Sulfonylureas soil degradation

The main soil degradation pathways of sulfonylurea herbicides are cleavage of the sulfonylurea bridge, O- and N-dealkylation reactions, aryl and aliphatic hydroxylation reactions, dehalogenation and ester hydrolysis. It is not within the scope of this chapter to discuss each of these in detail for all of the above-mentioned new sulfonylureas. Instead mesosulfuron-methyl is taken below as a general illustration of commonly found soil degradation pathways established within the sulfonylurea family. 

Conclusion. In this chapter, we have attempted a broad overview of the biochemical and environmental properties of the sulfonylurea herbicides. The history of these herbicides is distinguished by the apparently endless variation in structure leading to new weed control spectra, crop selectivities and soil degradation properties. They have already achieved significant use in j ctice, and it is likely that Dr. Levitt s discovery will continue to yield new herbicide tools offering viable solutions to the needs of world agriculture and of society. 

The use of methyl carbamates also provided a convenient means to prepare monoalkylamino heterocycles 34. These have been of interest because iticy impart more rapid soil degradation properties to the derived sulfonylureas, as discussed by Brown and Kearney in an earlier chapter of this work. 

Figure 7.11. Degradation in soil of sulfonylureas. Chlorsulfuron and DPX-M6316 contain a triazine ring.

Agrochemical Products. The principal thiophene derivative in herbicidal protection, one of a range of sulfonylurea herbicides, is Harmony [79277-27-3] (Du Pont) (60), based on the intermediate methyl 3-aminothiophene-2-carboxylate (9). The product is characterized by a rapid bio degradability in the soil. Many other thiophene derivatives have been shown to have agrochemical activity, but few of these have been developed to the commercial level. 

DineUi G., Di Martino E., and Vicari A. (1998) Influence of soil moisture and temperature on degradation of three sulfonylurea herbicides in soil. Agrochimica 42(1—2), 50-58. 

The last ten years have seen important developments in this respect in the three most important families of pesticides fungicides of the triazole group applied at about 100 g/ha, insecticides of the synthetic pyrethroid type at 20 g/ha and herbicides of the sulfonylurea type at 30 and even as little as 5 g/ha exert an effect which could be achieved with the pesticides of 15-20 years earlier only at rates of a few kilograms per hectare. These modern highly efficient preparations form only a small part of the selection of pesticides available today, but a rapid increase in their share of the total is to be expected as a result of purposeful research work. Another approach toward diminishing environmental contamination by chemicals is the development of new active substances which are less volatile, are degraded more rapidly or are more readily adsorbed by soil particles. 

Bossi, R., Vejrup, K., and Jacobsen, C. S., Determination of sulfonylurea degradation products in soil by liquid chromatography-ultraviolet detection followed by confirmatory liquid chromatography-tandem mass spectrometry, J. Chromatogr. A, 855, 575-582, 1999. 

Sulfonylurea herbicides (SUHs) are relatively new herbicides, introduced in the 1980s. Chlorsulfuron was the first sulfonylurea marketed in the United States, in 1982. World-wide, 19 sulfonylureas had been commercialized by 1994, and five more are being developed. This rapid increase is due to their very high and specific herbicidal activity, which results in extremely low application rates of 10 to 40 g/ha. Furthermore, as compared to other herbicides, sulfonylureas are less toxic and degrade more rapidly. Chemical structures of some representative sulfonylureas are presented in Figure 25.2. From a chemical point of view, these herbicides are labile and weakly acidic compounds. The common names, chemical formulas, water solubility, pKa, half-life in soil, and leaching potential through the soil (when available) of the most representative sulfonylureas are reported in Table 25.2. 

In the soil, there are two major pathways of sulfonylurea degradation [58] (a) chemical hydrolysis and (b) microbial degradation. The breakdown of sulfony-lureas in sterile soils is solely attributable to chemical hydrolysis, whereas breakdown in non-sterile soils is a combination of both microbial degradation and... 

Although studies on the degradation mechanism of SL-950 have just been started, judging from the fiu t that SL-950 has almost same dissociation constant (pKa 4.6) as SL-160, 81 950 is expected to follow a similar chemical degradation pathway. The short-residual properties of SL-160 have been described in the chapter of Haga et al. If 81 950 behaves similarly in soil, it will have a persistence profile that is very attractive for rotational cropping. Such properties may be considered important for future sulfonylurea herbicides. 

This chemical reactivity is a new mode of degradition, not seen in conventional sulfonylurea herbicides. It explains the short half-life (less than 2 weeks in soil) of SL-160, and provides a new strategy for overcoming the problem of carry-over related to sulfonylurea herbicides in some recropping situations, by means of quick chemical (not enzymatic) degradation. 

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