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Fenpropathrin controls many species of mites and insects on pome fruit, citrus, vines, hops, vegetables, cotton, ornamentals and glasshouse crops such as tomatoes.

ChEBI: Fenpropathrin is a cyclopropanecarboxylate ester obtained by formal condensation between 2,2,3,3-tetramethylcyclopropanecarboxylic acid and cyano(3-phenoxyphenyl)methanol. It has a role as a pyrethroid ester insecticide, a pyrethroid ester acaricide and an agrochemical. It is an aromatic ether and a cyclopropanecarboxylate ester. It is functionally related to a 2,2,3,3-tetramethylcyclopropanecarboxylic acid.

Fenpropathrin possesses only one chiral centre (at benzylic carbon) and therefore presents a much simpler stereochemical picture than that seen with most of the other pyrethroids. Most metabolic work has been conducted with the RS racemate. Solution and surface photochemistry, and degradation in water, soils, plants and animals, have been reported. The fate of the 3-phenoxybenzyl portion of the molecule is very similar to that reported for cypermethrin and other analogues. Degradation is mainly by ester cleavage and hydroxylation.

A detailed study of the kinetics of the hydrolysis of [¹⁴C-cyclopropyl]- fenpropathrin and [¹⁴C-benzyl]fenpropathrins howed that ester bond cleavage predominated over cyan0 group hydration (Takahashi et al., 1985a). Hydrolysis in a series of buffers gave the following DT₅₀ values at 25 °C: pH 7, >2 years; pH 9, 8 days; pH 10, <1 day. A base-catalysed process operates above pH 7. Products detected were 2,2,3,3-tetramethylcyclopropanecarboxylic acid (TMCA, 2), 3PBAl(3) and the amide (4) (Scheme 1).
Fenpropathrin (labelled as above) was subject to slow photodegradation in sunlight under various conditions with the following initial halflives: distilled water, >6 weeks; humic acid solution, 6 weeks; river water, 2.7 weeks; sea water, 1.6 weeks; 2% aqueous acetone, 0.5 day. Half-lives on three soils ranged from 1 to 5 days and on mandarin orange leaves it was 6 days (Takahashi et al., 1985b). The major products were TMCA, the amide (4) and 2-(3-phenoxybenzyl)-2-(2,2,3,3-tetraethylcyclopropyl)- acetonitrile (5). The latter product appeared to be unique to aqueous photolysis. By far the major product found on soil surfaces was the amide (4) but this was also found under dark conditions and is mainly a thermal product.
Many other minor products were detected indicating the occurrence of (i) hydroxylation at a methyl group, (ii) oxidation to 3PBA (7), (iii) 0-dephenylation to afford 3-hydroxybenzoic acid (8) and (iv) loss of CN as CO₂,. After 14 days, about 50% of the applied radioactivity was bound to a high organic matter soil. This was photochemically-induced, as less than 2% was bound in the dark. More recent studies using a xenon lamp (Katagi, 1993) indicated that formation of the amide was most efficient under drier conditions. Increased moisture, particularly in soil containing acidic binding sites, favoured ester cleavage.
Degradation in organic solvents and in thin films afforded similar results (Dureja, 1989). The pathways of photodegradation of fenpropathrin are illustrated in Scheme 1.