Herbicides, carbamate type

The term pesticide includes chemicals used to eradicate rodents (rodenticides), fungi (fungicides), weeds (herbicides) and insects (insecticides). These agrochemicals may be used directly on the medicinal plant crop itself, they may be used on crops growing adjacent to the herbs or they may occur as general environmental pollutants in soil, air or water. The presence of insecticide residues is of particular concern because those of the organochlorine type (DDT etc.) have been shown to cause cancers in animals and those of the organophosphate and carbamate types are potent cholinesterase inhibitors. 

This technique has been applied to the determination of chlorinated insecticides, carbamate insecticides and substituted urea type herbicides in soil and chloroaliphatic hydrocarbons in non-saline sediments. Separation is usually achieved on thin layers of silica gel or alumina. 

In many applications nowadays it is essential to link a mass spectrometer to the gas chromatography in order to achieve positive identification and sensitivity of analysis. Some 12 types of compounds are listed in Table 1.11(a) which are based on the application of this technique, viz. polyaromatic hydrocarbons, polychlorobenphenyls, dioxins, chloro, carbamate and triazine types of herbicides and pesticides, Diazinon, Dicamba, Imidazoline and Cyperquat herbicides and herbicide pesticide mixtures. 

Figure 20. A Structure of immobilized dihydroquinidine carbamate serving as chiral anion exchanger type CSP. B Chromatogram showing a direct resolution of the herbicide 2-(2,4-dichlorophenoxy)propanoic acid (Dichlorprop). Reprinted with permission from ref 140.

The next group of compounds that we want to look at are derivatives of carbamic acid (HO-CO-NH2), that is, the carbamates. Carbamates are widely used as herbicides and insecticides. The carbamate function exhibits both an ester and an amide-type functionality (Montgomery, 1997) ... 

A series of benzylnitramines were prepared by either nitration of the carbamates or N-alkylation of nitrourethane, followed by ammonolysis. These represent one-phenylnitramines, known broadleaf herbicides. Nastic responses and growth inhibition observed for this class suggested similarity to the auxin type herbicides. To determine if die preference for the (R) -configuration extended to nitramines, l-(2 6 -dichlorophenethyl)nitramine was resolved and the absolute configuration determined by asymmetric synthesis. A comparison of the (+) and (-) isomers in herbicidal and in vitro assays was performed and the results are discussed. 

Applied to beans (Phaseolus vulgaris L.) epicotyls the compounds do not affect the normal growth of the plant. In this they are similar to herbicides of the carbamate and urea types. They do not affect the germination of the seeds, their main effect being manifested on the growing young plants. After normal growth of... 

During the short time of their application in the early 1980s, DLI interfaces were often applied for substance-specific analysis [60] of various types of pesticides and herbicides (triazines, carbamates, organophosphorus compounds) [61-67], chlorinated phenoxyacetic acids, phenylureas, analides (alachlor, propachlor and aldi-carb) [63]. 

Methyl isocyanate, CHj—N=C=0, is used in the industrial synthesis of a type of pesticide and herbicide known as a carbamate. As a historical note, an industrial accident in Bhopal, India, in 1984 resulted in leakage of an unknown quantity of this chemical into the air. An estimated 200,000 people were exposed to its vapors, and over 2000 of these people died. 

Herbicides are the most widely used type of pesticides, and triazines, N-arylcarbamates, and chlorophenoxy acids are among the most important classes of herbicides. Herbicides are used to control unwanted grasses and weeds in a variety of crops, such as cotton, alfalfa, sunflower, sorghum, rice, and a variety of fruits and vegetables. The following experiment involves the analysis of atra-zine as an example of the triazines, 2,4-D for the chlorophenoxy acid herbicides, and chlorpropham for the carbamates. 

Many herbicides that are environmentally significant do not fall into the classifications described earlier. Nitroaniline herbicides are characterized by the presence of -NO2 and a substituted -NH2 group on a benzene ring as shown for trifiuralin in Figure 4.11. This class of herbicides is widely represented in agricultural applications and includes benefin (Balan ), oryzalin (Surflan ), and pendimethalin (Prowl ). Other types of herbicides include substituted ureas, carbamates, and thiocarbamates. 

A wide variety of chemicals have been used as herbicides, and have been potential water pollutants. One such compound is R-mecoprop. Other types of herbicides include substituted ureas, carbamates, and thiocarbamates ... 

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