Direct and induced phototransformation of mecoprop [2-(4-chloro-2-methylphenoxy)-propionic acid] in aqueous solution

Article abstract of DOI:10.1002/1526-4998(200012)56:12<1077::AID-PS247>3.0.CO;2-F

Mecoprop was irradiated under various conditions of pH, oxygenation and wavelengths in order to study the reactions involved in the phototransformation. Four main photoproducts were identified: 2-(4-hydroxy-2-methylphenoxy)propionic acid (I), o-cresol (II), 2-(5-chloro-2-hydroxy-3-methylphenyl)propionic acid (III) and 4-chloro-o-cresol (IV). When the anionic form of mecoprop was irradiated between 254 nm and 310 nm (UV-C or UV-B), I was the main photoproduct. At 254 nm its formation initially accounted for more than 80 percent of the transformation. It has not previously been reported in the literature. The reaction results from a heterolytic photohydrolysis. Product II accounted for only a low percentage of the transformation. The stoichiometry was different with the molecular form: the main photoproduct, III, resulted from a rearrangement after a homolytic scission. Products I, II and IV were also formed as minor photoproducts. Some other minor photoproducts were also identified. In contrast, IV was the main photoproduct under sunlight irradiation or when solutions were irradiated in near-UV light (UV-A). This wavelength effect is attributed to the involvement of an induced phototransformation; IV is also the main photoproduct when the phototransformation is induced by Fe(III) perchlorate or nitrite ions. In usual environmental conditions the excitation of the molecular form is negligible and the phototransformation is mainly due to induced photoreactions.

Full text of DOI:10.1002/1526-4998(200012)56:12<1077::AID-PS247>3.0.CO;2-F

Pest Management Science
Pest Manag Sci 56:1077±1085 (2000)
Direct and induced phototransformation of
mecoprop [2-(4-chloro-2-methylphenoxy)-
propionic acid] in aqueous solution
Laurence Meunier and Pierre Boule*
´
´
´
Laboratoire de Photochimie Moleculaire et Macromoleculaire, Universite Blaise Pascal, CNRS UMR 6505 (Clermont-Ferrand), 63177
`
Aubiere Cedex, France
Abstract: Mecoprop was irradiated under various conditions of pH, oxygenation and wavelengths in
order to study the reactions involved in the phototransformation. Four main photoproducts were
identi®ed: 2-(4-hydroxy-2-methylphenoxy)propionic acid (I), o-cresol (II), 2-(5-chloro-2-hydroxy-3-
methylphenyl)propionic acid (III) and 4-chloro-o-cresol (IV). When the anionic form of mecoprop was
irradiated between 254nm and 310nm (UV-C or UV-B), I was the main photoproduct. At 254nm its
formation initially accounted for more than 80% of the transformation. It has not previously been
reported in the literature. The reaction results from a heterolytic photohydrolysis. Product II
accounted for only a low percentage of the transformation. The stoichiometry was different with the
molecular form: the main photoproduct, III, resulted from a rearrangement after a homolytic scission.
Products I, II and IV were also formed as minor photoproducts. Some other minor photoproducts were
also identi®ed. In contrast, IV was the main photoproduct under sunlight irradiation or when solutions
were irradiated in near-UV light (UV-A). This wavelength effect is attributed to the involvement of an
induced phototransformation; IV is also the main photoproduct when the phototransformation is
induced by Fe(III) perchlorate or nitrite ions. In usual environmental conditions the excitation of the
molecular form is negligible and the phototransformation is mainly due to induced photoreactions.
