Phototransformation of linuron and chlorbromuron in aqueous solution

Article abstract of DOI:10.1002/(SICI)1096-9063(199712)51:4<413::AID-PS671>3.0.CO;2-9

The main photoproducts initially formed in the phototransformation of linuron and chlorbromuron in aqueous solution result from photohydrolysis, i.e. hydroxylation with release of halide ion, and from elimination of a methoxy group. Reductive debromination was also observed with chlorbromuron. The transformation is less specific than with diuron. The orientation of the reaction depends on the wavelength: short wavelengths (254 nm) favour demethoxylation and photohydrolysis in the meta position whereas, with 'black light', i.e. wavelengths longer than 330 nm, photohydrolysis in the para position is the main reaction observed.

Full text of DOI:10.1002/(SICI)1096-9063(199712)51:4<413::AID-PS671>3.0.CO;2-9

Pestic. Sci. 1997, 51, 413È418
Phototransformation of Linuron and
Chlorbromuron in Aqueous Solution
Valerie Faure & Pierre Boule
Laboratoire de Photochimie Moleculaire et Macromoleculaire (URA CNRS 433) Universite Blaise Pascal
(Clermont-Ferrand) F-63177 Aubiere Cedex, France
(Received 10 December 1996; revised version received 22 April 1997; accepted 13 August 1997)
Abstract: The main photoproducts initially formed in the phototransformation
of linuron and chlorbromuron in aqueous solution result from photohydrolysis,
i.e. hydroxylation with release of halide ion, and from elimination of a methoxy
group. Reductive debromination was also observed with chlorbromuron. The
transformation is less speciÐc than with diuron. The orientation of the reaction
depends on the wavelength: short wavelengths (254 nm) favour demethoxylation
and photohydrolysis in the meta position whereas, with “black lightÏ, i.e. wave-
lengths longer than 330 nm, photohydrolysis in the para position is the main
reaction observed.
Pestic. Sci., 51, 413È418, 1997
No. of Figures: 4. No. of Tables 4. No. of Refs: 14
Key words: linuron, chlorbromuron, chlorophenylureas, photolysis, photo-
transformation
1
INTRODUCTION
Monuron and diuron are stable in neutral water.
Their photochemical properties have been the subject of
several publications. The main products formed in the
photolysis of monuron result from substitution of Cl by
OH,1,2 oxidation or elimination of methyl groups,3 for-
mation of oligomers and biphenyl derivatives.1,4 The
formation of 4-chloroaniline, 4-chloroformanilide and
4,4@-dichlorocarbanilide has also been reported.3 The
presence of non-ionic surfactants eliminates ring
hydroxylation and enhances reductive dechlorination.5
The same e†ect was observed when methanol was
added to the solution.2
The main photoproducts identiÐed by Tanaka et al.6
in the photolysis of diuron in aqueous solution resulted
from the substitution of Cl by OH, and elimination or
oxidation of the methyl group. Reductive dechlori-
nation in the para position was also observed. Photore-
duction in the meta position, reported by Durand et
al.,7 may be related to the presence of a small amount of
methanol in the solution. From results published
Halophenylureas have been used for about 45 years as
herbicides. The most studied are monuron [3-(4-chlo-
rophenyl)-1,1-dimethylurea] and diuron [3-(3,4-
dichlorophenyl)-1,1-dimethylurea]. Linuron [Fig. 1,
3-(3,4-dichlorophenyl)-1-methyl-1-methoxyurea]
and
chlorbromuron [3-(4-bromo-3-chlorophenyl)-1-methyl-
1-methoxyurea] were introduced later, in 1962 and in
1966, respectively. Their elimination from the environ-
ment may involve chemical reactions (mainly hydrolysis
and oxidation), biotransformation and photolysis, so it
is important to know the relative importance of these
di†erent ways and to identify the products formed.
These products may have completely di†erent proper-
ties and might be more toxic than the initial compound.
* To whom correspondence should be addressed.
Contract grant sponsor: Centre National de la Recherche
ScientiÐque.
413
( 1997 SCI. Pestic. Sci. 0031-613X/97/$17.50. Printed in Great Britain

