Photochemistry of phenylurea herbicides and their reactions in the environment

ECOTOXICOLOGY

AND

ENVIRONMENTAL

Photochemistry

SAFETY

5, 503-5

12 ( 198 1)

of Phenylurea Herbicides in the Environment D. K~TZIASAND

Gesellschaft

fiir

Strahlen-

and Their

F. KORTE

und Umweltforschung, Institut ftir D-8042 Neuherberg, West Germany Received

Reactions

October

iikologische

Chemie.

5. 1980

INTRODUCTION Substituted phenylurea herbicides are widely used in agriculture. The herbicidal function of these compounds is based on their effect on plant photosynthesis. They are exposed to sunlight in various forms after application (adsorbed on soil, on aerosol particles, or also in the vapor state) and thus subject to photochemical reactions. The photochemistry of the herbicides has been investigated by several research groups in the past. It was shown that in the environment substituted phenylurea herbicides are decomposed by the uv radiation of the sun. The knowledge of the mechanism of the photochemical conversion and the nature of the decomposition or conversion products is of great interest for the ecotoxicological evaluation of these herbicides. In this paper data on the photochemistry of substituted phenylurea herbicides are presented and several typical reactions upon irradiation in a solid phase and on carrier material are discussed. Irradiation of the substituted phenylurea herbicides under anaerobic conditions and in deuterated solvents contribute to the elucidation of the mechanism of decomposition and help in understanding the significance of these reactions in the environment. PHOTOREACTION

OF THE SUBSTITUTED HERBICIDES

PHENYLUREA

Several commercial products of the substituted phenylurea herbicides are given in Table 1. In the photochemistry of substituted phenylurea herbicides there are four types of photochemical reactions: (a) photolysis of the -C-X bond on aromatics (X = Cl, Br), (b) photoeliminations (Norrish-Type II reaction), (c) photooxidations, and (d) photorearrangements. PHOTOLYSIS

OF THE

-C-X

BOND

IN AROMATICS

Rosen and Strusz (1968) found photolytical elimination of aromatically bound halogens under the influence of sunlight for metobromuron and linuron in aqueous solutions. Subsequently an OH group enters the aromatic nucleous instead of the 503

0147-6513/81/040503-10$02.00/O Copyright 0 1981 by Academic Press. Inc. All rights of reproduction in any form reserved

504

KOTZIAS

AND

TABLE

KORTE

1

PESTICIDAL UREAS Trade name

Diuron

Chemical

name

N’-(3,4-Dichlorophcnyl).N.N-dimethylurea

N’-(CChlorophcnyl).N,N-dimethylurea

Linuron

N’-(3.4.Dichlorophenyl)-N-methoxy-N-methylurea

Monolinuron

N’-(4-Chlorphenyl).N-methoxy-N-methylurea

Metobromuron

N’-(4-Bromophenyl).N-methoxy-N-methylurea

C’

:

,OCH3

;-C--N,CH3

Buturon

FellWOn

N’-Phenyl-N,N-dimcthylurea

Neburon

N’-O,CDichlorophenyl)-N-methyl-N-butylurea

Tribunil (methabaxthiazuron)

N’-(2.Bcnzthiazolyl)-N’-methyl-N-methylurea

C-1983 (tenoran; chloroxuron)

N’-[4-14-Chlorophcnoxy)phcnyl]-N,N-dimethylurea

halogen. On the example of linuron a directing influence of the -N / OCH3 \ CH3 group was reported for this reaction. The photolysis of the chlorine atoms in the

para position is more favored than in the meta position, under the influence of the &OCH3 \ CH3 group. Irradiation of the herbicides buturon and monolinuron in aqueous and methanolic solutions as well as in the solid phase and on carrier material (Kotzias, 1974; Kotzias et al., 1974) with wavelength above 290 nm (Pyrex filter) in the laboratory showed that also under these conditions a photochemical elimination of the aromatically bound halogen occurs. Mazzocchi and Rao (1972) studied the photo-

PHENYLUREA

HERBICIDE

505

PHOTOCHEMISTRY

CI &;;I* h x.t ’ct x0\ , H \- , ,-c-N:;; X-Cl,Br

x3

“O\/H FIG.

1. Photolysis

of the -C-X

bond.