# 2000 Society of Chemical Industry
Keywords: mecoprop; photolysis; photohydrolysis; photorearrangement; wavelength effect; induced photo-
transformation
1
INTRODUCTION
reaction induced by micro-organisms is a transforma-
tion into 4-chloro-2-methylphenol followed by ring
hydroxylation at the 6-position and ring opening.1
Only a few publications have been devoted to the
photochemical behaviour of mecoprop, and some
photoproducts have been identi®ed. Le Brun et al2
reported the formation of 2-methylphenol (o-cresol),
2-methylcyclohexadi-2,5-ene-1,4-dione (methylben-
zoquinone), 2-methyl-1-methoxybenzene, 2-methyl-
1,4-dihydroxybenzene (methylhydroquinone) and 4-
chloro-2-methylphenol (4-chloro-o-cresol), identi®ed
by gas chromatography-mass spectrometry coupling
(GC-MS) from a solution irradiated at 254nm or in
polychromatic light (200±600nm). Romero et al3 con-
cluded that photochemical transformation plays an
important role in the degradation of mecoprop on dry
or moist soil, but photoproducts were not identi®ed. It
has also been reported that mecoprop can be photo-
catalytically transformed very ef®ciently on TiO2 and
completely mineralised.4,5 The formation of several
intermediates such as a-ketopropionic acid, a-
Mecoprop [(Æ)-2-(4-chloro-2-methylphenoxy)pro-
pionic acid] is a selective systemic herbicide developed
in the 1950s which is absorbed by leaves with
translocation to roots. It is used for the post-
emergence control of broad-leaved weeds in many
cultivations. It is relatively soluble in water (734mg
litre 1) and may be found in surface waters.1
Mecoprop absorbs UV light of up to 310nm wave-
length, so that the overlap with sunlight is suf®cient to
induce its photodegradation, mainly in summer. This
is experimentally proved in the present work. Its pKa is
3.781 and, consequently, in natural waters more than
99% exists in the anionic form. Nevertheless the
photochemical behaviour of the molecular form
should be studied for comparison with other chloro-
aromatic pesticides and for a better understanding of
the mechanisms involved. Both molecular and anionic
forms have similar UV spectra, but not necessarily the
same photochemical behaviour. In plants mecoprop is
hydroxylated on the methyl group, but in soil the main
´
´
´
* Correspondence to: Pierre Boule, Laboratoire de Photochimie Moleculaire et Macromoleculaire, Universite Blaise Pascal, CNRS UMR 6505
`
(Clermont-Ferrand), 63177 Aubiere Cedex, France
E-mail: Pierre.BOULE@univ-bpclermont.fr
(Received 14 February 2000; revised version received 28 June 2000; accepted 26 July 2000)
# 2000 Society of Chemical Industry. Pest Manag Sci 1526±498X/2000/$30.00
1077

L Meunier, P Boule
hydroxypropionic acid and acetic acid was indicated
by nuclear magnetic resonance (NMR).
cylindrical mirror. The photon ¯ow in 100ml was
1
with uranyl
evaluated at 2.8Â1018 photons s
The aim of the present work was a kinetic and
analytical study of the phototransformation of meco-
prop in various conditions of pH (molecular or anionic
form), oxygenation, irradiation wavelength and sensi-
tization, in order to understand the mechanisms
involved and to compare them with those from other
chloroaromatic pesticides. From this basic study in
laboratory conditions the main photochemical path-
ways involved in environmental conditions can be
deduced and also what products may be formed if
arti®cial UV light is used for decontamination of
polluted waters.
oxalate.
Two other devices were used to study the in¯uence
of irradiation wavelength. One consisted of six
¯uorescent lamps (Duke GL 20W) emitting at wave-
lengths longer than 275nm with a maximum near
310nm. Mercury lines at 365, 405 and 436nm were
also emitted, but were not involved in the direct
phototransformation of mecoprop. The quartz reactor
was placed along the symmetry axis. The photon ¯ow
received in 21.5ml was evaluated at 9.9Â1016
photons s 1; this value is lower than the total photon
¯ow emitted by the lamp, since the actinometer
absorbs only a low percentage of the energy emitted
on lines 405nm and 436nm, but slightly higher than
the photon ¯ow really absorbed by the solution of
mecoprop.
2
MATERIALS AND METHODS
2.1 Reactants
Mecoprop (96% purity) was obtained from Zupa
Ï
In the other device, solutions were irradiated with
lamps (Philips HPW 125) and maintained at room
temperature with a cooling jacket. These lamps were
medium pressure mercury lamps ®tted with a black
bulb in order to select the line at 365nm. About 85, 6
and 2% of the light was emitted at 365nm, 334nm and
313nm respectively. With this device the photon ¯ow
was evaluated at 2.4Â1017 photons s 1 in 20ml using
the chemical actinometer Aberchrome 540.7 Potas-
sium ferrioxalate is too sensitive in these conditions
and uranyl oxalate is not suf®ciently absorbing at
365nm.
Ï
Krusevac (Yugoslavia). The commercial product,
which was slightly coloured, was puri®ed by recrys-
tallisation from water‡ethanol (1‡1 by volume) and
obtained as colourless needles. Its purity was con-
®rmed by NMR and high performance liquid chroma-
tography (HPLC) analyses.
