Pyrethroids hydrocarbon type

Build-up of the hydrocarbon type of pyrethroid 415 ensues from a phenyl acetonitrile [901, 902]. The missing carbon is introduced by means of a methyl Gignard reagent (Scheme 273). 

In the indoor environment, many types of products such as crystals, sprays and liquids are applied for active and preventative protection of insects. Insect sprays are particularly popular because they are easy to handle and can be combined with air fresheners. Commonly, the amount of active agents in these products is well below 2%. For example, a commercially available insecticide for indoor use may contain 0.25% tetramethrin, 0.05% D-phenothrin and 1% of the synergist pipero-nyl butoxide. Pyrethroids are also used as active agents in liquid products against furniture beetle. In addition, materials containing natural fibers are often equipped with synthetic pyrethroids as a precaution. Apart from the active ingredients, volatile components such as acetone, aliphatic hydrocarbons, cycloalkanes, branched alkanes C3-benzenes and dipropylene glycol monomethyl ether are usually present in insecticides. 

Second-generation insecticides are of three major types chlorinated hydrocarbons, organophosphorus compounds, and carbamates. Synthetic pyrethroids are a recent fourth type. A very dramatic decline of the chlorinated hydrocarbons in the late 1960s and the 1970s, while the use of organophosphates and carbamates increased. 

Unlike the chlorinated hydrocarbons and pyrethroids, the organophosphate insecticides tend towards hydrophilicity, and the more hydrophilic types are taken up by plants from the soil. These systemic insecticides have made a great contribution to selectivity because they poison only those insects that bite the plant. The organophosphates were later joined by the similarly acting carbamate insecticides, less hydrophilic but also less persistent. They are described in Section 13.3. 

Ever-increasing and widespread resistance presents a gloomy outlook for the most-used insecticides today. Cross-resistance of insects, in many localities, to the chlorinated hydrocarbons (both kinds) and the organophosphates has highlighted early indications that the pyrethroids, too, will be trapped in this net of multiple cross-resistance. All of these apparently diverse types of insecticide release acetylcholine at an important phase of their action, and some part of the trouble may be due to that. That so much resistance has arisen in so few years is traceable to overapplication of the insecticide, neglect of helpful climatic factors, timing, and unselective destruction of useful insect predators (e.g. birds and certain kinds of mites). 

Wether the latest aftermath in pyrethroid research, the non-ester variations of the ether, hydrocarbon or the silane types are sufficiently different or more advantageous to capture market shares has to be shown in the future. 

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