Formation

.1Hz0 -@-N+

of halogen

2

+

atoms

HX and substituted

phenyl

radicals.

chemistry of monuron after irradiation under anaerobic conditions in methanolic solution. After 24-hr irradiation at a wavelength of 254 nm, fenuron (dechloromonuron) could be isolated as the main product. In contrast to the experiments of Rosen et al. (1969), Crosby and Tang ( 1969) could not find a significant dehalogenation of the aromatic nucleus by irradiation of monuron in aqueous solutions with sunlight and with the uv light of a 40-W fluorescent lamp. The photolysis of the -C-X bond proceeds via a radical mechanism. Afterward, the aromatic C-ha1 bond is homolytically split up resulting in halogen atoms and substituted phenyl radicals (Fig. 1). The elimination of the halogen proceeds according to Henderson and Zweig (1967) via singlet states, as was shown on the example of the photoinduced cyclization of bromo- and chloro-substituted phenylnaphthaline. In contrast, the corresponding iodo derivative reacts via a triple state (Fig. 2). Plimmer and Hummer (1968) found for polyhalogenated aromatic compounds dissolved in methanol that photolysis of the halogen is dependent on its position in the aromatic ring and on the nature of other ring substituents. Thus o-chlorobenzoic acid reacts almost quantitatively to form benzoic acid whereas the yield is only 30% for p-chlorobenzoic acid and 5% for m-chlorobenzoic acid. The influence of other ring substituents on the photochemical dehalogenation decreasesfor ortho substituents in the sequence -COOH > -OCH3 > -CH,. For the para substituents it is -OCH3 & -COOH=CH3 and for the meta substituents -CHJ > -OCH3 9 -COOH. Upon irradiation in solutions, the photochemically produced radicals react predominantly with solvent molecules. Following the irradiation in the solid-phase products from the reaction of photo-excited molecules arise. The following intermolecular reactions with solvents, were observed: -Hydroxylation -Protonation -Phenylation

FIG. 2. Photoinduced

cyclization

of bromo-,

chloro-,

and iodo-substituted

phenylnaphthalene.

506

KOTZIAS

AND

KORTE

HYDROXYLATION After irradiation of buturon and monolinuron in aqueous and methanolic solutions (with the uv light of a Philips HPK 125-W mercury high-pressure lamp, Pyrex filter, X > 300 nm), for example, hydroxybuturon, hydroxymonolinuron, and p-hydroxyphenylmethylurea could be isolated as the main phenolic products in 15% yield (Kotzias, 1974; Kotzias et al., 1974). Phenolic derivatives also were identified after irradiation of linuron and monuron in aqueous solutions (Rosen et al., 1969). The hydroxylation of the aromatic nucleus in aqueous solutions is due to the reaction of the phenyl radicals with water molecules. In contrast, the formation of phenolic products after irradiation in methanolic solutions proceeds via the formation of peroxy compounds which arise through the reaction of the oxygen (dissolved in methanol) with the phenyl radicals (Fig. 3). A hydroxylation of the aromatic nucleus, which would be possible through the photochemically produced OH radicals from methanol does not take place. This is shown by the irradiation of monolinuron under Nz atmosphere in degassed methanolic solution. PROTONATION

AND

PHENYLATION

Protonation of the aromatics after halogen photolysis has been investigated predominantly in methanolic and cyclohexane solutions since both solvents are good proton donors. This is due to the fact that dissociation energy of the C-H bond of cyclohexane is 20 kcal lower than that of water. Table 2 shows the dissociation energy for several solvents frequently used in irradiation experiments. From this it can be seen that benzene is not a good H donor, contrary to primary alcohols and cyclohexane. Consequently protonation was observed only in small yields upon irradiation in benzene of halogen-substituted benzenes. The main reaction is phenylation of the aromatic nucleus with formation of a new C-C bond. The eliminated Cl and/or Br atoms react further in solution with solvent molecules under formation of HX acids. This leads to a change of the pH value of the solution during irradiation. PHOTOELIMINATION Upon irradiation of linuron, dehalogenation products, other at the N-l atom of urea. Thus above 290 nm in aqueous and urea as the main photoproduct.

HX

FIG. 3.

t

Reaction

metobromuron, monolinuron, and buturon, besides derivatives arise from the elimination of substituents monolinuron and buturon irradiated at wavelengths methanolic solutions result in p-chlorophenylmethyl

HO0

of the phenyl

radicals

with dissolved

oxygen.