4-Chloro-o-cresol (97%) and o-cresol (puriss), used
as analytical references, were obtained from Aldrich
and Fluka respectively. o-Cresol was sublimated
before use to eliminate traces of oxidation products.
Sodium nitrite RP Normapur and FeIII perchlorate
nonahydrate (>97%) used as photo-inducers were
purchased from Prolabo and Fluka respectively.
Water used for solutions was puri®ed with a Milli-Q
device (Millipore). Its purity was controlled by
measurement of the resistivity (>18MOcm). In
unbuffered solutions both forms of mecoprop (mol-
ecular and anionic) are generally present. To compare
the photochemical behaviour of both forms, solutions
were irradiated either at pH 2.15 (in the presence of
hydrochloric acid) or at pH 5.5 (phosphate buffer).
Solutions were also exposed in natural sunlight in a
`Pyrex' vessel. Experiments were carried out in
Clermont-Ferrand (latitude 46° N, altitude 420m)
in September.
2.3 Deoxygenation
In order to study the in¯uence of oxygen, solutions
were deoxygenated by argon bubbling for 50min. The
reactor was closed with a septum and samples were
collected with a syringe at different stages of the
photochemical transformation.
2.2 Irradiation
Several devices were used for the irradiation of
solutions. For the determination of quantum yields,
solutions were irradiated at 280nm with a xenon lamp
(1600W) equipped with a Schoeffel monochromator
providing a parallel beam. Band width at mid-height
was about 10nm. The photon ¯ow was evaluated at
2.4 Analyses
UV spectra were recorded on Varian Cary 3 spectro-
photometer.
HPLC analyses of irradiated solutions were carried
out on a Waters Chromatograph equipped with a
photodiode array detector 996 and an auto-sampler
717. Column: C18 (5mm) 250mmÂ4mm. Eluent:
acetonitrile‡water (35‡65 by volume). Water was
acidi®ed with acetic acid (1g litre 1) to prevent the
dissociation of mecoprop and products.
1
3.7Â1014 photonscm 2s with potassium ferrioxa-
late.6
Solutions were irradiated at 254nm in a device
consisting of a cylindrical mirror with an elliptic base, a
low-pressure mercury lamp (`Germicide Lamp') lo-
cated along one of the focal axes and a quartz
cylindrical reactor (2cm ID) located along the other
focal axis. The average photon ¯ow received in 30ml
was evaluated as 2.1Â1017 photons s 1, using uranyl
oxalate as the chemical actinometer.6 In order to iso-
late photoproducts, a solution (100ml) was irradiated
with six low pressure mercury lamps surrounded by a
For identi®cation, the main photoproducts, except
o-cresol and 4-chloro-o-cresol, were isolated from
irradiated solutions with a HPLC Gilson instrument
equipped with a C18 column, 50mmÂ21mm.
In order to con®rm the formation of o-cresol, solu-
tions were extracted by nitrogen bubbling, vapours
being trapped at 77K. Using this procedure it was
possible to isolate o-cresol from irradiated solutions as
1078
Pest Manag Sci 56:1077±1085 (2000)

Phototransformation of mecoprop in aqueous solution
an aqueous solution. o-Cresol was identi®ed by means
of UV and ¯uorescence spectrophotometry. An
authentic sample of known concentration was then
injected into HPLC to con®rm the identi®cation by
comparison of retention times and to quantify the
formation of o-cresol.
The products I and III were identi®ed by GC-MS
on Hewlett-Packard 5985 chromatograph (column
Machery-Nagel Optima 5; 25mÂ0.25mm ID) and by
1H NMR in hexadeuteroacetone on a Bruker AC 400
instrument.
3
RESULTS
3.1 UV absorption
Figure 2. HPLC chromatogram of an air-saturated solution of mecoprop
Molecular and anionic forms of mecoprop have similar
UV spectra. They absorb at wavelengths shorter than
310nm as shown in Fig 1. The maximum of
absorption of both forms is located at 280nm, but
the molar absorption coef®cient is slightly higher for
the anionic form (ε=1604M 1cm 1) than for the
molecular form (ε=1390M 1cm 1). The absorption
spectrum of mecoprop overlaps with that of sunlight in
summer, since the wavelength of sunlight starts at
about 295nm at sea level.8 The overlap is smaller, but
not negligible, in winter. It is shown experimentally
that mecoprop may be phototransformed in Septem-
ber. However, in environmental conditions some
induced photoreactions may also be involved.