414
V . Faure, P. Boule
Fig. 1. Structures of linuron, chlorbromuron and photoproducts.
recently it appears that photohydrolysis (i.e. hydroxyl-
ation with release of chloride ion) is initially almost
quantitative in dilute solution, but a wavelength e†ect
was observed.8 Para hydroxylation is a minor pathway
at 254 nm but is enhanced by increasing wavelength.8
Linuron and chlorbromuron have been much less
studied from the photochemical point of view. The
main photoproducts identiÐed by Rosen et al.9 in an
aqueous solution of linuron exposed to sunlight were
3-(3-chloro-4-hydroxyphenyl)-1-methoxy-1-methylurea
(13%), 3,4-dichlorophenylurea (10%) and 3-(3,4-
dichlorophenyl)-1-methylurea (2%). More recently Nick
and Scholer2 irradiated linuron and chlorbromuron at
254 nm. Several photoproducts were identiÐed. The
main one resulted from the cleavage of the N-methoxy
linkage with the formation of formaldehyde, but the
replacement of Cl by OH was also observed. In the
presence of an H donor like methanol or phenylurea
itself at high concentration these reactions compete with
the replacement of halogen atoms by hydrogen.
phototransformation in sunlight. The inÑuence of sub-
stituents on the photoreactivity was examined by com-
parison with results previously obtained with diuron.
2
MATERIALS AND METHODS
Linuron and chlorbromuron were provided by Chem.
Service. Dichloroaniline ([97%) was supplied by
Fluka.
3-(3,4-Dichlorophenyl)-1-methylurea was synthesized
from 3,4-dichlorophenylisocyanate and methylamine in
dry tetrahydrofuran.
The other photoproducts were identiÐed from their
[1H]NMR spectra recorded at 400 MHz on a Bruker
AC 400 and their mass spectra obtained on a VG MS
305 (Centre Regional de Mesures Physiques) or on an
E. B. Fison, 70E type (Service Central dÏAnalyses of
CNRS).
The aim of the present work was to analyse photo-
products formed in the photolysis of linuron and chlor-
bromuron at di†erent wavelengths in order to establish
if the irradiation at 254 nm is representative of the
Water used for solutions was puriÐed by a Milli Q
system.
Solutions were irradiated at 254 nm with a low-
pressure mercury lamp (germicide lamp) surrounded by

Phototransformation of halophenylureas in aqueous solution
415
a cylindrical mirror with an elliptical base, the reactor
in quartz and the lamp being located on both focus
axes.
several weeks. This evolution is enhanced by increasing
pH.
For irradiations between 300 and 350 nm, Ñuorescent
lamps type Duke GL 20E (maximum emission near
310 nm) were used. Solutions were also irradiated with
“black lightÏ lamps emitting about 85% of photons at
365 nm (mercury line) and about 7% at 334 nm. A
reactor in Pyrex was used for wavelengths longer than
300 nm.
3.1 Phototransformation of linuron
3.1.1 Analytical study
A typical chromatogram of a solution irradiated during
1 h at 254 nm is given in Fig. 3.
The four main photoproducts LP1ÈLP4 (Fig. 1) were
isolated and analysed by MS and [1H]NMR. They are
identiÐed from results presented in Table 1.
Irradiated solutions were analysed by HPLC on a
125 ] 4 mm
C
column using methanol ] water
18
(usually 60 ] 40 by volume) as eluent, associated to a
3.1.2 InÑuence of wavelength
Waters chromatograph with photodiode array detector.
The quantum yield was evaluated at 254, 277 and
296 nm. It is a little higher at 254 nm (0É05È0É06) than
at 277 and 296 nm (0É02). Recently the value 0É096 was
obtained by Nick and Scholer2 at 254 nm. At wave-
lengths longer than 300 nm it was not possible to
measure it because the absorption was too low.
However it was observed that the ratio of products
depended on wavelength between 254 and 365 nm as it
appears in Table 2.
3
RESULTS
The UV spectra of linuron and chlorbromuron are
similar, as shown in Fig. 2. The absorption is very low
at wavelengths longer than 300 nm.
It was observed that solutions of linuron and chlor-
bromuron are not as stable as solutions of diuron. For-
mation of dihalogenoaniline can be observed after
Long wavelengths favour para-hydroxylation, as was
reported for diuron.8 This e†ect is mainly important at
the limit of the absorption band i.e. at j [ 300 nm.
Fig. 3. HPLC chromatogram of
a solution of linuron
(10~4 M) irradiated during 1 h at 254 nm. Eluent
Fig. 2. UV spectra of (ÈÈÈ) linuron and (È È È È) chlorbro-
muron.
methanol ] water (52 ] 48 by volume). Detection 250 nm.
TABLE 1
MS and [1H]NMR Analyses of the Main Products formed by Irradiation of Linuron at
254 nm in an Air-Saturated Aqueous Solution (10~4 M)
MS
[1H]NMR in CD OD (d ppm)
3
(e-impact 70 eV )
(ref. CD HOD 3É5 ppm)
2
m/e
Cl number
H
H (d)
H (dd)
OCH (s)
CH
2
5
6
3
3
Linuron
LP1
LP2
LP3
LP4
248
230
230
204
218
2
1
1
2
2
8É01
7É66
7É43
7É92
7É91
7É59
7É04
7É35
7É55
7É54
7É64
7É36
7É06
7É41
7É40
3É93
3É90
3É95
È
3É35
3É35
3É30
È
È
2É95

