
235 ꢀMÀ1ÀHClÀNO2)
360 ꢀMÀ1), 362;
280 ꢀMÀ1ÀHClÀCO2)
7
342 ꢀMÀ1);
298 ꢀMÀ1ÀCO2)
8
9
332 ꢀMÀ1), 334;
312 ꢀMÀ1ÀHF)
298 ꢀMÀ1);
278 ꢀMÀ1ÀHF),
258 ꢀMÀ1À2ÂHF)
methanol), dppm: 8.09 ꢀd, 1H, J=9.1Hz), 8.00 ꢀd,
1H, J=2.5Hz) 7.09 ꢀdd, 2H, J=8.4Hz).
J=2.0Hz), 7.61 ꢀdd, 1H, J=7.9 and 1.5Hz), 7.24 ꢀd,
1H, J=7.9Hz), 7.03 ꢀd, 2H, J=9.3Hz) ꢀFig 3).
2.5.3 2-chloro-a,a,a-trifluoro-p-tolyl 4-nitrophenyl
ether,
2.5.4 4-Methoxynitrobenzene
Two 100-ml portions of aci¯uorfen ꢀ2Â10À4 M) were
irradiated at 254nm for 30min. The white precipitate
was recovered by ®ltration. [1H]NMR ꢀdeuterochloro-
form), dppm: 8.27 ꢀd, 2H, J=9.3Hz), 7.82 ꢀd, 1H,
A neutral solution of 5-methoxy-2-nitrobenzoic acid
ꢀ3.2Â10À4 M) was irradiated at 254nm for 25min
ꢀ30% conversion) and 4-methoxy-nitrobenzene ex-
tracted with dichloromethane. [1H]NMR ꢀdeutero-
374
Pest Manag Sci 57:372±379 ꢀ2001)

Photochemicaltransformation of aci¯uorfen
Figure 2. MS/MS spectrum of
product 4.
chloroform), dppm: 8.23 ꢀd, 2H, J=9.3Hz), 6.95 ꢀd,
2H, J=9.3Hz), 3.9 ꢀs, 3H).
seen at shorter wavelengths. In contrast, the formation
of 4 and 6 is not observed under these conditions.
The quantum yields of aci¯uorfen photolysis as a
function of the excitation wavelength and of pH are
given in Table 2. The quantum yield of photolysis is 10
to 30 times as high at 254nm as at 313nm.
3
RESULTS AND DISCUSSION
3.1 Photolysis of acifluorfen and related
compounds under laboratory conditions
3.1.1 Photolysis of acifluorfen
To obtain additional information on the mechanism
of photo-transformation, we studied the in¯uence of
additives. Oxygen was without effect on the reaction.
Bromide ion ꢀ2M), that was expected to facilitate the
inter-system crossing, doubled the quantum yield of
aci¯uorfen disappearance.
Many photoproducts are formed upon irradiation of
aci¯uorfen at 254nm. A typical HPLC±MS chroma-
togram is given in Fig 4. The main products are
5-hydroxy-2-nitrobenzoic acid 1, 4-nitrophenol 2 and
the decarboxylated product 5. Their relative amounts
as a function of pH are given in Table 2. It appears that
the formation of 1 and 2 on the one hand and 5 on the
other show opposing dependence on pH. The chemi-
cal yields of 1 and 2 are maximum in basic media
whereas that of 5 is maximum in neutral or slightly
acidic solutions.
3.1.2 Photolysis of related compounds: 5-methoxy-2-
nitrobenzoic acid and 2-nitrobenzoic acid
The irradiation of 5-methoxy-2-nitrobenzoic acid at
254nm in neutral and slightly acidic media yields
mainly 4-methoxynitrobenzene. At pH 12, the de-
carboxylation reaction is minor, 5-methoxy-2-nitro-
benzoic acid being converted mainly into 5-hydroxy-2-
nitrobenzoic acid 1, 4-nitrophenol and 4-methoxy-
phenol ꢀFig 5ꢀa)). 4-Nitrophenol and 4-methoxy-
phenol are necessarily secondary photoproducts.
Analysis of the kinetic curves indicates that 4-nitro-
phenol is produced from 1. Quantitative data are given
in Table 3. The quantum yield of 5-methoxy-2-
Several minor photoproducts, 4, 6, 7, 8 and 9, were
characterised with the help of HPLC±MS/MS ana-
lyses ꢀTable 1). Compound 4 was also identi®ed on
the basis of the [1H]NMR spectrum.
When aci¯uorfen is irradiated at 313nm, 1, 2 and 5
are again the main photo-products. In these experi-
mental conditions, 3 can be detected, whereas it is not
Figure 3. [1H]NMR spectrum of
product 5.
Pest Manag Sci 57:372±379 ꢀ2001)
375