4
(concentration=4.85Â10 M; pH 5.5) irradiated 3min at 254nm.
Conversion about 59%.
products is affected neither by oxygen nor by the
irradiation wavelength between 254nm and 350nm.
(Absorption is very low at l>310nm).
Product I is clearly the main primary photoproduct.
Its UV spectrum, obtained on a photodiode array
detector, is similar to the spectrum of mecoprop. It
was isolated by preparative HPLC and identi®ed by
MS and 1H NMR as 2-(4-hydroxy-2-methyl-
.
‡
phenoxy)propionic acid (Fig 3). m/z: 196M
3.2 Irradiation of the anionic form
4
A solution 4.94Â10 M mecoprop buffered at pH 5.5
was irradiated at 254nm in the presence and in the
absence of oxygen. Similar UV spectra were observed
in both cases. The absorption increases and a weak
absorption band appears between 300 and 400nm
(Fig 1). In both cases two photoproducts appear on
the HPLC chromatogram (Fig 2).
The same photoproducts are formed when solutions
are irradiated between 275 and 350nm in the presence
or in the absence of oxygen. Thus the formation of
Figure 1. Evolution of the UV spectrum of an air-saturated solution of
4
Figure 3. Structures of mecoprop and main photoproducts.
mecoprop (concentration=4.94Â10 M; pH 5.5) irradiated at 254nm.
Pest Manag Sci 56:1077±1085 (2000)
1079

L Meunier, P Boule
(26%); main fragments 123 (loss of CH3-CH-CO2H)
(86%); 95 (loss of COCH3-CH-CO2H) (21%); 45
(CO2H) (91%). 1H NMR 400MHz (acetone D6)
dppm: 6.69 (H6), d, J=8.7Hz; 6.64 (H3), d, J=3Hz;
6.55 (H5), dd, J=3 and 8.7Hz; 4.65 (CH), q; 2.17
(CH3 on the ring), s; 1.55 (CH3 chain), d. (s=singlet;
d=doublet; dd=double doublet; q=quadruplet).
This product has not been reported in previous work.
Product II was extracted from the irradiated solu-
tion by nitrogen bubbling and isolated as an aqueous
solution as described in Section 2.4. It was identi®ed
as o-cresol.
Solutions of known concentrations of products I
and II were prepared from product I isolated by
preparative HPLC and from the commercial com-
pound, respectively. Calibration of HPLC with these
solutions allowed a comparison between the kinetics of
I and II formation and the disappearance of meco-
prop. It appears from Fig 4 that the yield of I is initially
about 85% when the air-saturated or deoxygenated
solution is irradiated at 254nm. Product II is a minor
product, but its formation is more ef®cient in the
absence of oxygen, which is not really surprising since
it is a reduction product. Similar results were obtained
when solutions were irradiated in the range 275±
350nm. The quantum yield was evaluated at 0.75 in
an air-saturated and deoxygenated solutions irradiated
at 280nm.
However, the HPLC chromatogram was more com-
plex and at least two new photoproducts, III and IV
were formed. The same photoproducts were obtained
by irradiation in the range 275±350nm (Fig 5). The
quantum yield was measured in monochromatic
parallel beam at 280nm. It is higher in air-saturated
solution (f=0.34) than in deoxygenated solution
(f=0.23).
Product IV was identi®ed as 4-chloro-o-cresol (Fig
3) by comparison of UV spectra and retention time
with those of a commercial standard.
Product III was isolated by preparative HPLC and
identi®ed by MS and 1H NMR 400MHz spectro-
metry. It was found to have the same molecular weight
as product I, but was a chlorinated compound. m/z:
.
.
‡
‡
196, M (48%); 198, M (16%); 168 (loss of CO)
(100%); 170 (loss of CO) (33%); 133 (loss of Cl and
CO) (55%); 105 (loss of Cl and CH3-CH-CO) (81%);
77 (62%); 51 (65%).
