Chloroacetamides and the Occurrence of Pesticides in Groundwater
In some regions where soybeans and corn are grown intensively, atrazine has yielded its status as the herbicide of choice to one of the chloroacetamides, which are derivatives of chloroacetic acid, ClCH2COOH, in which the —OH group is replaced by an amino group —NR^. The most prominent herbicides of this type are alachlor, metolachlor, and acetochlor. These three compounds differ only in minor variations in the complicated organic groups R; and R2 attached to the amino nitrogen. Alachlor is a carcinogen in animals, and metolachlor is suspected of being one as well. The EPA has proposed that the use of alachlor, metolachlor, atrazine, and simazine be carefully managed in areas where they are used intensively since they represent a significant risk to groundwater.
chloroacetic acid general structure of chloroacetamide herbicides
Generally, the concentrations of these herbicides in waterways that drain agricultural land peak in May and are nondetectable by the end of summer; however, all are somewhat toxic to fish. Alachlor and its degradation products have been detected in groundwater that lies under com fields. Metolachlor is known to degrade in the environment by the action of sunlight and of water. The chloroacetamides degrade by reaction with water since their amide unit undergoes hydrolysis, producing an amine and chloracetic acid:
R^N—C—CH2C1 + 11—OH-» RjR2N—H + HO—C—CH2C1
Atrazine and its metabolite and metolachlor were the agricultural herbicides most often detected in streams and shallow groundwater in both urban and agricultural areas according to an investigation by the U.S. Geologic Survey in the 1990s. Domestic herbicides found most often were the triazines simazine and prometon. Insecticides found in highest concentrations— principally carbaryl and the organophosphates diazinon, malathion, and chlorpyrifos—were higher in urban than rural regions, presumably because of domestic usage. More than 95% of the streams, and 50% of the groundwater samples, were found to contain at least one pesticide at detectable levels. Research in Switzerland has found levels of atrazine, alachlor, and other agricultural pesticides in rainwater that exceed drinking water standards. Presumably the pesticides evaporated from farm fields.
Glyphosate is an example of a phosphonate, a class of compounds that are structurally similar to organophosphates except that one oxygen of the four that surround phosphorus is missing and is replaced by an organic group, in this case a methylene group, —CH2—, attached to the simple amino acid glycine.
Glyphosate is widely used as a herbicide, e.g., as the commercial product Roundup. It is rather nontoxic: its LD50 values are high for both oral and dermal routes of exposure, although acute ingestion of or exposure to large quantities of it is fatal. Dermal and oral absorption of it is small, and it is eliminated essentially unmetabolized. Glyphosate is nonresidual, and there is no evidence that it bioaccumulates in animal tissue or is carcinogenic or teratogenic. The same is true of its initial breakdown product, the substance corresponding to cleavage of the rightmost NB—CH2 bond in the structure above.
Glyphosate operates by inhibiting the synthesis of amino acids containing the aromatic benzene ring, which in turn prevents protein synthesis from occurring. Although it kills almost all plants, some strains of soybeans have been genetically altered using biotechnology so that they are resistant to glyphosate; consequently, it can be used as a weed killer in the growth of the crop (see Box 10-2). Its advantages in growing soybeans are that it replaces several different herbicides and that only one application is required, though the total volume of herbicide used is not reduced substantially. Its greater tendency to stay adsorbed on soil means that it has a lesser tendency to occur in runoff and subsequently in water supplies than do the herbicides atrazine and alachlor, which it replaces. The evidence gathered so far indicates that glyphosate is a relatively benign herbicide.
Phenoxy weedkillers were introduced at the end of World War 11. Environmentally, the by-products contained in commercial products of such herbicides are often of greater concern than the herbicides themselves, as we shall see in Chapter 11. For that reason, we begin by discussing the chemistry of phenol, the fundamental component of these compounds.
Phenols are mildly acidic; in the presence of concentrated solutions of a strong base like NaOH, the hydrogen of the OH group is lost as H ' (as occurs
Genetically Engineered Plants
In 1940 the world population was 2.3 billion people; by 1985 it had more than doubled, and it now exceeds 6.5 billion. Fortunately, beginning in the 1940s, a "green revolution" in agriculture took place that allowed the world to feed this burgeoning population. Extensive development and use of pesticides (many of which have been mentioned in this chapter) and fertilizers, along with irrigation and plant breeding programs, led to dramatic increases in the yield per acre of crops. Total worldwide cram production increased from 600 million metric tons in 1950 to more than 1600 million metric tons by 1985. Since 1995, production has leveled off at 1800-2000 metric tons. However, the human population continues to grow and is expected to reach 9 billion by 2050.
Since the 1980s, talk of a second green revolution has centered about genetically engineered plants. Traditional crossbreeding of wheat plants over many years has resulted in plants that yield two to three times more grain than previously existing varieties and are more resistant to pests and diseases. Genetic engineering of plants offers the possibility of doing these same things and additional feats in much less time and with more selectivity than traditional crossbreeding.
Genetic engineering involves taking a portion of the DNA from one species and inserting it into the DNA of another, unlike species. One striking example of this technique has been to take the human DNA (gene) that codes for the synthesis of the protein insulin and insert it into that of bacteria, thus allowing the bacteria to produce insulin. This results in the production of human insulin to be used for medical purposes.
Transgenic plants have been produced which have enhanced resistance to herbicides, drought, pests, salinity, and frost, as well as improved taste and nutritional value. The best-known examples of herbicide-resistant plants that have been developed are known as Roundup Ready. Roundup, as was previously mentioned, is a commonly used broad-spectrum herbicide. Monsanto, its manufacturer, has developed and patented genetically altered seeds for soy, corn, alfalfa, sorghum, canola, and cotton which grow into plants that are resistant to destruction by Roundup. Fields planted with these crops can be sprayed indiscriminately to destroy weeds, with little concern for destruction of the crop.
The use of transgenic plants has been widely adopted in the United States. In 2005, 87% of all soybean acreage in the United States was planted with transgenic crops, followed by cotton at 79% and com at 52%. The top five countries in growing transgenic crops in 2005 were the United States, Argentina, Brazil, China, and Canada.
Although transgenic plants offer the possibility of improving upon what nature has provided us, there are significant concerns about these organisms, especially in Europe. Concerns include:
• the use of greater quantities of herbicides, since there is less fear of destroying a crop from the indiscriminate application of the herbicide;
• the spread of herbicide resistance to related plants that become "super weeds"; and
• the decrease in genetic diversity of crops as farmers all use the same seeds.
In addition to these concerns, genetically engineered grains have not resulted in substantial increases in crop yields.
with any common acid) and the phenoxide anion, C6H50 , is produced in the form of its sodium salt:
The 0~Na+ group is a reactive one, and this property can be exploited in order to prepare molecules containing the C—O—C linkage. Thus if an R—CI molecule is heated together with a salt containing the phenoxide ion, NaCl is eliminated and the phenoxy oxygen links the benzene ring to the R group:
C6H50~Na+ + CI—R-»QHj—O—R + NaCl
Such a reaction is the most direct commercial route to the large-scale preparation of the herbicide, introduced in 1944, whose well-known commercial name is 2,4,S-T. Here (in the reaction immediately above) the R group is acetic acid, CH3COOH, minus one of its methyl group hydrogens, so that R = —CH2COOH, and the CI—R reactant is CI—CH2COOH. Then, according to the reaction, C6H5—O—CH2COOH, called phenoxyacetic acid, is obtained as an intermediate in the production of the actual herbicides.
In the commercial herbicides, some of the five remaining hydrogen atoms of the benzene ring in phenoxyacetic acid are replaced by chlorine atoms.
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