Description
Pyridaphenthion is a pale yellow solid, mp 54.5–56 ?C,
vp 0.00147 mPa (20 ?C). Solubility in water is 100 mg/L
(20 ?C). It is very soluble in acetone, methanol, and diethyl
ether. Log Kow = 3.2.
Uses
Pyridaphenthion is used to control a wide range of chewing and
sucking insects and mites in rice, vegetables, fruit and ornamentals.
Definition
ChEBI: Pyridaphenthion is an organic thiophosphate, an organothiophosphate insecticide and a pyridazinone. It has a role as an EC 3.1.1.7 (acetylcholinesterase) inhibitor and an agrochemical. It is functionally related to a 6-hydroxy-2-phenylpyridazin-3-one.
Safety Profile
Poison by intraperitoneal route. Moderately toxic by ingestion and skin contact. When heated to decomposition it emits very toxic fumes of SOx, POx, and NOx. Used to control chewing and suckmg insects on rice, fruits, vegetables, and cereals.
Metabolic pathway
Pyridaphentluon is photolytically transformed into the oxon and phenylmaleic
hydrazide. In the mouse its metabolism involves oxidative
dearylation, oxidative desulfuration to the oxon, de-ethylation and conjugation
of phenylmaleic hydrazide produced as the ultimate product of
stage I metabolism and is typical of many phosphorothioates. A marked
difference between metabolism in mice and rats was shown, with rats
excreting no desethylpyridaphenthion and excreting all of the phenyl-maleic
hydrazide in an unconjugated form.
Metabolism
More than 70% of the dose
administered to rats and mice was excreted within 24 h
in the urine. The major metabolites excreted are phenylmaleic
hydrazide and desethyl pyridafenthion-oxon. The
half-life in soil is 11–24 d.
Toxicity evaluation
The acute
oral LD
50 for rats is 769–850 mg/kg. Inhalation LC50 (4 h)
for rats is >1.13 mg/L air.
Degradation
The photolysis of pyridaphenthion on a glass slide irradiated with both
short wavelength UV light and light of wavelength >300 nm was investigated
by Tsao et al. (1989). Samples were irradiated for 48 hours at
unrecorded light intensity as thin films spread on petri dishes or in solution
in water, methanol or hexane under an oxygen atmosphere. The
effects of photosensitisers was also assessed in the thin film experiments.
The photoproducts were analysed by TLC, GC-MS and
1H NMR
spectroscopy. Under thin film conditions pyridaphenthion was photodegraded
more quickly with light >300 nm than by 254 um light and was
degraded faster as a thin film than in solution. The main photoproducts
from the glass surface and aqueous solution were pyridaphenthion oxon
(2) and phenylmaleic hydrazide (3) (Scheme 1); the only other products
were small amounts of polymers. Photosensitisers accelerated photolysis
markedly with the most effective being a plant-derived natural product,
cis-dehydromatricaria ester. It is notable that a simple chemical oxidation
using 3-chloroperoxybenzoic acid gave the same two products, 2 and 3.
