Photochemical behaviour of dichlorprop [(±)-2-(2,4-dichlorophenoxy)propanoic acid] in aqueous solution

Article abstract of DOI:10.1002/ps.537

Aqueous solutions of dichlorprop were irradiated under different conditions of pH, wavelength and oxygenation. The photochemical behaviour was found to be complex and many photoproducts were formed. However, at low concentrations the main photoproducts we

Full text of DOI:10.1002/ps.537

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Pest Management Science
Pest Manag Sci 58:845–852 (online: 2002)
DOI: 10.1002/ps.537
Photochemical behaviour of dichlorprop
[(ꢀ)-2-(2,4-dichlorophenoxy)propanoic acid]
in aqueous solution
Laurence Meunier, Emmanuelle Gauvin and Pierre Boule*
´
´
´
Laboratoire de Photochimie Moleculaire et Macromoleculaire, Universite Blaise Pascal (Clermont-Ferrand), CNRS UMR 6505, F - 63177
`
Aubiere cedex, France
Abstract: Aqueous solutions of dichlorprop were irradiated under different conditions of pH,
wavelength and oxygenation. The photochemical behaviour was found to be complex and many
photoproducts were formed. However, at low concentrations the main photoproducts were 4-chloro-
pyrocatechol, 2-chlorophenol, 4-chlorophenol and 2,4-dichlorophenol. Some other photoproducts
were identified, namely 2-(4-chloro-2-hydroxyphenoxy)propanoic acid, 2-(3,5-dichloro-2-hydroxy-
phenyl)propanoic acid and 2,4-dichlorophenyl acetate. From comparison with results previously
obtained with mecoprop [2-(4-chloro-2-methylphenoxy)propanoic acid] it appears that the presence
of a chlorine atom in position 2 on the ring strongly modifies the photochemical behaviour.
# 2002 Society of Chemical Industry
Keywords: dichlorprop; chlorophenols; photolysis; dichlorohydroxyphenylpropanoic acid; chlorophenoxypro-
panoic acid
1
INTRODUCTION
appearance of DCPP dispersed on Petri dish surfaces
or on dry (or moist) soil and exposed to sunlight, but
the photoproducts were not identified. On soil the
phototransformation is retarded by the presence of
peat, which leads the authors to the conclusion that
organic matter has no sensitizing effect on the dis-
appearance of DCPP.
Dichlorprop [(ꢀ)-2-(2,4-dichlorophenoxy)propanoic
acid; DCPP] is a selective systemic herbicide absorbed
by the leaves with translocation to the roots.1 Its
solubility in water is 350mg litreꢁ1 at 20°C.1 DCPP
may thus be present in surface waters and there be
exposed to sunlight irradiation. Its pKa was evaluated
as 3.00 at 20–25°C, so in natural waters it will mainly
be present in the anionic form. The R form is more
active than the S form, and the compound is used both
as a racemic mixture and the pure R-form (dichlor-
prop-P). It can be assumed that both forms have the
same photochemical behaviour. DCPP is an analogue
of 2,4-D but, in contrast with the latter, only a few
publications deal with its phototransformation. Ac-
cording to Climent and Miranda,2 six different photo-
products were identified by means of GC–MS and
GC–FTIR techniques in irradiated aqueous air-
saturated solutions: 2-chlorophenol, 4-chlorophenol,
2,4-dichlorophenol (main photoproduct), 2,4-di-
chlorophenyl acetate, the lactone of 2-(4-chloro-2-
hydroxyphenoxy)propanoic acid and 2-(2-chloro-
phenoxy)propanoic acid. Under argon, mono- and
dichlorophenol are formed together with 2-(2-chloro-
phenoxy)propanoic acid and a decarboxylation pro-
duct, 2,4-dichlorophenylethyl ether. The lactone
results from a thermal cyclisation of the corresponding
hydroxylated acid.
