Benzonitrile herbicides

Other examples of nonionic compounds (Fig. 10, Table 3) are the phenyl-amide herbicides (e. g., Diphenamid, moderately water soluble and nonvolatile), thiocarbamate, and carbothioate herbicides (e. g., Thiobencarb, low water solubility, high vapor pressure, relative mobility in soil systems) and benzonitrile herbicides (e.g., Dichlobenil, low water solubility, low vapor pressure, relative immobility in most soils) [151]. 

Benzonitriles. Although there are presently three benzonitrile herbicides used commonly, dichlobenil (Table XI) is the only one which is discussed here. Bromoxynil and ioxynil (Table IV) are hydroxybenzo-nitriles their hydroxy groups are ionizable, and they were discussed in Ionic Pesticides because of their acidic properties and low vapor pressures. 

Nitriles are cyanide-substituted carbo lic acids, which occur naturally and synthetically, and are of the general siructure, R-CN. The naturally occurring nitriles are found in higher plants [94-97], bone oils, insects [98], and microorganisms [99,100] the synthetic ones are used industrially in benzonitrile herbicides [101], as organic solvents and in the synthesis of polymers, plastics [102,103], synthetic fibers, resins, and dye stuffs. 

L. (2010) Hydrolysis of benzonitrile herbicides by soil actinobacteria and metabolite toxicity. Biodegradation, 21, 761-770. 

Holtze, M.S., Sorensen, S.R., Sorensen, J., and Aamand, J. (2008) Microbial degradation of the benzonitrile herbicides dichlobenil, bromoxynil and ioxynil in soil and subsurface environments - insights into degradation... 

Dichlobenil, another benzonitrile herbicide, had no effect on any of the algae studied by Cullimore (1975) except Hormidium barlowi and H. flaccidum. These were 50% inhibited by one to two ppm of the herbicide. 

Vesela A, Franc M, Pelantova H, Kubac D, Vejvoda V, ulc M, Bhalla T, Mackova M, Lovecka P, Jaml P, Demnerova K, Martinkova L. Hydrolysis of benzonitrile herbicides by soil actinobacteria and metabolite toxicity [J]. Biodegradation 2010, 21 (5) 761-770. 

MTD and MTD approaches will be discussed and compared using QSAR of insecticidal benzoylphenyl-ureas, DDT-type analogs and benzylchrysanthemates, herbicidal benzonitriles and nitrophenols, and plant-growth regulating phenoxypropionic acids. 

The phenolic derivatives indicated in Figure 8.1 are also bound to the same binding niche on PS II as the triazines (Oettmeier, 1992). However, they have a somewhat different inhibition pattern than the classical family of PS II herbicides (e.g., triazines and ureas) and, therefore, were regarded as a separate family with a somewhat different mode of action (Van Rensen et al., 1978 Trebst and Draber, 1986). It is now clear that they just orient somewhat differently in the same binding niche, as discussed below. Although the phenolics are photosynthesis inhibitors, dinoseb and the halogenated benzonitriles also inhibit respiration. 

Some good inhibitors of the Hill reaction, however, do not contain the carbonyl oxygen-nitrogen moiety. Examples are the dinitroanilines, diphenylethers, 2,4-dinitrophenols, halogenated benzonitriles, and pyridinols. Hence, the postulates proposed are not all inclusive. Three of these herbicides are phenols. Under physiological pH s, the molecules can be expected to be ionized, and it may be the ionized form of the molecule that binds to the receptor. 

The transformation of the herbicide bromoxynil (3,5-dibromo-4-hydroxy-benzonitrile) was studied upon direct photolysis in water and in the presence of added nitrite. The addition of nitrite reduces the transformation rate of the substrate, most likely due to competition for absorption of radiation, but induces the formation of nitro derivatives (3-bromo-4-hydroxy-5-nitrobenzonitrile, 4-hydroxy-3-nitrobenzonitrile) [140]. 

There are a number of organic acids that interfere with plant metabolism. Dicamba (3,6-dichloro-2-methoxybenzoic acid) and tricamba are herbicides which interfere with protein synthesis, while picloram (4-amino-3,5,6-trichloro-2-pyriidinecarboxylic acid) interacts with nucleic acid synthesis in susceptible plants. Dicamba is primarily eliminated in the urine due to its hydrophilic properties. Some benzonitrile derivatives are also employed as herbicides, including dichlobenil (2,6-dichlorobenzonitrile), bromoxynil (3,5-dibromo-4-hydroxybenzonitrile), and ioxynil. Linuron (N -(3,4-dichlorophenyl)-N-methoxy-N-methylurea) is a phenyl urea while trifluralin (a,a,a-trifhiro-2,6-dinitro-N,N-dipropyl-p-toluidine) is a dinitroaniline. The major concern with the substituted ureas is induction of hepatic microsomal enzymes which could modulate the disposition of simultaneously exposed chemicals metabolized by the same enzymes. Finally, one of the most common herbicides presently in use is glyphosphate, or Roundup , a phospho-... 

Koopman and Daams (1965) investigating the structure-activity relationship of substituted benzonitrile, were the first to report that compounds easily converted into nitriles may also be efficient herbicides. 

WEBER Interaction of Organic Pesticides Acetamide, Benzonitrile, and Ester Herbicides... 

Treatment with the herbicide dichlobenil (2.6-dichloro-benzonitrile) (17)- In. this case the hydroxycinnamic acid derivatives are converted into ligninlike compounds. With either treatment the concentration of hydroxycinnamic acids in the tissue was very low and the PAL level increased. Phenylalanine and cinnamic acid added together with the Mn or dichlobenil inhibited the increase in PAL level. 

The NHase/amidase in R. erythropolis A4, a strain used to hydrolyze a wide spectrum of nitriles [16], was recently applied to the biotransformation of benzonitrile analogs used as herbicides (Figure 11.4) and the products and parent compounds were compared in terms of their acute toxicides [17]. In other rhodococcal strains, the same compounds, apart from dichlobenil, can also be hydrolyzed in a direct pathway catalyzed by a nitrilase [18,19]. It was demonstrated that the hydrolysis of the nitriles cannot itself be considered a detoxification. The two-step transformation may be especially important in the natural degradation of these compounds because unlike nitrilases, NHases and amidases are often constitutive enzymes, and their producer strains form the typical constituents of soil microflora [17, 20]. 

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