Urea herbicides monuron

TPNH) and oxygen to demethylate a number of closely related N,N-dimethylcarbamates (21). The same particulate system, however, exhibited a low order of activity on the urea herbicides monuron, diuron, and fenuron. Apparently, substitution of a nitrogen atom for the oxygen atom of the ester linkage to form the corresponding urea substantially decreased the velocity of the reaction. 

DIMETHYL-N -(4-CHLOROPHENYL)UREA 1,1-DIMETHYL-3-(p-CHLOROPHENYL)UREA HERBICIDES, MONURON KARMEX xMONURON HERBICIDE KARMEX W. MONURON HERBICIDE LIROBETAREX MONUREX MONURON MONUROX MONURUON MONUURON NCI-C02846 TELVAR TELVAR MONURON WEEDKILLER USAF P-8 USAF XR-41... 

The urea herbicides do not inhibit seed-germination to any extent, or the growth of nonphotosynthetic tissue, nor do they interfere with dark-fixation reactions. Monuron-treated bean-leaves which were fed 14C02 in the light, however, decreased in sucrose content (as percent of ethanol-soluble materials) from 70 to 2% in 72 hours.185 Most of the carbon-14 label... 

SE Katz. Determination of the substituted urea herbicides linuron, monuron, diuron, neburon and fenuron in surface waters. J Assoc Off Anal Chem 49 452-456, 1966. 

Sheets (5), in reviewing the disappearance of substituted urea herbicides from soil, found that inactivation occurs under soil conditions favorable for the growth of microorganisms but takes place slowly or not at all in dry or autoclaved soil. Burschel and Freed (6), reviewing work relating to 2,4-D and amitrole as well as monuron, state that the data indicate that ultimate breakdown is caused by microbiological attack. 

Introduced in 1951 by du Pont in the USA monuron (CMU, 6) was the first urea herbicide. Diuron (DCMU, 7) was introduced by the same company in 1956. 

The selectivity of these compounds can be attributed to several factors. Because of their acyl group, their mode of penetration into the plants differs substantially from that of the other urea herbicides. The acyl group is slowly split off in the soil, resulting in diuron and monuron, and they exert their action in these forms. Owing to their poor solubility in water and strong adsorption on the soil, they remain in the upper 10 cm of the soil, and are not toxic to deep-rooted crop plants. 

Urea herbicides, in addition to causing the degradation of photosynthetic tissues, inhibit root growth. Muzik et al. (1954), and Voderberg (1961) describe this action of CMU and monuron. 

Field tests with monuron, linuron and with diuron (Belasco and Pease, 1969 Maier-Bode, 1971) unequivocally showed that under natural conditions no azo compounds are formed in the soil in analytically detectable quantities neither could they be detected by gas chromatography (detectability 0.01 ppm) in cultured crops grown on soils treated with urea herbicides. 

In recent experiments mass spectrography has been used to identify 3-(2-hydroxy-4-chlorophenyl)-1,1-dimethylurea and 3-(2-hydroxy-4-chlorophenyl)-1 -methylurea in bean leaves as metabolites of monuron (Lee et al., 1973). This is, in the case of urea herbicides, experimental proof of the hydroxylating (oxidative) detoxication process, performed together with N-dealkylation by the microsomal oxidase enzyme system. 

Wallnofer et al. (1974) investigated the microbial degradation of some urea herbicides by Rhisopus japonicus and showed that this microorganism only demethylates monuron, fluometuron and monolinuron, while from buturon it splits off the l-methyl-2-propynyl group. 

The photolytic degradation of urea herbicides also proceeds in several steps. In addition to hydrolysis reactions, degradation products hydroxylated in the ring are formed in photonucleophilic reactions. In the case of monuron and linuron, hydrolysis is even preceded by the exchange of the chlorine at< ms of the ring for hydroxyl groups (Rosen et al., 1969 Crosby and Tang, 1969). 

FIGURE 25.11 GC with a fused-silica capillary column and a NPD detector of some urea herbicides after conversion to their methylated forms. Symbol explanation A, monuron B, isoproturon C, chlorotoluron D, linuron E, diuron F, methabenzthiazuron G, tebuthiuron. (From Scott, S., Analyst, 118, 1117, 1993. With permission.)... 

