Phenoxyacetic acid pesticides

Phenoxyacetic acid pesticides, those based on the fungicide phe-noxyacetic acid (122-59-8). 

Chloroorganic and phosphoroorganic pesticides, and derivatives of phenoxyacetic acid... 

Applications of this technique are growing and include the analysis of mixtures of pesticides and herbicides, including oiganophosphorus, phenoxyacetic acid, caibamate, urea types, mixtures of various types of organic compounds, alkylbenzene sulphonates, polyethylene glycols, nonylphenyl ethoxylates, dioctadecylmethyl ammonium, ozonisation products and chlorination products. 

Devine et al. [188] adjust the water sample (1L) to pH2 with hydrochloric acid and extract it with benzene (100, 50 and 50mL). The extract is dried over sodium sulphate, concentrated to 0.1 mL and methylated by the addition of diazomethane in ethyl ether (lmL). After lOmin, the volume is reduced to about O.lmL, acetone is added and an aliquot is analysed by gas chromatography on one of three columns (1) 5% SE-30 on 60-80 mesh Chromosorb W at 175°C, (2) 2% QF-1 on 90-100 mesh Anakron ABS at 175°C or (3) 20% Carbowax 20m on 60-80 mesh Chromosorb W at 220°C In each instance nitrogen is the carrier gas and detection is by electron capture. The minimum detectable amount of pesticide in water was 2 parts per 109 for MCPA (4-chloro-2-methyl-phenoxyacetic acid) and 0.01-0.05 parts per 109 for 2,4-D (2,4-dichloro-... 

Larose and Chau [189] state that owing to the similar retention times of several common phenoxyacetic acid type herbicides the alkyl esters are subject to incorrect identification if several herbicides are present. Also, the sensitivity obtainable by means of electron capture detection of the alkyl esters by some herbicides, such as MCPA and MCPB is very poor and therefore the method is generally not suitable for the determination of these compounds in water. In addition, the methyl ester of MCPA has a very short retention time close to the solvent front and is prone to interference from sample coextractives, which usually appear in this region. In fact the MCPA methyl ester often cannot be detected even at higher levels because of overlapping with coextraction peaks when the same gas chromatographic parameters as for the determination of organochlorine pesticides are used. Hence other derivatives have been considered. 

Phenoxyacetic acid is the parent chromophore of a number of commonly used pesticides. If ring-halogenated, they can be regarded as derivatives of haloalkoxybenzenes. The simplest model molecules of this series are the haloanisoles. The photochemistry of 4-chloro- and 4-fluoroanisole has been studied previously, and it was shown that in aqueous solution photohydro-... 

As Table I illustrates, the chemical classes represented by the pesticides studied include thiophosphates [0,0-diethyl-o-p-nitrophenyl phos-phorothioate], carbamates [1-naphthyI-N-methylcarbamate], dinitrophe-nols [2,4-dinitro-o-sec-butylphenol and 2,4-dinitro-o-cyclohexylphenol], and chlorophenoxy acids [2,4-dichlorophenoxyacetic acid, 2,4,5-trichloro-phenoxyacetic acid, and 2-(2,4,5-trichlorophenoxy)propionic acid]. In addition, a number of molecularly related nitrophenols have been studied to establish the effects of molecular geometry and substituent groups on adsorption of pesticide-type materials. 

P-chloro-o-cresol is the precursor for the pesticides MCPA (2-methyl-4-chloro-phenoxyacetic acid), MCPP (2-methyl-4-chloro-phenoxy propionic acid) and MCPB (2-methyl-4-chloro-phenoxy butyric acid). MCPP is also known as mecoprop. 

In addition to the amount of pesticide present, the degradation rate could be affected by the availability of the chemical for degradation. Ogram et al. (30) have recently presented evidence suggesting that only the 2,4-D (2,4-dichloro phenoxyacetic acid) in soil solution, but not that adsorbed on soil colloids, could be degraded by soil microbes both in soil solution and sorbed on soil colloids. Other considerations should also be given to the nature and quantity of soil microbial biomass present in relation to nutrient availability (26,28,31) and the adaptability of microbes, either by natural selection or by genetic manipulation, to attack and utilize the pesticide chemical (32). 