1
The H NMR 400MHz spectrum of product III
corresponds to a mixture of two compounds III1 and
III2 (Fig 3) approximately in the ratio 3:2. III1 dppm
(acetone D6): 7.03 (H3 and H5), s; 4.03 (CH), q; 2.23
(CH3 on the ring), s; 1.45 (CH3- chain), d. III2 dppm
(acetone D6): 7.25 (H), d, J=2.4Hz; 7.18 (H), d,
J=2.6Hz; 6.80 (CH), q; 2.27 (CH3- ring), s; 1.56
(CH3- chain), d. It was concluded that III2 resulted
from the lactonisation of III1 during isolation and
drying. To corroborate this identi®cation, it was
shown experimentally that the proportion of III2
increased as the mixture was heated to dryness.
3.3 Irradiation of the molecular form
4
Solutions of 4.80Â10 M mecoprop were irradiated
at 254nm in a solution buffered at pH 2.15 in order to
study the photochemical behaviour of the molecular
form (pKa 3.78). Similar results were obtained in air-
saturated and in deoxygenated solutions, showing that
oxygen plays a minor role in the reaction. An increase
of UV absorption was observed as well as the forma-
tion of a new weak absorption band between 300 and
400nm, as was the case in the less acidic solutions.
The irradiation of an air-saturated aqueous solution
of product III in the range 275±350nm led to the
formation of a main product of slightly shorter HPLC
retention time, characterised by strong absorption
bands located at 261nm and 365nm. This product,
which was not formed in deoxygenated solution, is
most likely to be the quinonic derivative (V; Fig 3)
since it is well known that the irradiation of 4-
Figure 4. Kinetics of mecoprop disappearance and formation of products I
Figure 5. HPLC chromatogram of an air-saturated solution of mecoprop
4
4
and II in an air-saturated solution (concentration=4.85Â10 M; pH 5.5)
(concentration=5.03Â10 M; pH 2.15) irradiated 5min between 275 and
irradiated at 254nm.
350nm. Conversion about 41%.
1080
Pest Manag Sci 56:1077±1085 (2000)

Phototransformation of mecoprop in aqueous solution
chlorophenol in air-saturated solution leads to the
formation of p-benzoquinone in high yield.9,10 The
strong absorption at 365nm is probably due to an
internal H-bond.
Product V was observed as a minor secondary
photoproduct in irradiated air-saturated solution of
mecoprop (Fig 5). Another minor secondary photo-
product was identi®ed as methyl-p-benzoquinone.
3.4 Irradiation in sunlight and at 365nm
4
An unbuffered 5.82Â10 M solution of mecoprop
was exposed to sunlight in September. The half-life of
mecoprop was evaluated at 8 days. Product IV has the
kinetics of a secondary photoproduct and it accumu-
lates in the solution (Figs 6a and b). It can be deduced
that the irradiation wavelength has a signi®cant
in¯uence on the orientation of the reaction.
Figure 7. Kinetics of mecoprop disappearance and formation of 4-chloro-
4
o-cresol (IV) in an air-saturated solution (concentration=5.03Â10 M; pH
In order to con®rm this wavelength effect, a 4.85Â
2.15) irradiated at 365nm.
4
10 M solution of mecoprop buffered at pH 5.5 was
irradiated with HPW 125 lamps emitting about 85%
of photons on the mercury line 365nm and a small
percentage at 313 and 334nm (see Section 2.2). The
phototransformation was very slow, but about 13% of
mecoprop was transformed after 15 days. The main
photoproducts detected were IV, I and II. Only
disappearance of mecoprop was clearly self-acceler-
ated (Fig 7) and it is noteworthy that IV corresponded
to the main peak on the chromatogram of a solution
irradiated with HPW 125 lamps, whereas it was a
minor photoproduct when the molecular form was
irradiated at short wavelengths.
photoproducts IV and I were detected in a solution
4
of mecoprop in the molecular form (5.03Â10
M;
pH 2.15) irradiated under the same conditions. The
Product V absorbs between 300 and 400nm and it
may be assumed to play a role in the phototransforma-
tion of mecoprop at longer wavelengths. To demon-
4
strate this, an air-saturated solution of III (3.2Â10
M; pH 2.2) was irradiated between 275 and 350nm.
The formation of V was observed (Solution A). The
rate of formation of IV was then compared between
4
two solutions (B and C) of mecoprop (4.71Â10 M)
irradiated under the same conditions at 365nm, the
only difference between B and C being the presence of
25% of solution A in solutionC. It was noted that the
formation of IV was ®ve times higher with solution C
than with solution B, indicating that the presence of V
induced the formation of IV. Similarly it was observed
that the formation of IV may be induced by the
addition of a small amount of methylbenzoquinone to
a solution of mecoprop.
