Pesticides compounds

An extensive pesticide properties database was compiled, which includes six physical properties, ie, solubiUty, half-life, soil sorption, vapor pressure, acid pR and base pR for about 240 compounds (4). Because not all of the properties have been measured for all pesticides, some values had to be estimated. By early 1995, the Agricultural Research Service (ARS) had developed a computerized pesticide property database containing 17 physical properties for 330 pesticide compounds. The primary user of these data has been the USDA s Natural Resources Conservation Service (formerly the Soil Conservation Service) for leaching models to advise farmers on any combination of soil and pesticide properties that could potentially lead to substantial groundwater contamination. 

Polychlorinated Pesticides. A once substantial but now diminished use for DCPD is in the preparation of chlorinated derivatives for further use or synthesis into pesticide compounds (see Insectcontrol technology). Soil permanence and solubiUty of the products in human fatty tissues have considerably restricted the use of these compounds. The more prominent chlorinated pesticides were aldrin, dieldrin, chlordane, and heptachlor, all of which use hexachorocyclopentadiene as a starting material. Aldrin and dieldrin are no longer used in the U.S. Chlordane and heptachlor are stiU produced, but only for export use. 

To analyze pesticides from the sample, several GC techniques were used GC with FID and EC detectors and GC/MS with external standards. Pesticides are mostly analyzed using split/splitless technique where higher amount of injected solution exits the gas chromatograph without decomposing therefore by quantification of the several pesticides in the filter, we found out how harmful is exposing analysts to pesticide compounds during the GC analysis. 

The first of these environmentally-important parameters can be expressed as a partition coefficient. In aqueous solution many, but not all pesticide compounds exhibit strong affinity for soil organic matter or concentrate in the lipid phase of soil organisms. Some, notably the cationic group, also exhibit marked affinity for clay or other mineral surfaces. An overall partition (or distribution) coefficient (kD) can be defined ... 

The concentration of many pesticide compounds in soils is substantially reduced by degradation processes before they can be leached. Half-lives are normally quoted for each compound in a fertile clayey-loam soil and are normally less than one year and, nowadays, in many cases less than one month. Degradation may be by chemical hydrolysis in the case of some compounds and by bacteriological oxidation in the case of many others. However, certain compounds are either relatively resistant to such degradation or the derivatives of partial hydrolysis/oxidation may be equally toxic as the original compounds. 

The sorption and degradation characteristics listed for most pesticide compounds in terms of partition coefficients and half-lives relate only to a (standard) fertile, organic clayey soil and must not to be taken as representative of the permeable sandy soils widely developed on aquifer outcrops. Thus leaching of 1% of original application rates, and perhaps significantly higher, could easily occur for certain compounds on permeable soils. 

Several hundred-pesticide compounds of diverse chemical structures are widely used in the United States and Europe for agricultural and non-agricultural purposes (Fig. 10). Some are substitutes for organochlorines, which were banned due to their toxicity, persistence, and bioaccumulation in environmental matrices. According to a report published by the US-EPA, a total of 500,000 tons of pesticides was used in 1985 [144, 145, 148]. As far as specific pesticides are concerned, worldwide consumption of Malathion and Atrazine in 1980 amounted to 24,000 and 90,000 tons, respectively [149,150]. In the Mediterranean countries, 2100 tons of Malathion (active ingredient) were sprayed during the same period compared to 9700 tons in Asia [150]. 

After a pesticide is manufactured in its relatively pure form (the technical grade material) the next step is formulation - processing a pesticide compound into liquids, granules, dusts, and powders to improve its properties of storage, handling, application, effectiveness, or safety [9]. The technical grade material may be formulated by its manufacturer or sold to a formulator/ packager. 

Molecular size. The physical size of complex molecules often limits the approach of enzymes and reduces the rate at which organisms can break down the compound. Many pesticide compounds and their isomers are of large and complex structure, making them resistant to degradation. Examples are some carbamates and carboxylic acid-based compounds. 

Molecular structure. In general, aliphatic (straight and cyclic) compounds are more degradable than aromatic compounds. Thus some pesticide compounds and parts of some molecules can be degraded easily while other parts cannot. In some cases, partial degradation will occur, but the pesticidal activity of the waste stream may not be reduced significantly if the toxic components of the compound are bioresistant. 

Substitutions. The substitution of elements other than carbon in the molecular chain often make the compound more resistant. Esters and epoxides, salts, and so on are more resistant than the base pesticidal compound. 

Inherent in this book is the promise of a methodology that will make rational design of safe and target-specific pesticide compounds a reality. We look forward to seeing whether and how rapidly that promise is fulfilled. 

The activity spectrum of a pesticidal compound is often determined by the physical properties of the compound. For example, systemic insecticides usually require both water and lipid solubility since it is necessary for them to pass through... 