416
V . Faure, P. Boule
TABLE 2
TABLE 4
InÑuence of Wavelength on the Phototransformation of
Linuron in Air-Saturated Aqueous Solution
InÑuence of Wavelength on the Phototransformation of
Chlorbromuron Irradiated in Air-Saturated Aqueous Solu-
tion (10~4 M)
Photoproduct
j (nm)
L P1
L P2
L P3
L P4
Photoproduct
j (mm)
CP1
CP2
CP3
CP4
254
]
]
]]
]
]
]
Da
]]
D
295È350
254
275È350
“Black lightÏ
]
]
]]
]
]
]
]
]]
]
“Black lightÏ
(85% 365 nm
7% 334 nm)
Sunlight
]]
]
D
D
a D \ detected as a minor product.
thoxylation and meta-hydroxylation. (Table 4). It was
also noted that at 254 nm CP4 is the main photo-
product in the absence of oxygen and that deoxy-
genation increases the ratio CP2/CP1.
Results in sunlight are not available, but it can be
assumed that they are intermediate between the two last
lines of Table 4.
The quantum yield was evaluated at 0É05 at 254 nm
in good agreement with results of Nick and Scholer.2 It
is lower at 277 nm.
Wavelength also inÑuenced demethoxylation, which
was induced to a greater extent by excitation in the
main absorption band (230 \ j \ 270 nm) than by exci-
tation between 270 and 300 nm. Demethoxylation can
be related to the formation of formaldehyde which was
experimentally detected in solutions irradiated at
254 nm using its reaction with chromotropic acid.
3.2 Phototransformation of chlorbromuron
3.2.1 Analytical study
Four photoproducts CP1ÈCP4 (Fig. 1) were isolated by
HPLC in an air-saturated solution (1É2 ] 10~4 M) irra-
diated at 254 nm. The main peak observed by detection
at 250 nm corresponds to CP4. These were identiÐed by
MS and NMR and results are given in Table 3. The
presence of Cl or Br is easily deduced from mass
spectra.
4
DISCUSSION AND MECHANISMS
The meta-hydroxylation on the ring with dechlorination
is a common reaction observed with chlorobenzenes, 3-
halophenols and diuron. It results from a nucleophilic
substitution involving a molecule of water (Fig. 4a) and
for this reason it has been called photohydrolysis.10h12
With linuron and chlorbromuron it is less selective than
with diuron irradiated at short wavelengths.
3.2.2 InÑuence of wavelength
As was observed with linuron para-hydroxylation is the
main reaction observed at long wavelengths whereas the
excitation of 254 nm (main UV band) enhances deme-
The fact that the ratio para-hydroxylation/meta-
hydroxylation increases with increasing wavelength was
TABLE 3
(a) MS and (b) [1H]NMR Analyses of the Main Products formed by Irradiation of Chlorbromuron at 254 nm in an Air-Saturated
Aqueous Solution (10~4 M)
(a) MS
(e-impact 70 eV )
m/e
Cl number
Br number
CP1
CP2
CP3
CP4
230, 232
274, 276
214, 216
1
0
1
1
0
1
0
1
262, 264, 266
(b) [1H]NMR
Solvent
NH
H
H
H
H
OCH
CH
2
4
5
6
3
3
chlorbromuron
CD OD
È
8É90 (s)
È
8É60 (s)
8É80 (s)
8É30 (s)
8É03 (d)
8É00 (d)
7É58 (d)
7É51 (d)
7É83 (t)
7É93 (d)
È
È
È
È
7É79 (d)
7É58 (d)
6É96 (d)
7É32 (d)
7É26 (t)
7É49 (d)
7É58 (dd)
7É52 (dd)
7É28 (dd)
6É98 (dd)
7É53 (td)
7É25 (dd)
3É95 (s)
3É70 (s)
3É90 (s)
3É75 (s)
3É75 (s)
È
3É35 (s)
3É10 (s)
3É35 (s)
3É10 (s)
3É10 (s)
2É54 (s)
3
(CD ) CO
3 2
CP1
CP2
CP3
CP4
CD OD
3
(CD ) CO
3 2
(CD ) CO
3 2
7É02 (ddd)
È
(CD ) CO
3 2