An air-saturated solution of methylbenzoquinone
4
(1.07Â10 M) was irradiated at 365nm in the
4
presence of mecoprop (4.75Â10 M), or in its
absence. The formation of methylhydroquinone was
observed in both cases, but it was clearly more ef®cient
in the presence of mecoprop, which was transformed
into product IV. It can be deduced that methylbenzo-
quinone induces the transformation of mecoprop and
that the formation of IV results from the oxidation of
mecoprop. This result is in good agreement with the
phototransformation of mecoprop induced by FeIII.
The involvement of singlet oxygen was ruled out since
5
no formation of IV was observed when a 3Â10
M
solution of Rose Bengal was irradiated in visible light
Figure 6. (a) HPLC chromatogram of an unbuffered solution of mecoprop
4
in the presence of 3.9Â10 M mecoprop.
4
(concentration 5.82Â10 M) exposed 23 days to sunlight in September;
The results obtained in direct photolysis are
summarised in Table 1.
conversion about 98%. (b) Kinetics of mecoprop disappearance and
formation of 4-chloro-o-cresol (IV).
Pest Manag Sci 56:1077±1085 (2000)
1081

L Meunier, P Boule
Table 1. Comparison of initial transformation rates of mecoprop under the different conditions used
Photon ¯ux
Initial transformation rate
(air-saturated solutions)
1
Conditions of irradiation
(photon cm 3s
)
4
1
`Germicide' lamp, 254nm
7.1Â1015
ꢀ4.6Â1015a
1.2Â1016
Anionic form: 1.8Â10 M min
4
1
1
1
Molecular form: 0.57Â10 M min
4
1
6 GL 20W lamps, max 310nm‡mercury lines
3 HPK 125 lamps, max at 365nm
Sunlight, September
Anionic form: 0.75Â10 M min
4
Molecular form: 0.43Â10 M min
4
1
Anionic form: 0.075Â10 M day
4
Molecular form: 0.18Â10 M day
4
1
Unbuffered solution: 0.21Â10 M day
a
See Section 2.2.
sources of hydroxyl radicals.15±17
h n
3.5 Induced phototransformation
FeIII salts absorb in the range 300±400nm and may
induce the photooxidation of organic substrates
present in the solution. Several processes may be
involved, depending on the substrate.11±13 In the
.
.
NO2 ‡ H2O ! NO ‡ OH ‡ OH
h n
.
.
HNO2 ! NO ‡ OH
pKa of HNO2 ˆ 3:37
present work, FeIII perchlorate (10 M) was used to
3
h n
.
.
induce the degradation of mecoprop in acidic solution
4
NO3 ‡ H2O ! NO2 ‡ OH ‡ OH
(5.03Â10 M; pH 2.15) irradiated at 365nm. The
acidi®cation was necessary to prevent the precipitation
of FeIII hydroxide. After 30min about 83% of
mecoprop was transformed, whereas direct photolysis
is negligible under these conditions. It clearly appears
in Fig 8 that the main product is 4-chloro-o-cresol (IV)
The quantum yield of photolysis is signi®cantly
higher with HNO2 (about 0.45) than with NO2
(0.015±0.07 according to the wavelength) and NO3
(0.009±0.017).17 Nitrite ions were preferred to nitrate
ions in the present work since they absorb at longer
wavelengths (maximum at 352nm) than nitrate ions
(maximum at 302nm) and it is possible to excite them
selectively at 365nm in the presence of mecoprop. A
further advantage is that nitrite ions have a higher
that accumulates in the solution.
Methanol was used as a quencher of OH to identify
.
the role of these radicals in the reaction. In the
presence of methanol (20ml litre 1, about 0.49M)
transformation is inhibited by about 80%, whereas
molar absorption coef®cient and higher quantum
.
.
yield, but unfortunately NO2 ions are good OH
.
about 99% of OH is quenched. (Evaluated from the
1
rate constant 9.7Â108
M
1s
for the reaction
quenchers and the production of OH in a solution is
.
OH‡CH3OH).14 It was deduced that hydroxyl
radicals contribute to the formation of IV, but
probably another pathway is also involved.
only ef®cient at low concentrations of nitrite.