PHENYLUREA

HERBICIDE

PHOTOCHEMISTRY

507

TABLE 2 DISSOCIATIONENERGYFOR

SEVERAL

SOLVENTS

kcal/mol HO-H CH,O-H C&O-H HOCH2-l-l CsH5-H Cycle-C,H,,-H ccl,-Cl

119 100 100 90

103 94 68

Because p-chlorophenylmethylurea was formed under aerobic as well as unaerobic conditions, a photooxidative decomposition of the methyl- or isobutinyl group of the monolinuron or buturon, which could have led to this product, is improbable. Furthermore, because a direct photolysis of the

bond following absorption of the uv radiation can be excluded under these conditions, the formation of these products must proceed according to another mechanism. A reaction mechanism for the formation of p-chlorophenylmethylurea has been proposed, according to which the formation of these products proceeds as a photoelimination reaction (Norrish-Type II reaction) via a cyclic transitional state (six ring) (Fig. 4) (Kotzias, 1974). The photoelimination was predominantly observed with ketones and aldehydes containing H atoms at the y position. The n - a* transition at the carbonyl group that is photochemically induced by the uv radiation evokes the photoelimination reaction, in which a y-hydrogen atom is removed from the methoxy- and/or isobutinyl group of the monolinuron or buturon (Fig. 4). The y-hydrogen shift and the subsequent fi splitting leads to product C. After the photoreaction, p-chlorophenylmethylurea arises through intramolecular proton migration from the Enolform C. Ruzo et al. ( 1974) also reported photoelimination as a possible degradation mechanism following the irradiation of the compounds I-(4-chlorophenyl)-3-(2,6dichlorobenzoyl)urea and I -(4-chiorophenyl)-3-(2,tSdiRuorobenzoyl)urea (Fig. 5) in methanol and benzene and in the solid phase. Figure 5 shows the reaction mechanism suggested by Ruzo et al. for the photochemical degradation of XA and Xg. They found that oxygen had no influence on the photoelimination reaction and thus conclude that the Norrish-Type II fragmentation of the two compounds XA and Xa does not proceed via triplet states. THE ROLE OF OXYGEN Crosby and Tang (1969) as well as Tanaka et al. (1977), have been able to isolate a seriesof oxidation products (Fig. 6) after irradiation of monuron in aqueous solutions with sunlight and with the uv light of a Hannovia 450 high-pressure lamp

508

KOTZIAS

)c=GT-, y-z.

FIG. 4. Photoelimination

reaction

AND

KORTE
tj

of a substituted

‘2

t-,x

S’

--+;c=o

phenylurea

(Norrish-Type

II reaction).

(Pyrex filter, X > 290 nm) (Fig. 6). The dimethylamino group of monuron is gradually and oxidatively degraded and p-chlorophenylurea arises as the end product. The irradiation of solid 2-14C-labeled IV’-4-(4-chlorophenoxy)phenyl-N,N-dimethylurea (chloroxuron) with uv light resulted in 90% loss in 13 hr. The demethylated products, IV’-4-(4-chlorophenoxy)phenyl-IV-methylurea and N’-4-(4-chlorophenoxy)phenylurea (Fig. 6), were identified in the reaction mixture. Of the original radioactive label, 64% was evolved as carbon dioxide (Geissbtihler et al., 1963). Sakriss et al. (1976) also isolated a series of oxidation products after irradiation of methabenzthiazuron (Table 1). Photooxidation products were also isolated and identified by Kotzias et al. (1974) after irradiation of buturon and monolinuron in solution and on carrier material. To study the influence of oxygen on the photochemical degradation of substituted phenylurea herbicides, irradiations in degassed methanolic solutions under N2 atmosphere were studied. The example of buturon and monolinuron (Kotzias, 1974;

xA= cl +,=F

‘A

-

-

hJ

Cl

FIG. 5. Photochemical degradation rophenyl)-3-(2,6-difluorobenzoyl)urea

-

+

of 1-(4-chlorophenyl)-3-(2,6-dichlorobenzoyl)urea (Ruzo et al., 1974).

and I-(4-chlo-

PHENYLUREA

HERBICIDE

509

PHOTOCHEMISTRY

Cl

Cl

FIG. 6. IPhotodecomposition

products

of monuron

in aqueous

solutions

(Crosby

and Tang,

1969).