The anions of concern to agriculture include Cl-, HCOJ, NOJ, SO, HPO -, H2PO4, OH-, and F-. In addition, some micronutrients (HjBOij-, MoO -, and HAsO -) and heavy metals (CrO -) exist as anions in soils, as do some pesticides, such as the dissociated phenoxyacetic acids (2,4,5-T and 2,4-D). Molecular species of interest include NH3, undissociated weak acids such as H3BO3 and H4Si04, and the undissociated forms of many pesticides (DDT, 2,4,5-T, ant 2,4-D.) The study of anionic and molecular retention by soils has been the subject of increasing research in recent years. 

Since then, materials containing Ti02 and carbon phases have been referred for the photocatalytic degradation of aqueous solutions of several model molecules followed, including phenol and phenol derivatives [13,15,17-20,22,26,27,31, 32,48-50,52,60,68,69,72,74,75,93,99,115,125,138,140,145,147,158,166,176,183, 184,187-189,193,194], other molecules of humic acids class [53], phenoxyacetic acids [85,86], insecticides or pesticides [16,33,34,147,150], and dyes, especially those used in the textile industry [28,29,42,63,68,76,81,82,85,105,122,126,137, 148,184,185], just to cite a few representative examples. 

Still another possibility involves the solvated electrons mentioned previously. Most, if not all, of the demonstrated photoreduction reactions of pesticides in water take place with substances likely to form hydrated electrons—e.g., NAA, fenac, the phenoxyacetic acids, and the chlorinated benzoic acids. Although they have measurable independent lifetimes, hydrated electrons eventually undergo reactions such as those shown in Equations 5, 6, 7, and 8 ( 34,56). 

Figure 5.9 Root concentration factor in barley roots for substituted phenoxyacetic acids as a function of pH. The lines represent predictions based on the ion-trap mechanism. O, 2,4-D ( ), 3,5-D. [Reproduced from G. G. Briggs, R. L. O. Rigitano, and R. H. Bromilow, Physico-chemical factors affecting uptake by roots and translocation of weak acids in barley Pesticid. Sci. 19, 101. Copyright 1987, Society of Chemical Industry. Reproduced with permission granted by John Wiley and Sons, Ltd on hehalf of the S.C.I.]...

So different types of pesticides, i.e., carbamates [47, 48], chloro-phenoxyacetic acids [49], phenyl- and sulfonylureas [50-54], halogenated triazines [46], as well as non-ionic surfactants [55], polycyclic aromatic hydrocarbons (PAH) [56-58] and polar pharmaceutical compounds [59] were determined. This technique first used a steel wire which later was substituted by a Kapton ribbon. However, it was soon replaced by the particle-beam interface, because the complex mechanical device led to considerable difficulties with the endless, continuously moving belt... 

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]. 

Since the 1930s almost all the pesticides which have been developed are organic compounds, with notable examples being DDT, the chlorinated phenoxyacetic acid and bipyridyl herbicides, and glyphosate. 

In 1939, the highly efficient contact insecticide DDT (an abbreviation of the technically incorrect name dichlorodiphenyl-trichloroethane) was introduced to the market. Its use, as well as the use of several other organochlorine compounds, spread worldwide in the following years. Important active substances that appeared on the market in the period after World War II include carbamate insecticides and herbicides. Also significant was the discovery of herbicidal phenoxyacetic acids, which represent the first group of the so-caUed hormonally active pesticides. Around the middle of the last century a number of other biologically active substances were discovered, many of which, such as herbicides based on substituted urea, s-triazines (1,3,5-triazines), quaternary ammonium salts or insecticidal synthetic pyrethroids and many others, are still used in many countries around the world. 

Pesticides are beUeved to be carcinogenic by multiple mechanisms including genotoxicity, tumor promotion, immunotoxicity, and hormonal action. This subject is addressed in some detail in an excellent review paper on pesticides and cancer [80], Exposures to individual pesticides have been associated with some cancers. For example, phenoxyacetic acids, of which 2,4-D is the most widely used and researched, and chlorinated phenols carry risks of developing soft tissue sarcoma and non-Hodgkin s lymphoma [82, 83], It should be noted, however, that the authors of... 

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