The phototransformation of DCPP may be com-
pared with the photodegradation of 2,4-dichloro-
phenoxyacetic acid (2,4-D), reported by Crosby and
Tutass.4 Solutions of 2,4-D were irradiated at 254nm
in artificial polychromatic light or in sunlight. The
main photoproduct resulting from UV irradiation was
2,4-dichlorophenol, but the formation of 2-chloro-4-
hydroxy- and 4-chloro-2-hydroxyacetic acids was also
reported. Over long irradiation times 2,4-D was found
to be converted into complex polymer-like humic
substances, after dechlorination and elimination of the
side chain. No pH effect was reported. The failure to
detect 2,4-dichlorophenol in sunlight irradiation of
2,4-D seems to be inconsistent with the wavelength
effect observed on the degradation behaviour of
MCPA [4-chloro-2-methylphenoxyacetic acid]5 and
mecoprop [2-(4-chloro-2-methylphenoxy)propanoic
acid].6 It was recently reported that 4-chloro-2-
methylphenol accumulates in higher amounts when
solutions of MCPA or mecoprop are exposed to
sunlight than when they are irradiated at 254nm or
in the range 290–350nm.
Romero et al3 studied the photochemical dis-
´
´
´
* Correspondence to: Pierre Boule, Laboratoire de Photochimie Moleculaire et Macromoleculaire, Universite Blaise Pascal (Clermont-
`
Ferrand), CNRS UMR 6505, F-63177 Aubiere cedex, France
E-mail: Pierre.Boule@univ-bpclermont.fr
(Received 8 January 2002; accepted 22 April 2002)
# 2002 Society of Chemical Industry. Pest Manag Sci 1526–498X/2002/$30.00
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L Meunier, E Gauvin, P Boule
The photocatalytic transformation of 2,4-D on
titanium dioxide or zinc oxide has been the subject
of several publications. Under these conditions com-
plete mineralization can be obtained. Several inter-
mediate photoproducts have been identified,
particularly 2,4-dichlorophenol,7–10 but also 4-chloro-
phenol,7 chlorohydroquinone,7,8 2,4-dichloro-6-
six low pressure mercury lamps (‘germicidal lamps’).
Duke GL 20W lamps emitting between 275 and
436nm, with a maximum at 313nm, were used for
irradiation of DCPP in the range 275–315nm.
Solutions were also irradiated in ‘black light’ with
lamps providing a main emission at 365nm. These
lamps (Mazda MAW 125w) emit about 85% of
photons at this wavelength, but they have a minor
emission on mercury lines located at 313, 334 and
405nm.11 Sunlight irradiations were carried out in
Clermont-Ferrand (latitude 46°N; altitude 420m) in
September, ie near the equinox.
hydroxyphenoxyacetic
acid,10
3,5-dichloropyro-
and
catechol,8,10 2,4-dichlorophenylformate10
4-chloropyrocatechol.8 With MCPA the major
product initially formed in the photocatalytic trans-
formation of MCPA is 4-chloro-2-methylphenol.5
The aims of the present work were:
2.4 Analyses
(1) to study the effect of substituents on the photo-
chemical behaviour of phenoxyacetic acids;
Irradiated solutions were analysed by HPLC with a
Waters 996 chromatograph equipped with a C18
column, 250mmꢃ4.6mm, a photodiode array detec-
tor and an autosampler controlled by a Millennium
chromatography manager. The separation of products
was difficult: in isocratic mode using aceto-
nitrileþwater (50þ50 by volume) DCPP and 2,4-
dichlorophenol were separated, but products of short
retention time were not, whereas in gradient mode
(20–90% acetonitrile) a good separation was obtained
for short retention time products, but not for 2,4-
dichlorophenol. In both cases water was acidified with
acetic acid (1.0ml litreꢁ1) to prevent dissociation of
ionisable products of pKa less than 8.
(2) to compare the photochemical behaviour of both
neutral and anionic forms of DCPP, since a
significant difference between the two forms was
observed in the cases of MCPA and mecoprop;
(3) to study the influence of the irradiation wave-
length on the orientation of the reaction.
2
MATERIALS AND METHODS
2.1 Chemicals
Chemicals were purchased as follows: dichlorprop,
99%, Riedel-de Hae¨n; 2,4-dichlorophenol, ꢂ95%,
HPLC, Fluka; 2-chlorophenol and 4-chlorophenol,
99%þ, Aldrich; hydroquinone, 99.5%, Merck;
p-benzoquinone, >99%, Merck; 4-chlorocatechol
(4-chloropyrocatechol), TCI.
Gas chromatography–mass spectrometry (GC–MS)
coupling experiments were carried out on Hewlett–
Packard 5985 and 5989 apparatus equipped with a
capillary column Machery–Nagel Optima 5 capillary
column, 25mꢃ0.25mm ID.
