Quinalphos

The Uses of Quinalphos

Quinalphos is used to control both chewing and sucking insects and mites in a large number of different crops.

Definition

ChEBI: Quinalphos is an organic thiophosphate and an organothiophosphate insecticide. It has a role as an EC 3.1.1.7 (acetylcholinesterase) inhibitor, an acaricide and an agrochemical. It is functionally related to a quinoxalin-2-ol.

Safety Profile

Poison by ingestion, inhalation, skin contact, parenteral, and intraperitoneal routes. Experimental reproductive effects. Mutation data reported. An insecticide. When heated to decomposition it emits very toxic fumes of NOx, POx, and SOx

Metabolic pathway

In methanol solution, quinalphos undergoes photodegradation reactions of isomerization, oxidation, and deesterification, together with other degradation reactions, resulting in several degradation products. However, the photolysis of quinalphos in ethanol solution yields the two products, O,O-diethyl-O-(3- ethoxyquinoxalin-2-yl)phosphorothioate and O,O- diethyl-O-[3(1-hydroxyethyl)-quinoxalin- 2yl]phosphorothioate, both of which are derived from the reaction of the photoexcited quinalphos molecule with the solvent.

Degradation

Quinalphos is susceptible to hydrolysis. The DT₅₀ values at pH 3,6 and 9 were 23,39 and 26 days, respectively (PM). Quinalphos is very susceptible to acid hydrolysis but stable under mildly basic conditions, having a halflife ten times that of parathion at pH 11 (Schmidt, 1972). Quinalphos was degraded in distilled water (pH 6.0), rain water (pH 6.8) and tap water (pH 7.8) with DT₅₀ values of 80,50 and 60 days, respectively (Dureja ef aI., 1988). The photolysis of quinalphos in ethanolic solution irradiated by a medium pressure mercury vapour lamp (λmax 366 nm) with a glass filter to screen out light of <290 nm gave photoproducts principally derived from reaction with the solvent. The main photolysis products were 3-ethoxyand 3-hydroxyethyl-quinoxaline derivatives of quinalphos (not shown in Scheme 1). A minor product of photodegradation was quinalphos oxon (2) (Pusino et al., 1989). When quinalphos was irradiated with a high pressure mercury vapour lamp (λmax 254 nm) in a quartz reactor many more photoproducts were detected, although whether this information is relevant to photodegradation in the environment is debatable since light of wavelength ﹤290 nm is generally absent at the Earth's surface. Using this method of irradiation, Dureja et al. (1988) examined the photolysis of quinalphos in hexane and methanol solution, as a thin film on a glass plate and on the soil surface. Analysis of the photoproducts was by MS and ¹H NMR spectroscopy after separation by TLC and GC. The rates of photodecomposition in aqueous solution and on soil surfaces exposed to sunlight were also measured, although in these experiments the nature of the photoproducts were not determined. In methanol solution, three major pathways of photolysis were apparent: (a) de-esterification to afford quinoxolin-2-o1(3), (b) thionethiolo rearangement to O,O-diethyl S-quinoxalin-2-yl phosphorothioate (4), which was not actually detected and was presumably de-esterified to give quinoxoline-2-thiol (5) which dimerised to the disulfide (6) and the sulfide (7), and (c) oxidation of the P=S sulfur to afford the oxon (2) as shown in Scheme 1. In addition, the de-esterification of the phosphorothioate moiety and additional alkyl transfer from quinalphos gave triethyl phosphate (S), O,O,O-triethyl phosphorothioate (9) and O,O,S-triethyl phosphorodithioate (10). A number of other photoproducts were identified which were formed via reaction of the methanol solvent with quinalphos (1) and quinoxolin-2-o1 (3) but because of their non-relevance to photoproducts produced under environmental conditions they are not included in Scheme 1.
Irradiation of quinalphos as a thin film on glass produced quinalphos oxon (2) and quinoxolin-2-o1(3) as the main photoproducts. The products from the soil irradiation experiments were identified as quinalphos oxon (2), the rearranged product O,O-diethyl S-quinoxalin-2-yl phosphorothioate (4), quinoxolin-2-o1(3), quinoxoline-2-thiol(5) and the sulfide (7). Routes for photodegradation of quinalphos in methanol solution, as a thin film and on soil surfaces are shown in Scheme 1. Half-lives for the photodecomposition of quinalphos by sunlight in water varied from 23 to 30 days and on soil surfaces from 2 to 5 days.