General Aspects

The organophosphorus (OP) insecticides are esters of phosphorus, mainly phosphates, phosphorothionates, phosphorothiolates, phosphorodithioates, phosphonates, and phosphoramidates.83 These compounds were developed in the late 1930s by Gerhard Schrader, a German chemist working for IG Farben. Some of the synthesised organophosphorus compounds were extremely toxic,

General formulae

Sarin (30)

Cyclosarin (31)

Tabun (32)

General formulae

Sarin (30)

Cyclosarin (31)

Tabun (32)

Figure 2.2 General structures for OP insecticides and main nerve gases.

and led to the development of nerve gases such as sarin 30, cyclosarin 31 and tabun 32.84-86 These compounds were classified as ''weapons of mass destruction'' by the Chemical Weapons Convention, which was signed in 1993 to ban the manufacture and use of chemical weapons.87 Almost all countries have now joined the Convention. The use of organophosphorus compounds as pesticides began in the 1930s and took off in the 1960s, when they were considered as an alternative to organochlorines.

Active organophosphorus insecticides have the general formulae shown in Figure 2.2, where R1 and R2 are short-chain alkoxy, alkyl or amino groups, mainly either methoxy or ethoxy groups, and Y is a displaceable group, including alkoxy, thioalkoxy, vinyloxy, aryloxy and heteroaryloxy groups.

Nowadays, among the various families of pesticides that are being used all over the world, organophosphorus pesticides are the most widely used group, accounting for more than half of the total world market.88,89 Over 100 000 OP compounds have been screened as insecticides, of which about 100 are marketed in estimated quantities of ca. 2 x 105 ty-1.90 As an example, in the USA market, the use of organophosphates, as a percent of total used insecticides, increased from 58% in 1980 to 70% in 2001.91 The main families of organophosphorus insecticides are depicted in Figures 2.3-2.5.

Organophosphorus insecticides are available with a wide range of properties. Some have a prolonged persistence, such as parathion 46 and azinphos-methyl 52, whereas malathion 42 has a short residual action.92 Compounds such as monocrotophos 35, malathion 42, parathion 46, and chlorpyrifos 49 have a broad spectrum of action. By contrast, trichlorfon 57 is a selective insecticide.93 Some can be used as plant systemic insecticides such as demeton-S 36 and dimethoate 38.94 Others, such as famphur 43 and phosmet 54, act as animal systemic insecticides.95,96

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