Kotzias 290 nm) under anaerobic conditions than in the presence of oxygen. Photooxidation products were not identified in these experiments. The oxygen can intervene in a photochemical reaction in different ways. Since the oxygen molecule possesses two unpaired electrons in the ground state, is present in the triplet state, and is paramagnetic, it is able to quench triplet states due to its long life span. Furthsermore, oxygen can intervene in radical processes because of its biradical character. It induces the formation of peroxy compounds which are reduced to the corresponding phenols in the subsequent reaction (Fig. 3). Based on the fact that buturon and monolinuron are degraded faster upon irradiation under anaerobic conditions than in the presence of oxygen, it can be stated that in the 77- X* transition of the photoelimination, triplet states are involved and oxygen causes quenching. Therefore the photoelimination in the presence of oxygen is impeded in the sense that as a result of the photo induced n - T* transition in the carbonyl group, no hydrogen elimination in the cyclic transition state (Fig. 4) occurs. PHOTOREARRANGEMENTS The skeleton of the urea molecule stays intact, both by dehalogenation and photoelimination. The irradiation energy of wavelengths above 290 nm is not sufficient to break the stable amide bond. Mazzocchi and Rao (1972) examined the reaction behavior of monuron and fenuron upon irradiation under anaerobic conditions in methanolic solutions with uv light of 254 nm. They isolated and identified several photorearrangement products showing a splitting of the 0 -N-CH bond (Fig. 7). The formation of the photorearrangement products proceeds analogous to the photoanilide rearrangement. Under the influence of the high-energy

510

KOTZIAS

AND

KORTE

uv radiation, dechlorination of monuron and splitting of the -NH-CObond takes place, and the resulting radicals react further to form products 1 and 2 (Fig. 7). Since such photorearrangements only proceed under the influence of high-energy radiation (X < 290 nm) they are not of high relevance to the tropospheric environment. REACTIONS

ON CARRIERS

As a further step in the direction of simulating environmental conditions, Kotzias (1974) irradiated the two 14C-labeled substituted phenylurea herbicides, buturon and monolinuron, in concentrations of 1 ppm in sterilized, moist soil with wavelengths above 290 nm. The photochemical conversion under these conditions proceeded analogously to that in aqueous solution. As main products, p-chlorophenylmethylurea and hydroxybuturon were isolated from buturon and p-chlorophenylurea. p-Chlorophenylmethylurea and hydroxy monolinuron were formed from monolinuron. After 10 hr irradiation, about 5.4% of the buturon and 31.2% of the monolinuron had reacted. Under standardized test conditions for the comparative photomineralization of silica gel absorbed chemicals the rate for monolinuron is 38.3% (Kotzias et al., 1979). et al.

REACTIONS

IN DEUTERATED

SOLVENTS

To determine how far the photochemical reactions elucidated after irradiation of substances like buturon and monolinuron in an organic solvent could be of significance to the environment. The phenylurea herbicides were irradiated in tetradeuteromethanol, hexadeuterobenzene, and hexadeuteroacetone for 4-5 hr with uv light (h > 290 nm). After irradiation in methanol, p-chlorophenylmethylurea as well as dechlorobuturon and dechloromonolinuron could be isolated. p-Chlorophenylmethylurea contains no deuterium atom, therefore no reaction with solvent molecules took place. Dechlorobuturon and dechloromonolinuron each contained a deuterium atom. From this it

FIG. 7. Photodecomposition products of monuron upon irradiation under methanolic solutions with uv light of 254 nm (Mazzocchi and Rao, 1972).