2,4-Dichlorophenyl acetate was prepared from
2,4-dichlorophenol and acetyl chloride according to
the following procedure. 2,4-Dichlorophenol (10mM)
was dissolved in diethyl ether (c 40ml). Triethylamine
(c 12mM) was added to the mixture, followed by slow
addition of acetyl chloride (10mM). A white pre-
cipitate was formed but disappeared on addition of
water. 2,4-Dichlorophenyl acetate was obtained by the
evaporation of ether phase. Yield c 90%.
UV spectra were recorded on a Cary 3 spectro-
photometer (Varian).
3
RESULTS
3.1 Spectrophotometric properties
Under natural conditions DCPP is present in the
anionic form. Its maximum UV absorption band is
located at 283nm, with a molar absorption coefficient
of 1890 (ꢀ50)Mꢁ1cmꢁ1 (Fig 1(a) and (b), full line).
The absorption overlaps with sunlight between 290
and 310nm, which is sufficient for a slow photolysis to
be observed in sunlight (see Section 3.5). The absorp-
tion is negligible at wavelengths longer than 315nm.
The neutral form, observed at pH 2.0 has a very
similar UV absorption band with a maximum at
283nm. The molar absorption coefficient (1650
(ꢀ50)Mꢁ1cmꢁ1) is just a little lower than that for the
anionic form.
2.2 Test solutions
Solutions were prepared in water purified in a Milli-Q
system to a resistivity of 518MOcm. In order to
compare the photochemical behaviour of both mol-
ecular and anionic forms, solutions were acidified at
pH 2 with hydrochloric acid or buffered at pH 7.2 with
phosphates. To control the influence of oxygen,
solutions were deoxygenated by bubbling with argon
for 20min.
2.3 Irradiation
The devices used to irradiate solutions at different
wavelengths have been described elsewhere.6 Mono-
chromatic irradiations in parallel beam at 283nm
(maximum of absorption of DCPP) were used for the
measurement of quantum yields. This wavelength was
obtained with a xenon lamp equipped with a mono-
chromator. Irradiations at 254nm were obtained with
3.2 Quantum yield of photolysis
The quantum yields were measured at 283nm in a
monochromatic parallel beam, the incident photon
flow being measured with chemical actinometry
(potassium ferrioxalate) and the transformation rate
with HPLC. The values obtained are shown in Table
1. The quantum yields for the two forms do not differ
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Photochemical behaviour of dichlorprop in aqueous solution
275–436nm (Fig 1(b)). The two irradiation devices
have slightly different photon flows; the lower trans-
formation rate at l>275nm is mainly due to the lower
absorption of the solution. In both cases a significant
increase of absorption was observed during the
experiment. The appearance of a new absorption band
near 260nm when the solution was irradiated at
l>275nm is consistent with the formation of quinonic
products. These products, which may play the role of
photoinducers, cannot accumulate when solutions are
irradiated at 254nm.
3.3.2 Analytical and kinetic studies
Under irradiation in the range 275–436nm, four
photoproducts, 4-chloropyrocatechol (Fig 2; 1),
2-chlorophenol (2), 4-chlorophenol (3) and 2,4-di-
chlorophenol (4), were identified by comparison of
their HPLC retention times and their UV spectra
obtained with photodiode array detector with those of
standards. On the HPLC chromatogram in gradient
mode given in Fig 3, product 4 was not separated, and
separation was obtained using the isocratic mode as
reported in Section 2.4.
The kinetics of formation of photoproducts 1 to 4
are given in Fig 4. The formation of 2, 3 and 4 was
previously reported by Climent and Miranda,2 but no
information was given about kinetics and mechanisms.
The formation of 4-chloropyrocatechol (1) from
DCPP has not previously been reported, but it was
reported by Crosby and Tutass4 in the photolysis of
2,4-dichlorophenoxyacetic acid (2,4-D). The kinetics
of the formation of 1, 2 and 3 are those expected of
primary photoproducts, whereas the formation of 4
was not observed during short irradiation times.
Products 5 and 6 were not identified. They do not
correspond to any of the reference compounds
injected in HPLC, hydroquinone, p-benzoquinone,
hydroxybenzoquinone (obtained in solution by
photolysis of p-benzoquinone) or 2,4-dichlorophenyl
acetate. They did not accumulate sufficiently to be
isolated. Their UV spectra obtained with photodiode
array detector had maxima at 264 and 275nm,
respectively. The latter is consistent with a phenolic
derivative. It might correspond to 2-(4-chloro-2-
hydroxyphenoxy)propanoic acid, as reported by
Climent and Miranda.2
Figure 1. Evolution of the UV–visible spectrum of an air-saturated solution
of dichlorprop (DCPP) 4.98ꢃ10ꢁ4 M in the anionic form (pH 7.2).