Phototransformation of halophenylureas in aqueous solution
417
Fig. 4. Proposed mechanisms of photo-induced reactions.
tentatively explained, with diuron, by the involvement
of an enolic form, since it was observed neither with 3,4-
dichlorophenol nor with diuron methylated on both
nitrogen atoms8 and because the anionic form is
expected to absorb at longer wavelengths. This explana-
tion, which was not deÐnitively proved, can be applied
to linuron and chlorbromuron, but this e†ect is not so
clear as with diuron because of the competition between
factants.5 It is attributed to a substitution involving the
formation of a radical after electron transfer from the
solvent (SN . ET . Ar* substitution), more efficient with
2
Br than with Cl (Fig. 4c).
photohydrolysis
(demethoxylation and reduction).
and
the
other
reactions
CONCLUSION
Demethoxylation associated with formation of form-
aldehyde is more efficient by irradiation in the absorp-
tion band centered at 242È245 nm than by excitation in
the lowest excited state. A Norrish-type II mechanism is
most likely. (Fig. 4b); it was previously proposed in the
case of monolinuron.13h14 It can be noted that deme-
thoxylation is more efficient than photohydrolysis when
linuron and chlorbromuron are excited in the second
absorption band centred near 245 nm, but this wave-
length e†ect cannot be correlated with bond energies
since the energy of the OÈN bond is lower that the
energy of the CÈCl bond.
The photoreactivity of halogenophenylureas depends on
the substituents on nitrogen and on the ring, but pho-
tohydrolysis of the C-halogen bond occurs in all cases.
Linuron and chlorbromuron can be photolysed at
254 nm and at wavelengths representative of sunlight,
but the transformation is signiÐcantly di†erent and irra-
diations at 254 nm cannot be used to study the photo-
chemical transformations in environmental conditions.
The para-hydroxylated derivative is the main product
obtained in near-UV radiation, whereas other reactions
are favoured by shorter wavelengths. The ratio meta/
para hydroxylation increases with decreasing wave-
length. The methoxy group is more easily eliminated
than the methyl group, and the reaction is more efficient
at short wavelengths, i.e. by irradiation in the main
(second) absorption band. Photoreduction of the ring is
Photoreduction of the ring was observed with chlor-
bromuron but not with linuron. Previously it was
observed with diuron and monuron when the solution
contained organic substances such as alcohol7 or sur-

418
V . Faure, P. Boule
dimethylurea (monuron) in aqueous solution. J. Agric.
Food Chem., 29 (1981) 1153È8.
5. Tanaka, F. S., Wein, R. G. & Mansager, E. R., E†ect of
nonionic surfactants on the photochemistry of 3-(4-chlo-
rophenyl)-1,1-dimethylurea in aqueous solution, J. Agric.
Food Chem., 27 (1979) 774È9.
6. Tanaka, F. S., Ho†er, B. L. & Wein, R. G., Photolysis of
3-(3,4-dichlorophenyl)-1,1-dimethylurea (diuron) in dilute
aqueous solution. T oxicol. Environ. Chem., 11 (1986)
261È9.
7. Durand, G., Barcelo, D., Albaiges, J. & Mansour, M., On
the photolysis of selected pesticides in the aquatic
environment. T oxicol. Environ. Chem., 31–32 (1991) 55È
62.
8. Jirkovsky, J., Faure, V. & Boule, P., Photolysis of diuron.
Pestic. Sci., 50 (1997) 42È52.
9. Rosen, J. D., Strusz, R. F. & Still, C. C., Photolysis of
phenylurea herbicides. J. Agric. Food Chem., 17 (1969)
206È7.
10. Boule, P., Guyon, C., Tissot, A. & Lemaire, J., Photo-
transformation en milieu aqueux dilue. J. Chim. Phys., 82
(1985) 513È16.
11. Boule, P., Guyon, C. & Lemaire, J., Photochemistry and
environment. VI. Direct phototransformation of chlorop-
henols and interaction with phenol on UV exposure in
easier with Br than with Cl and can be attributed to a
radical mechanism. Our results on irradiation at
254 nm are in good agreement with those of Nick and
Scholer.2
It is noteworthy that the presence of a methoxy
group on nitrogen reduces the stability of the solution
in the dark. With both linuron and chlorbromuron a
slow hydrolysis leading to the formation of dihaloge-
noaniline was observed.
ACKNOWLEDGEMENTS
This work was mainly supported by Centre National de
la Recherche ScientiÐque (CNRS) by the intermediate of
the “Groupe de Recherche ExosolsÏ. The authors are
grateful to A. Cuer and C. Tixier (University Blaise
Pascal, Clermont-Ferrand) for the synthesis of 3-(3,4-
dichlorophenyl)-1-methylurea.
aqueous solution. T oxicol. Environ. Chem.,
97È110.
7 (1984)
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