🧯 Organophosphate Insecticides II
Additional organophosphate insecticides with emphasis on characteristics, toxicology, and safe handling.
This lesson introduces major organophosphate insecticides and summarizes their key chemical and practical characteristics.
PHORATE, PHOSALONE, DIMETHOATE AND QUINALPHOS
a) Phorate (Thimet)
(0, 0-diethyl-S-2-ethylthiomethyl phosphoro dithioate)
Derivative of Dithiophosphoric acid.
Phorate is produced by reacting dimethyl dithiophosphoric acid with
formaldehyde and ethyl mercaptan at room temperature.
The second method is by condensation of chloromethyl sulphide with sodium
dithiophosphoric acid.
Phorate is a clear liquid. B.P. 100°C, highly soluble in most organic solvents. It
is unstable to hydrolysis. In acid medium it is more stable. It is easily oxidized to the
corresponding sulphoxide which is resistant to hydrolysis and hence persists on plants
for long time providing insecticidal action.
Phorate has both systemic and contact insecticidal action and is a very toxic
compound. Phorate is absorbed and translocated in plants. It is oxidatively
metabolized. Phorate is employed for control of aphids, carrot fly, fruit fly and wire worm
in potatoes.
b) Phosalone (Zolone)
(0, 0-diethy-S-(6-chloro-2, 3-dihydro-2 oxobenzoxazol-3-yl) methyl
phosphorodithioate).
Derivative of Dithiophosphoric acid
Phosalone is a systemic insecticide and acaricide used in citrus and orchard
fruits. It is also used to control aphids in cereals, oilseed, rape and brassicas. It is also
used for testing seeds to protect the seedling from insect damage. LD50: 135 mg/kg.
Phosalone is produced by condensation of sodium or ammonium
diethyldithiophosphoate with 6-chloro-3 chloromethyl benzoxazolone.
It is practically insoluble in water. It is a white crystalline substance, m.p. 45
17°C. It is relatively stable in acid medium but in alkaline medium it is rapidly hydrolyzed
to the 6-chloro-benzenehexazolone diethyl thiophosphoric acid and formaldehyde.
c) Dimethoate (Rogor, phosphamide, cygon)
(0, 0-dimethyl-S-methyl carbamoylmethyl phosphoro dithioate).
Derivative of Dithiophosphoric acid
Dimethoate is a systemic and contact insecticides and acaricide, produced by
reacting salts of dimethyldithiophosphoric acid with N-methylchloroacetamide in aqueous
medium in the presence of some organic solvents.
It is also produced by reacting dithiophosphate with aqueous methylamine at low
temperature.
Pure dimethoate is a white crysatalline substance with camphor like odour. The
technical material is a yellowish brown (amber) coloured oily liquid with sulphurous acid
smell. It is highly soluble in water and most organic solvents. Dimethoate is thermally
unstable and on heating it decomposes.
d) Quinalphos (Ekalux) or (Bayrusil)
Derivative of Thiophosphoric acid.
Quinalphos (0, 0diethyl-0- (2-quinoxalinyl) phosphorothioate) is obtained by
condensation of 0-phenylenediamine with the hemi-acetal of glyoxylate.
The compound is highly active against biting and sucking insects and has an
LD50: 70 mg/kg rat (oral).
Quinalphos developed by Bayer AG (1969) is prepared by reaction of 0
phenylenediamine, chloroacetic acid and 0, 0-diethyl phosphorochloride thioate.
Quinalphos is a broad-spectrum contact and systemic insecticides, applied as
spray to control pests in cereals, brassicas and other vegetables. The mammalian
toxicity is quite high (LD50: 70 mg/kg) but the compound is degraded in plants within a
few days of application.
Summary Cheat Sheet
| Topic | Key exam point |
|---|---|
| Main class | Organophosphates with emphasis on toxicology and safety |
| Common examples in lesson | Phorate, phosalone, dimethoate, quinalphos |
| Mode of action | AChE inhibition remains the common toxicological basis |
| Safety issue | LD50, toxicity, and protective handling are major exam points |
| Antidote association | Atropine is the classical antidote reference |
| Environmental point | Persistence is lower than many organochlorines, but misuse still causes risk |
| Field caution | Dose, timing, and protective gear are critical |
| Exam distinction | Toxicology-focused OP questions differ from use/classification-focused OP questions |
| Poisoning clue | Cholinergic symptoms are linked to OP poisoning |
| Trap | Do not assume all organophosphates have identical hazard level or persistence |
References
3 sources • [1] [2] [3]
References
Principles of Soil Science and Agricultural Chemistry — Standard BSc Agriculture Textbook
BookLesson Doubts
Ask questions, get expert answers