D Vialaton et al
Figure 4. HPLC–MS chromatogram
from acifluorfen (2.0Â10À4 M)
irradiated at 254nm in basic medium:
(A) ESI detection in negative mode;
(B) UV detection, ldet =300nm.
Table 2. Photolysis of acifluorfen (ACF) in water (2.0Â10À4 M)
Chemical yield ꢀ%)
l
pH
ΦACF
Conversion extent
1
2
5
254nm 1.5 0.0016
3.8 0.0022
50%
6.2 1.9 25
4.8 2.9 51
2.9 2.2 50
7.1 0.0020
12.4 0.0023
43 10
1.7
313nm 2.0 0.0002
4.0 0.000061
15%
6
7
2
1
5
28
35
12.0 0.00012
43
Traces
nitrobenzoic acid photolysis is as low as that of
aci¯uorfen and twice as low in very acidic media
compared with neutral or basic media.
Figure 5. Photolysis of (a) 5-methoxy-2-nitrobenzoic acid,
(b) 2-nitrobenzoic acid.
The excitation of 2-nitrobenzoic acid yields nitro-
benzene as the sole detectable photoproduct ꢀchemical
yield=70±80%). The quantum yield of 2-nitrobenzoic
acid phototransformation is 0.0014 within the pH
range 1±12 ꢀFig 5ꢀb)).
nant in neutral and acidic media, and scission of the
ether bond with the formation of phenols, which is
mainly observed in basic solution. The two reactions
show opposing pH dependence and are likely to take
place from a common intermediate. The same pattern
of reaction is observed in the case of 5-methoxy-2-
nitrobenzoic acid.
3.1.3 Mechanism of acifluorfen photolysis
The analytical study shows that aci¯uorfen undergoes
several types of photochemical rearrangement. The
main ones are photo-decarboxylation, which is domi-
Chemical yield ꢀ%) ꢀafter a conversion extent of 50%)
pH Φ5ÀMeOÀ2ÀNBA 4-MeO-nitro-benzene
1
4-nitrophenol 4-methoxy-phenol
1
0.0019
0.0041
0.0034
0.0036
15
53
55
6
<1
<1
<1
9
1
1
1
7
Ð
Ð
Ð
2
3.7
7.3
Table 3. Photolysis of 5-methoxy-2-nitrobenzoic
acid (5-MeO-2-NBA) in water (2.1Â10À4 M;
254nm)
12
376
Pest Manag Sci 57:372±379 ꢀ2001)

Photochemicaltransformation of aci¯uorfen
Photo-decarboxylation occurs for many compounds
containing the carboxyl moiety through homolytic or
heterolytic cleavage of the carboxylic group.4 For
example, photo-decarboxylation of aryl alkyl carboxy-
lates in aqueous solution has been shown to be mainly
a homolytic reaction taking place after electron photo-
ejection. In the case of nitrophenylacetates, the photo-
decarboxylation was shown to be heterolytic, involving
the intermediary formation of the triplet excited states
of nitrophenyl acetates and nitrobenzyl carbanions.5±7
Pusino and Gessa1 found a strong inhibiting effect
of oxygen on the quantum yield of aci¯uorfen
photolysis and deduced that a homolytic reaction after
electron photo-ejection was operating. For our part,
we have not observed any oxygen in¯uence either on
the quantum yield of aci¯uorfen disappearance or on
the formation of the main photoproducts, but we
found that the photolysis of aci¯uorfen was accelerated
by the addition of bromide ion ꢀ2M). This effect can be
interpreted as a heavy atom effect, suggesting the
involvement of the triplet in the reaction. This triplet
should have a short lifetime, since no oxygen effect on
the reaction was observed. On the basis of these
results, a heterolytic mechanism seems plausible.
The scission of the ether linkage is clearly favoured
by an increase of hydroxyl ion concentration and
appears to be a displacement reaction of phenoxide
by OHÀ. Nucleophilic substitutions of MeOÀ by
OHÀ have been reported in the literature in the case
of methoxynitrobenzenes and methoxynitrobi-
phenyls.8±11 They result from the activation of the
aromatic ring by the electron-withdrawing nitro group.
We propose that this nucleophilic substitution com-
petes with the photo-decarboxylation from the triplet
excited state ꢀFig 6).
In contrast to aci¯uorfen and 5-methoxy-2-nitro-
benzoic acid, 2-nitrobenzoic acid undergoes only
photo-decarboxylation within the pH range 1±12.
This result shows that decarboxylation is possible at
high as well as at low pH values. It may suggest that the
non-observation of decarboxylation at high pH in the
case of aci¯uorfen and 5-methoxy-2-nitrobenzoic acid
is due to the existence of a competitive reaction.
The photo-product 3 is not detected at 254nm. Its
formation is only observed when aci¯uorfen is
irradiated at 313nm. By contrast, we observed the
formation of 4 at 254nm but not at the longer
wavelength. Compound 4 is therefore likely to result
from the secondary photolysis of 3.
Two other reactions are observed at 254nm along
with photo-decarboxylation and cleavage of the ether
bond: photo-hydrolysis with formation of 7 and a
photo-Claisen rearrangement with formation of the
isomer 6 ꢀFig 6). The loss of the chloride ion is a
Figure 6. Photolysis of acifluorfen
(ACF): (a) photo-decarboxylation,
(b) photo-Claisen rearrangement,
(c) photo-hydrolysis.
Pest Manag Sci 57:372±379 ꢀ2001)
377