anaerobic

conditions

in

PHENYLUREA

HERBICIDE

511

PHOTOCHEMISTRY

can be cloncluded that the reductive dechlorination of monolinuron and buturon proceeds under participation of the solvent. So the significance of this reaction for the environment is doubtful. In fact, dechloromonolinuron and dechlorobuturon could not be identified under outdoor conditions (Freitag, 1977; Haque et al., 1976, 1977). Also in the irradiation in hexadeuterobenzene and hexadeuteroacetone no deuterated pl-chlorophenylmethylurea was formed. However, among others, deuterodechloromonolinuron and deuterodechlorobuturon were found. The irradiations in deuterated solvents show that the formation of p-chlorophenylmethylurea proceeds via the process of photoelimination and not by an intermolecular reaction. INFLUENCE OF THE SOLVENT ON PHOTOCHEMICAL DEGRADATION The photochemical conversion of substituted phenylurea herbicides in solution strongly depends on the polarity of the solvent and its affinity to the reactant. About 66% of monolinuron is photochemically converted within 23 hr irradiation in aqueous solutions. In methanolic solutions, within the same time and under the same conditions it is converted to about 94%. The behavior of the herbicide buturon is similar to that of monolinuron. The photochemical degradation of buturon proceeds faster in pure methanol than in a methanol/water mixture as solvent. The photochemically induced T) - 7r* transition in the carbonyl group is hypsochromically shifted in polar solvents due to the easing of the H bonding with the solvent molecules, because the groundstate becomes stabilized. Thus the photochemical 7 - K* transition occurs at shorter wavelengths. COMPARISON OF ABIOTIC AND BIOTIC CONVERSION SUBSTITUTED PHENYLUREA HERBICIDES

OF

Several compounds which formed upon irradiation of substituted phenylurea herbicides were also isolated from plants and soil. Freitag (1977) as well as Haque et al. (1976, 1977) after application of monolinuron and buturon, respectively, on potatoes and wheat, isolated the derivatives hydroxymonolinuron, hydroxybuturon, p-chlorophenylmethylurea, and p-chlorophenylmethyl carbamate. These could be formed metabolically or by abiotic reactions. After application to rats, Ernst and Bohme (1965) found the compounds p-chlorophenylurea, 2-hydroxy-4-chlorophenylurea, and 2-hydroxy-4-chlorophenylmethylurea as monuron metabolites. Essentially, three processes take place during the biotic conversion of substituted phenylurea herbicides, namely dealkylation, hydroxylation to phenolic products, and the formation of anilines. SUMMARY

AND

CONCLUSIONS

The photochemical behavior of a number of substituted phenylurea herbicides envisaged above leads to the following conclusions: (a) Substituted phenylurea herbicides are degraded by uv and sun radiation; the photochemical degradation is influenced by oxygen. (b) The absorbed energy is sufficient to split the -C-X bond on the aromatic ring and to induce further photochemical processes. The photolysis of the -C-X

512

KOTZIAS

AND

KORTE

bond on the aromatic ring depends on other ring substituents as well as on the substituents at the N-l atom in the urea molecule. Upon irradiation in methanolic solutions, predominantly H abstraction occurs. In aqueous solutions phenolic products are mainly formed; in benzene solutions phenylation takes place under linkage of a new -C-C bond. (c) Irradiation in the pure solid phase (on glass) results in products which are formed by oxidation as well as by reaction of photoexcited phenylurea molecules. After the irradiation of several substituted phenylurea herbicides on carrier material (dry silica gel and soil), the reaction products of the irradiation in the solid phase can be isolated. In the presence of water phenolic products are also formed. (d) A relationship between the photochemical degradation and the chemical structure of the substituted phenylurea herbicides was substantiated by experiments under various conditions. This knowledge could be advantageously utilized in the development of new compounds in the class of substituted phenylurea herbicides. ACKNOWLEDGMENT We

are

indebted

to Mr.

R. Viswanathan

for

helpful

discussions

and

his interest

in the

work.

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D. G., AND

TANG,

C. S. ( 1969).

Food

ERNST, W., AND B~HME, C. (1965). FREITAG, D. (1977). Thesis, University

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L. (1963). Weed Rex 3, 277. KORTE, F. (1976). J. Environ.

HAQUE, A., WEISGERBER, I., KOTZIAS, HENDERSON, W. A., JR., AND ZWEIG, KOTZIAS, D. (1974). Thesis, University

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AND KORTE, F. (1974). Chemosphere 4, 161. LOTZ, F., NITZ, S., AND KORTE, F. (1979). Chemosphere

M. P. (1972). J. Agr. Food Chem. 20, 957. B. E. (1968). Photochemistry of Herbicides: Meeting, San Francisco. Amer. Chem. SOL, Washington, D.C. J. D., AND STRUSZ, R. (1968). J. Agr. Food Chem. 16, 568.

MAZZOCCHI, PLIMMER,

Nat.

KLEIN, KLEIN,

D., AND

G. (1967).

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R. G. (1977).

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