Relatively few rearrangements are noted in the spectra of these tin pesticide compounds which fragment in a simple manner. 

R.L. Frost, R.W. Parker, J.V. Hanna, Detection of the pesticide compound-1080 (sodium monofluoroacetate) using F-19 nuclear magnetic-resonance spectroscopy. Analyst 114 (1989) 1245-1248. 

Environmental. The high lipophilicity of the cydodienes and the prolonged persistence of dieldrin and heptachlor epoxide (soil half-lives 2—10 yr) have resulted in severe environmental contamination. These compounds are bioaccumulated from water to fish up to 100,000- to 300,000-fold and are ubiquitous in human fat and milk. Oxychlordane [26880-48-8], mirex, and chlordecone are also bioaccumulative. The cydodienes are extremely toxic to fish with LC5Qs (ppm) to trout and bluegill of endrin, 0.001-0.002 endosulfan, 0.001-0.003 diddrin, 0.003-0.015 aldrin, 0.006-0.01 heptachlor, 0.03-0.026 and chlordane, 0.022—0.095. The LD5Qs to pheasant and mallard are aldrin 16.8 and 520, dieldrin 79 and 381, and endrin 1.6 and 5.6 mg/kg. As indicated by their rat oral LD - s, they are also extremdy toxic to small mammals in fact, endrin has been used as a rodenticide (see Pesticides). Compounds, eg, aldrin and heptachlor, which have unsubstituted double bonds, readily add oxygen to form epoxides in plant and animal tissues and are preferentially concentrated and stored in animal fats. Aldrin epoxide (dieldrin) and heptachlor epoxide are more stable (half-lives on alfalfa of seven to eight days) than aldrin and heptachlor (half-lives on alfalfa of less than one day). 

Typically, only 0.01-10% of the mass of pesticide compounds applied to fields is detected in streams [91]. The remaining 90-99% of pesticides adsorb to soil, percolate into groundwater, or volatilize [79]. The major degradates of the most heavily used herbicides found in surface water have not been studied widely. Many chemical properties of pesticides affect the amounts transported to streams. In general, acetanilide herbicides are more soluble in water, and thus more mobile than are the triazines [92], The solubilities of sulfonated degradates of acetanilides (ethane sulfonic acid, or ESA), can be 10 times the solubility of the parent compound [93]. The greater mobilities of the degradates of the acetanilides (amide family) can explain these com-... 

Fig. 10.4. Correlation of pK of some pesticide compounds with COSMO-RS dissociation energies [133]. Confidential parts of the molecules are hidden. The three yellow symbols denote three carboxylic acids that were treated for comparison. All other methods failed on this data set.

Repeated Dose Dermal Tests. Twenty-one to 28-day dermal tests are particularly important when the expected route of human exposure is by contact with the skin, as is the case with many industrial chemicals or pesticides. Compounds to be tested are usually applied daily to clipped areas on the back of the animal, either undiluted or in a suitable vehicle. In the latter case, if a vehicle is used, it is also applied to the controls. Selection of a suitable solvent is difficult because many affect the skin, causing either drying or irritation, whereas others may markedly affect the rate of penetration of the test chemical. Com oil, methanol, or carboxymethyl cellulose are preferred to dimethyl sulfoxide (DMSO) or acetone. It should also be considered that some of the test chemical may be ingested as a result of grooming by the animal, although this can be controlled to some extent by use of restraining collars and/or wrapping. 

As shown by the structures in Figure 15.9, a bipyridilium compound contains two pyridine rings per molecule. The two important pesticidal compounds of this type are the herbicides diquat and paraquat other members of this class of herbicides include chlormequat, morfamquat, and difenzoquat. Applied directly to plant tissue, these compounds rapidly destroy plant cells and give the plant a frostbitten appearance. However, they bind tenaciously to soil, especially the clay mineral fraction, which results in rapid loss of herbicidal activity so that sprayed fields can be planted within a day or two of herbicide application. 

The agency also considered the toxic effects that result from chronic exposure to chemical substances. The results of chronic oral feeding studies of 2-years duration on 220 compounds have shown that only five of the 220 chemicals exhibited toxic effects below 1 mg/kg. All five of the chemicals that were toxic at levels below 1 mg/kg, on a dietary basis, were pesticides, compounds that would, based on their pesticidal activity, be expected to be more toxic than most substances (Frawley, 1967). However, even among these 5 pesticides, none exhibited toxic effects at dietary concentrations below 0.1 mg/kg. 

Fig. 16.5 Pesticide compounds in a well in Florida. (After Lewallen, 1971.) Contamination was caused by pesticide-contaminated soil in the backfill and in sediment washed into the well. Cleaning of the well and repair of the casing removed the contamination and the well recovered.

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