D Vialaton et al
Figure 7. Photolysis of acifluorfen
(1.4Â10À5 M) in solar light; (*) pure
water; (~) water containing humic
acids (10mg litreÀ1); (*) natural water.
Inset A: formation of product 5 in the
course of the irradiation Inset B: plot of
ln (C/C0) vs t.
typical reaction of halogenoaromatics in water12 and
was expected to occur with aci¯uorfen. The photo-
Claisen rearrangement takes place only at short
wavelengths as is generally observed.13
obtained in solar light are in good accordance with
these obtained with arti®cial light: decarboxylation is
the main reaction in neutral medium. Humic acids do
not affect the rate of aci¯uorfen photolysis. This shows
that aci¯uorfen is not sensitive to oxidant species
photogenerated by humic substances ꢀ1O2, RO2, HO2
or oxidant triplets).14,15 However, the transformation
of aci¯uorfen is photo-induced by components other
than dissolved organic matter contained in natural
waters. We suspect nitrate ions are responsible
because they are generally found in the aquatic
environment and are known to photo-produce the
very oxidant hydroxyl radicals.16,17 Even in these
conditions, the direct photolysis of aci¯uorfen remains
the main route of photo-transformation in solar light.
.
.
3.2 Phototransformation of acifluorfen in natural
conditions
Aci¯uorfen ꢀ5mg litreÀ1) was dissolved in pure water
and exposed to solar light at Clermont-Ferrand
ꢀlatitude 46°N) during the ®rst days of June. The loss
is equal to 35% after 80h of irradiation ꢀFig 7). The
decarboxylated product 5 is the main photoproduct. It
accounted for 40% of converted aci¯uorfen until the
irradiation reached 33h ꢀFig 7, insert A). At longer
irradiation times, the concentration of 5 in solution
decreases, indicating that a secondary photolysis
occurs. When aci¯uorfen is dissolved in water contain-
ing humic acids ꢀ10mg litreÀ1), the rate of photolysis is
not affected. Lastly, aci¯uorfen was irradiated in a
natural water sampled in a dam. In this case, the rate of
photolysis is increased ꢀFig 7). Again, 5 is the main
photoproduct, accounting for 23% of converted
aci¯uorfen at the maximum of formation.
4
CONCLUSION
We have studied in detail the mechanism of photo-
transformation of aci¯uorfen under arti®cial light. The
identi®cation of minor photoproducts was made
possible with the help of HPLC±MS/MS. In neutral
and acidic media, photo-decarboxylation is the main
reaction pathway, but photo-hydrolysis and photo-
Claisen type rearrangement also occur. In basic media,
a nucleophilic substitution of phenoxide by OHÀ
competes with decarboxylation. The same type of
reaction is observed with 5-methoxy-2-nitrobenzoic
acid. When exposed to solar light, aci¯uorfen is photo-
decomposed. Direct photolysis is the main reaction
pathway even in the presence of humic acids and in
natural water. In all cases, the decarboxylated photo-
product is the major reaction product.
In order to evaluate the half-lives ꢀt1/2), ln ꢀC/C0)
was plotted against t assuming ®rst-order kinetics. As
expected, these plots were linear ꢀFig 7, insert B).
From the slopes that correspond to k, we computed
t
1/2 using the following relationship:
t1=2 ˆ ln 2=k
and found 133h for experiments conducted in pure
water and water containing humic acids and 88h for
experiments with the natural water. Taking into
account that aci¯uorfen received 13h of sunshine per
day gives values of t1/2 in practical terms of 10 days and
6.8 days, respectively.
ACKNOWLEDGEMENTS
The authors acknowledge the ®nancial support given
to this work by the EC-JRC Environment Institute
From the analytical point of view, the results
378
Pest Manag Sci 57:372±379 ꢀ2001)

Photochemicaltransformation of aci¯uorfen
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the reacting species in the nucleophilic photosubstitution
reaction of m-nitroanisole. Tetrahedron Lett 15:1261±1264
ꢀ1973).
under contract 14089-1998-06 F1EI ISP FR and they
wish to thank Dr B Larsen for his help and assistance
with LC±MS/MS.
10 Cornelisse J, Lodder G and Havinga E, Pathways and inter-
mediates of nucleophilic aromatic photosubstitution reactions.
Rev Chem Intern 2:231±265 ꢀ1979).
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