4
A 5.0Â10 M solution of mecoprop with 5Â
4
10 M sodium nitrite and acidi®ed at pH 2.15 was
Nitrite and nitrate ions are also photochemical
irradiated at 365nm. Several photoproducts were
formed but only product IV accumulated in the
solution. The ef®ciency increased with increasing
irradiation time, as shown in Fig 9, and it may be
assumed that IV is not only formed as a primary
photoproduct. By comparison with a solution kept in
the dark it was established that this transformation
did not result from a thermic reaction of HNO2. The
rate of formation of IV was signi®cantly higher at pH
2.15 than at pH 5.15, due to the fact that the
photolysis of HNO2 is more ef®cient than the
photolysis of NO2 .
4
DISCUSSION AND MECHANISMS
4.1 Direct excitation
The formation of I (hydroxylated photoproduct) is
almost quantitative with the anionic form of mecoprop
excited in aqueous solution. This reaction is not
in¯uenced either by oxygen or by wavelength in the
range 254±310nm. Similar photoreactions have been
3
Figure 8. HPLC chromatogram of a solution of FeIII perchlorate (10 M)
4
and mecoprop (concentration=5.03Â10 M) irradiated 15min at 365nm.
Conversion about 65%.
1082
Pest Manag Sci 56:1077±1085 (2000)

Phototransformation of mecoprop in aqueous solution
is more ef®cient with the anionic form than with the
molecular form.
The formation of product III from the molecular
form of mecoprop results from a rearrangement after
homolytic scission of the ether bond. A similar re-
arrangement was reported in the case of 2-naphthoxy-
acetic acid.21 The formation of a lactone occurs when
the product is isolated and dried (Fig 10(ii)).
With the molecular form of mecoprop, product IV
was detected as a minor photoproduct in air-saturated
or deoxygenated solutions irradiated between 250 and
350nm. Its formation is self-accelerated and more
ef®cient in the presence of oxygen in solutions
irradiated in near-UV. Both direct and induced
reactions are probably involved. The direct formation
of IV is more ef®cient when irradiating the molecular
form, as it is for the formation of product III. It may be
attributed to a reaction between the two radicals
initially formed (Fig 10(iii)).
4
Figure 9. Phototransformation of mecoprop (5.0Â10 M) induced by
4
excitation of nitrite ions (concentration 5.0Â10 M) at 365nm (pH 2.15).
reported with chlorobenzene,18 3-chlorophenol,19 3-
chloroaniline and chlorpropham.20 It results from a
heterolytic photohydrolysis (Fig 10(i)). This reaction
The induction by quinonic compounds was estab-
lished experimentally. It is consistent with the in¯u-
ence of oxygen, the self-acceleration and the
Figure 10. Mechanisms of direct
phototransformations.
Pest Manag Sci 56:1077±1085 (2000)
1083

L Meunier, P Boule
Figure 11. Mechanisms of induced
phototransformations.
accumulation of product IV in solutions irradiated in
the near-UV. (Fig 10(iv)).
formation of I and III, which is very slow in sunlight
or with lamps emitting mainly at 365nm. The
formation of I induces a decrease of pH that favours
the further formation of III. The latter, which is a
derivative of 4-chlorophenol, is easily oxidised into a
quinonic compound which plays the role of photo-
inducer for the formation of IV. Quinonic compounds,
particulary V, are transformed into hydroquinone
derivatives. The latter is photooxidised into a quinone
that consequently plays a photocatalytic role in the
formation of IV. This mechanism explains the high
accumulation of IV in solutions irradiated in the near-
UV. The formation of methylbenzoquinone from IV
can also participate in the induced phototransforma-
tion that is consistent with the self-accelerated forma-
tion of IV.
The minor formation of o-cresol from the anionic
form of mecoprop is tentatively explained by an
internal charge transfer that induces the reduction of
the ring (Fig 10(v)).
4.2 Induced phototransformation
The main reaction observed when FeIII perchlorate is
excited in the presence of mecoprop is the formation of
4-chloro-o-cresol (IV). The excitation of FeIII salts
induces oxidation. Two pathways may be assumed
(Fig 11): oxidative decarboxylation involving the
formation of a complex between FeIII and substrate
either in the ground state or in the excited state,11 or
.
the intermediate formation of OH radicals:12,13
hn
.