The first generation of urea herbicides is represented by types such as monuron and linuron (see Table 29.12). These compounds in general interfere with the photosynthetic process, as they inhibit electron transport in the photosystem II process of the plant. They have varying degrees of selectivity, depending upon the structure. Some owe their selectivity to physical properties such as water solubility and soil absorption. If the crop is a deep-rooted perennial, the shallow rooted annual weeds are controlled by using a herbicide that is not easily leached down into the root zone... 

Research had confirmed that no parent simazine residues were found in treated com plants, and additional data on the dissipation pathway of simazine needed to be developed. Research also indicated that triazines interfered with the photosynthetic process on susceptible growing weeds, as evidenced by the appearance of chlorotic leaves. Steps were undertaken to elucidate simazine s dissipation pathway and herbicidal mode of action. In Basel, Dr. Gast (1958) showed that the accumulation of starch by common coleus (Coleus blumei Benth.) plants was inhibited from treatment with 2-chloro-4,6-bis-(alkyl-amino)-triazines due to the inhibition of sugar synthesis. At the same time, Moreland et al. (1958) found weed control activity could be reduced by supplying carbohydrates to the plants through their leaves and that simazine was a strong inhibitor of the Hill reaction in photosynthesis. Exer (1958) found that triazines inhibited the Hill reaction as strongly as urea of the CMU (monuron) type. 

Triazine (e.g., atrazine, simazine) and substituted urea (e.g., diuron, monuron) herbicides bind to the plastoquinone (PQ)-binding site on the D1 protein in the PS II reaction center of the photosynthetic electron transport chain. This blocks the transfer of electrons from the electron donor, QA, to the mobile electron carrier, QB. The resultant inhibition of electron transport has two major consequences (i) a shortage of reduced nicotinamide adenine dinucleotide phosphate (NADP+), which is required for C02 fixation and (ii) the formation of oxygen radicals (H202, OH, etc.), which cause photooxidation of important molecules in the chloroplast (e.g., chlorophylls, unsaturated lipids, etc.). The latter is the major herbicidal consequence of the inhibition of photosynthetic electron transport. 

The determination of the vapor pressure of various compounds was reviewed by Wiedemann (49). He discussed the determination of vapor pressure by TG techniques based on the Knudsen effusion method. The sample holder that was employed is illustrated in Chapter 3 (Figure 3.6). For some measurements, a Pyrex glass cell having a diameter of about 15 mm was used. Four organic compounds were studied p-chlorophenyl-AT. Af-dimethyl urea (Monuron. a herbicide), p-phenacetin, anthracene, and benzoic acid, in the temperature range of 250-400 K. The vapor-pressure curves of these compounds, in the range from 0 10 Torr, are shown in Figure 4.49. The AHs values calculated were Monuron. 27.4 p-phenacetin 27.6 anthracene, 20.1 and benzoic acid. 20.7 kcalmole. 

Due to the fact that the arylutea type herbicides, such as diuron and monuron, did not inhibit the wild type bacterial reaction centers, the predictions have been based mainly upon mutations of the Qp-binding domain, which was affected by interaction with diuron. For example, the characterization of the herbicide-resistant mutants from Bps. viridis has revealed that one of the mutants, T4 (Tyr L222 to Phe) was sensitive to the urea type inhibitors similar to the D1 protein of PSII reaction centre. The semiquinone-iron electron paramagnetic resonance (EPR) signal of Qp in viridisTA mutants was also similar to that reported for photosystem II. 

The carbamate and urea derivatives used as pesticides are commonly prepared with phosgene. The substituted ureas are mainly used as herbicides, while carbamates are used as insecticides and acaricides. It is estimated that 93% of all carbamates and 92% of all urea-based herbicides on the U.S. market are commercially synthesized with phosgene [206]. Of the substituted ureas, Monuron and Diuron are the most important pesticides in volume terms, and of the carbamates. Car-baryl (Sevin) is the single most important chemical. Eptam (S-ethyl-N,N-dipropyl... 

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