Herbicide bipyridilium

Wong, R.C. and J.B. Stevens. 1986. Bipyridilium herbicide toxicity in vitro comparative study of the cytotoxicity of paraquat and diquat toward the pulmonary alveolar macrophage. Jour. Toxicol. Environ. Health 18 393-407. 

Mulugeta, M. and N. Megersa. 2004. Carrier-mediated extraction of bipyridilium herbicides across the hydrophobic liquid membrane. Talanta 64 101-108. 

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. 

Paraquat, which was registered for use in 1965, is the most used of the bipyridilium herbicides. With a toxicity rating of five, it is reputed to have been responsible for hundreds of human deaths. 11 Exposure to fatal or dangerous levels of paraquat can occur by all pathways, including inhalation of spray, skin contact, ingestion, and even suicidal hypodermic injections. Chronic health effects from long-term exposure are reputed to include pulmonary effects, skin cancer, and Parkinson s disease.12 Despite these possibilities and its widespread application, paraquat is used safely without ill effects when proper procedures are followed. 

Figure 15.9 The two major bipyridilium herbicides (cation forms).

Free Radical Mechanisms. The appearance of phytotoxic symptoms only in the light after treatment of plants with herbicides such as diuron and atrazine prompted some investigators to propose "light-activation1 hypotheses, the formation of toxic substances, or the formation of reactive free radicals. However, except for the strong documentation on the formation of free radicals by bipyridiliums, there is no direct evidence that toxic components are formed from an interaction between a herbicidal Hill inhibitor and light (2). ... 

Although the adsorption of this class of organics on soils and soil minerals has been studied in some detail, there are actually few examples of organic cations that are used as pesticides or occur as pollutants in soils. The best known examples are the two bipyridilium quaternary salts, diquat and paraquat, herbicides that are very soluble in water because of their ionic nature. The structure of the chloride salt of paraquat is diagrammed below ... 

Paraquat and diquat are both extremely effective non-selective contact herbicides which have been used globally for the last 40 to 50 years. Unforhinately, the very property which makes them so efficient at destroying weeds also lends itself to the destruction of animal tissues and both are extremely toxic to humans. Despite their rather different struchires (Figure 1), both are members of the bipyridilium (sometimes referred to as dipyridilium) family of compoimds. Moreover, their structural differences mean that their toxicity manifests itself in different ways even though the underlying mechanism is the same for both. 

Bipyridilium pesticides, those including or based on bipyridinium such as diquat or paraquat, both herbicides. 

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