☠️ Organophosphate Insecticides I

Organophosphate insecticides are widely used but require careful handling due to high mammalian toxicity. This lesson outlines major compounds, preparation notes, and mode of action.

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MONOCROTOPHOS, PHOSPHAMIDAN, MALATHION AND CHLORPYRIPHOS

a) Monocrotophos (Azodrin)

Derivative of phosphoric acid

Monocrotophos (Dimethyl-1 methyl-2-methyl carbamoyl-venyl phosphate)

(Monocrotophos)

LD50: 21 mg/g

b) Phosphamidon (Dimecron)

(2-chloro-2diethyl carbamyl-1-methylvinyl dimethyl phosphate)

Derivative of Phosphoric Acid

Phosphamidon is produced by reacting equimolecular quantities of trimethyl

phosphite with boiling solution of diethylamide of dichloroacetic acid in chlorobenzene.

O CH3

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(CH3O)3P+CH3CO C Cl2 CON(C2H5)2(CH3O)2-P-O-C=CCl-CON-(C2H2)2+CH3Cl

Phosphamidon is very similar to mevinphos in toxicity has a broad spectrum of activity

against biting and sucking pests and spider mites.

LD50: 10 mg

Pure phosphamidon is colourless. Commercial product is bright violet due to the

presence of a dye. B.P. 70°C, with a faint pleasant odour.

Highly soluble in water, alcohol, acetone, slightly soluble in saturated

bydrocarbons like hexane and insoluble in kerosene. Does not clogg the nozzles and

hence suited to ultra low volume and low volume sprays.

 - Phosphamidon is stable in neutral and weak acidic aqueous solutions.

 - It is rapidly hydrolysed in alkaline medium.

 - Stable for 2 years when stored in closed containers.

 - It is compatible with most pesticides except alkaline ones.

 - When mixed with captan it is synergistic. Toxicity is reduced when mixed

with copper oxychloride.

 - When insects feed on this, the choline esterase is inhibited and the insect

gets killed.

 - Used as a systemic to control sucking pests in cotton.

c) Malathion

(0, 0-dimethyl S-(1, 2, dicarbethoxy) ethyl phosphorus dithioate)

Derivatives of Dithiophosphoric acid

Dithiophosphoric acid + Maleic acid  Malathion

Malathion is obtained by addition of dimethyl dithiophosphoric acid to maleic acid

ester in the presence of basic catalyts.

Malathion was introduced in 1950 by the American Cyanamid company. It is an

important and widely used contact insecticide and acaricide for the control of aphids, red

spidermites, leafhoppers and thrips on a wide range of vegetable and other crops. It

was important in the history of development of OP insecticides since it was the first

member with a broad spectrum of contact insecticidal activity combined with the

remarkably low mammalian toxicity (LD50: 1300 mg/kg rats). Malathion is also valuable

to control insect vectors eg. Mosquitoes and can be used as a substitute for

organochlorine insecticides.

Pure malathion is a colourless liquid boiling at 120°C. It is sparingly soluble in

water but highly soluble in most of the organic solvents except in saturated

bydrocarbons.

Malathion on prolonged heating at 150°C is isomerised and goes over to thiolo

isomers.

d) Chlorpyriphos (Dursban)

(0, 0-diethyl 0-(3, 5, 6 trichlorophyridine-2 yl phosphorothioate)

Derivative of Thiophosphoric acid

Chlorpyrifos is used for mosquito control but may also be employed against

ectoparasites on domestic animals.

LD50: 135-163 mg/kg rat oral.

Metallic ions in soils often interact with organophosphorus insecticides; the cupric

ion is a very effective calalyst for the degradation of some organophosphorus esters,

such as diazinon and chlorpyrifos.

Chlorpyrifos has low acute mammalian toxicity in the 2000-5000 mg/kg range.

Many organophosphorus insecticides contain heterocyclic moieties with nitrogen

heterocycles. The only important pyridine derivatives are chlorpyrifos and chlorpyrifos

methyl. Chlorpyrifos is a very valuable contact insecticide some 3500 tonnes were used

in USA in 1982. It has a wide spectrum of activity, by contact, ingestion and vapour

action. It is moderately persistant and retains its activity in soil for 2-4 months and is

valuable against mosquito and fly larvae, cabbage root fly, aphids and codling and winter

moths on fruit trees.

Chlorpyrifos has become one of the most widely applied insecticides in homes

and restaurants against cockroaches, and other domestic pests. It is a comparatively

safe insecticide. Chlorpyrifos methyl (R=CH3) is quite volatile and is used to control

insects in grain stores.

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Summary Cheat Sheet

Topic	Key exam point
Main class	Organophosphate insecticides
Mode of action	Inhibit acetylcholinesterase (AChE)
Common examples	Monocrotophos, phosphamidon, malathion, chlorpyriphos
Chemical basis	Derivatives of phosphoric or thiophosphoric acid
Toxicology note	Generally high acute toxicity risk compared with many older botanicals
Use theme	Widely used in insect pest management across crops
Residue caution	Safety interval and correct dosage are critical
Antidote theme	Atropine is classically linked with OP poisoning management in toxicology discussions
Exam distinction	Organophosphates differ from carbamates even though both affect AChE
Trap	Do not confuse chlorpyriphos/malathion class with organochlorines or carbamates

References

3 sources • [1] [2] [3]

[1]

Indian Council of Agricultural Research (ICAR)

Official

[2]

Principles of Soil Science and Agricultural Chemistry — Standard BSc Agriculture Textbook

Book

[3]

Department of Agriculture & Farmers Welfare

Website