(a) irradiation at 254nm; (b) irradiation at l>275nm.
significantly, and only a small enhancing effect of
oxygen on the transformation rate was observed with
the neutral form, which can be explained by the
involvement of photooxidation reactions. Quantum
yields are significantly lower than with mecoprop.6
This effect of the introduction of a second chlorine
atom on the ring has previously been reported in the
case of chlorobenzenes and chlorophenols.12 It dis-
courages the generation of photoproducts.
3.3 Phototransformation of the anionic form
3.3.1 Spectrophotometric study
Another photoproduct, 7, was observed when an
HPLC chromatogram was recorded at 250nm. This
product has strong absorption bands at 258 and
365nm. Traces of hydroquinone and benzoquinone
were also detected, but they were not observed when
the solutions were irradiated at 254nm.
An air-saturated solution of DCPP (4.98ꢃ10ꢁ4M),
was irradiated at 254nm (Fig 1(a)) and in the range
The same photoproducts were obtained in solutions
irradiated in the absence of oxygen. Except for
compound 7, they were formed in significantly lower
amounts and 4 was not formed in deoxygenated
solutions irradiated at l>275nm. It can be concluded
that photoproducts 5 and 6 do not result from an
oxidation. The UV spectrum of 7 is consistent with a
quinonic derivative.
Table 1. Quantum yield of disappearance of DCPP
irradiated at 283nm (initial concentration 5.2ꢃ10ꢁ4 M)
pH (form)
Oxygenation
Quantum yield
2.0
Air-saturated
Deoxygenated
Air-saturated
Deoxygenated
0.020
0.015
0.015
0.015
(neutral)
7.2
(anionic)
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L Meunier, E Gauvin, P Boule
Figure 2. Structure of DCPP and photoproducts.
1, 4-chloropyrocatechol; 2, 2-chlorophenol;
3, 4-chlorophenol; 4, 2,4-dichlorophenol;
8, 2-(3,5-dichloro-2-hydroxyphenyl)propanoic acid.
3.4 Phototransformation of the neutral form
3.4.1 Spectrophotometric study
were irradiated at 254nm, since such compounds are
degraded at this wavelength.
The irradiation of the neutral form at pH 2.0 also
induced an increase in UV absorption. As was ob-
served with the anionic form, a new band near 260nm,
consistent with quinonic derivatives, appeared clearly
when solutions were irradiated at l>275nm. The
increase in absorption was less typical when solutions
3.4.2 Analytical and kinetic study
Photoproducts 1 to 4 also appeared on the HPLC
chromatogram of a solution (4.96ꢃ10ꢁ4M; pH 2.0)
irradiated at 254nm or at l>275nm, but the pro-
portions were different, as can be seen by comparing
Figs 4 and 5. Photoproducts were also analysed by
GC–MS after extraction with diethyl ether. Products
identified are shown in Table 2.
The product which has a retention time of 7.3min
results from the lactonisation of 2-(4-chloro-2-hy-
droxyphenoxy)propanoic acid and the product at
8.3min from the lactonisation of the product of re-
arrangement, 2-(3,5-dichloro-2-hydroxyphenyl)pro-
panoic acid (Fig 2; 8). The formation of this product
has not previously been reported in the case of
dichlorprop, but it is in good agreement with results
obtained with mecoprop irradiated in acidic solution.6
Products 6, 7, hydroquinone, benzoquinone and
hydroxybenzoquinone were also detected in solutions
irradiated in the range 275–436nm.
3.5 Irradiation in near UV light
A wavelength effect has been reported in the case of
the phototransformation of mecoprop, the photo-
chemical behaviour being significantly different when
solutions are irradiated at wavelengths shorter than
310nm and in near-UV light. For this reason,
Figure 3. HPLC chromatogram (gradient mode) of an air-saturated
solution of DCPP (4.98ꢃ10ꢁ4 M; pH 7.2) irradiated at l>275nm, up to
62% transformation. Detection at 280nm. The broad band between 17 and
20min results from the gradient. 1, 4-chloropyrocatechol;
2, 2-chlorophenol; 3, 4-chlorophenol; 5 and 6, unidentified photoproducts.
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