‡
Fe3‡ ‡ H2O ! Fe2‡ ‡ OH ‡ H
5
CONCLUSIONS
The latter is most probably involved, since the
reaction is inhibited by methanol and the formation of
IV was also observed when the transformation was
induced by nitrite ions. However it is not the only
pathway, since the formation of IV was observed when
The formation of the main primary photoproducts
resulting from the phototransformation of mecoprop is
not in¯uenced either by oxygen or by wavelength in
the range 254±310nm, but depends on pH (different
photochemical behaviour of molecular and anionic
forms): heterolytic photohydrolysis is the main reac-
tion observed with the anionic form, whereas a
photochemical rearrangement is the main pathway
with the molecular form. A number of other photo-
products were identi®ed: o-cresol, 4-chloro-o-cresol
and quinonic derivatives. The photoreactivity of the
molecular form is interesting for comparison with
other chloroaromatic pesticides, but generally it plays
a minor role under environmental conditions.
.
.
OH was 99% eliminated. In acidic solution OH is
assumed to react with ÐCHÐ.
4.3 Irradiation in the near-UV—wavelength effect
The photochemical transformation when solutions are
irradiated at wavelengths longer than 300nm was not
expected. However, such a wavelength effect has
previously been reported in the case of MCPA (4-
chloro-2-methylphenoxyacetic acid),22 although it was
not explained. From present results, the formation of
4-chloro-o-cresol as the main photoproduct may be
explained by the excitation of quinonic photoproducts
formed by oxidation of III. The ®rst step of the
reaction in unbuffered solution is necessarily the
Different photochemical reactivity was observed
when solutions were irradiated in sunlight or near-
UV light. Under these conditions 4-chloro-o-cresol is
formed as the main photoproduct. This phenomenon
con®rms the wavelength effect previously reported in
1084
Pest Manag Sci 56:1077±1085 (2000)

Phototransformation of mecoprop in aqueous solution
Central Europe and the adjacent North Sea. Chemosphere
17:985±994 (1988).
the case of 4-chloro-2-methylphenoxyacetic acid. It is
attributed to reactions induced by quinonic com-
pounds formed as intermediates.
9 Lipcynska-Kochany E and Bolton JR, Flash photolysis/HPLC
method for studying the sequence of photochemical reactions:
application to 4-chlorophenol in aerated aqueous solution. J
Photochem Photobiol, A Chem 58:315±322 (1991).
10 Oudjehani K and Boule P, Photoreactivity of 4-chlorophenol in
aqueous solution. J Photochem Photobiol, A Chem 68:363±373
(1992).
The transformation of mecoprop can be induced by
FeIII salts and nitrite ions, leading to the formation of
4-chloro-2-methylphenol as the main product. An
oxidation of the acetic moiety by hydroxyl radicals may
be suggested.
11 Andrianirinaharivelo S, Mailhot G and Bolte M, Photodegrada-
tion of organic pollutants by complexation with transition
metals (Fe3‡ and Cu2‡) present in natural waters. Trends
Photochem Photobiol 3:361±369 (1994).
Mecoprop does not accumulate in environmental
conditions and induced reactions play a major role in
the phototransformation.
12 Mazellier P, Mailhot G and Bolte M, Photochemical behavior of
the iron(III)/2,6-dimethylphenol system. New J Chem 21:389±
397 (1997).
ACKNOWLEDGEMENTS
13 Mazellier P, Jirkovsky J and Bolte M, Degradation of diuron
photoinduced by iron(III) in aqueous solution. Pestic Sci
49:259±267 (1997).
The authors are grateful to Dr A Topalov (Institute of
Chemistry, Novi Sad, Yugoslavia), for providing re-
crystallised mecoprop, and to D Harakat (University
of Clermont-Ferrand), for his ef®cient assistance in
mass spectrometry.
14 Buxton GV, Greenstock CL, Helman WP and Ross AB, Critical
review of rate constants for reactions of hydrated electrons,
hydrogen atoms and hydroxyl radicals (ÁOH/ O ) in aqueous
Á
solution. J Phys Chem Ref Data 17:513±886 (1988).
15 Daniels M, Meyers RV and Belardo EV, Photochemistry of the
aqueous nitrate system. I. Excitation in the 300-mm band. J
Phys Chem 72:389±399 (1968).
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