Phoxim

Description

Phoxim is a light yellow liquid, Solubility in water is 1.5 mg/L (20 ?C). It is readily soluble in most organic solvents and slightly in aliphatic hydrocarbons. Log Kow = 3.38. It is relatively slowly hydrolyzed in aqueous media; DT₅₀ (22 ?C) at pH 4, 7, and 9 is 26.7, 7.2, and 3.1 d, respectively.

Chemical properties

yellowish oily liquid, Specific gravity: 1.176 (20°C)

The Uses of Phoxim

Insecticide.

The Uses of Phoxim

Phoxim is used to control stored product insect pests in silos and barns. It is also used to control household insects such as ants and other public health pests. Agricultural targets include caterpillars and soil insects in maize, vegetables, cotton and potatoes.

Safety Profile

Poison by ingestion. An experimental teratogen. When heated to decomposition it emits very toxic fumes of CN-, NO,, PO,, and SO,. See also NITRILES.

Metabolic pathway

The primary metabolic step in the metabolism of phoxim in most biological systems is hydrolysis to the oxime, which may occur either oxidatively or via oxidative desulfuration followed by hydrolysis of the oxon which is much more hydrolytically labile. Stage II metabolism in plants involves the oxime group of the oxaminophenylacetonitrile moiety being reduced to an amine followed by its conjugation with malonic acid. Phoxim is one of a quite large group of insecticidal organophosphates which generates tetraethyl pyrophosphate and its thio analogues under the influence of light. These are compounds which are very active as acetylcholinesterase inhibitors and highly toxic to mammals.

Metabolism

Phoxim is oxidatively desulfurated to the oxon, which inhibits housefly AChE 270 times as quickly as bovine AChE. In mammals, the oxon is immediately hydrolyzed into diethyl hydrogen phosphate. The direct cleavage of the oxime ester bond of phoxim, the hydrolytic transformation of the nitrile group into carboxyl, and deethylation also contribute to the low mammalian toxicity. Elimination is very quick, 97% of the dose being excreted in the urine and feces in 24 h. Degradation in soils is also very rapid. By photochemical reactions, the thiooxime phosphate isomer, tetraethyl pyrophosphate, and its monothio analog are produced in small amounts on foliage.

Toxicity evaluation

The acute oral LD₅₀ for rats is >2000 mg/kg. Inhalation LC50 (4 h) for rats is >4.0 mg/L air. NOEL (2 yr) for rats is 15 mg/kg diet (0.75 mg/kg/d). ADI is 1 μg/kg b.w.

Degradation

Phoxim is relatively slowly hydrolysed in water. The half-lives at pH 4,7 and 9 were 26.7, 7.2 and 3.1 days, respectively (PM). Drager (1977) reported a half-life of 145 min in pH 11.5 50% aqueous propanol at 37 °C. It is gradually decomposed under UV irradiation (PM).
The photodecomposition of [³²P]phoximon cotton leaves and glass plates was investigated by Drager (1971) who characterised the more hydrophobic organic-soluble components by TLC, GLC and ¹H NMR and MS of the purified products. The cotton plants were irradiated with natural sunlight and mercury-vapour lamps at an average light intensity of 3500-4000 lux. On both glass plates and irradiated cotton leaves, phoxim was degraded relatively quickly to one major hydrophobic component, the concentration of which reached a maximum after about one day. This material was identified as the thiono-thiolo rearranged compound O,O-diethyl a-cyanobenzylideneamino-thiophosphorothioate (2). This transformation is an activation step as 2 is a highly active acetylcholinesterase inhibitor. No metabolite 2 was formed on cotton leaves placed in the dark so it can be considered to be a photochemical product rather than a plant metabolite. Additional minor metabolites were tetraethyl pyrophosphate (3) and O,O,O',O'-tetraethyl thiophosphorothioate (4) which formed to a greater extent on the glass plates rather than the cotton leaves.