METHIDATHION

Methidathion is a non-systemic organophosphate insecticide and acaricide with contact and stomach action; its use as a pesticide is banned in the European Union and the USA, while registration status in other jurisdictions must be verified with competent authorities.
Methidathion acts by inhibiting acetylcholinesterase at nerve synapses, causing continuous nerve stimulation, paralysis, and death in target insects, and has been applied historically to citrus, apple, cherry, peach, walnut, almond, cotton, sunflower, olive, alfalfa, safflower, artichoke, ornamental plants, and grapevine against scale insects, aphids, spider mites, whiteflies, and leafrollers.
Methidathion earned its US EPA Class I and Restricted Use Pesticide (RUP) designation precisely because of its exceptional efficacy against a broad range of resistant insect and mite species that proved difficult to control with less potent alternatives.

CAS Number: 950-37-8
EC Number: 213-449-4
Molecular Formula: C₆H₁₁N₂O₄PS₃
Molecular Weight: 302.33 g/mol

Synonyms: DMTP, Supracide, Ultracide, Suprathion, Somonil, Medathion, Metidation, Faat, Sphat, OMS 844, NC2964, GS-13005, GS 13005, Hionate, Geigy 13005, Geigy GS 13005, Fisons NC 2964, Cobracide, Somonic, Surpracide, S-2,3-dihydro-5-methoxy-2-oxo-1,3,4-thiadiazol-3-ylmethyl O,O-dimethyl phosphorodithioate, S-[(5-methoxy-2-oxo-1,3,4-thiadiazol-3(2H)-yl)methyl] O,O-dimethyl phosphorodithioate, 3-(dimethoxyphosphinothioylsulfanylmethyl)-5-methoxy-1,3,4-thiadiazol-2-one, phosphorodithioic acid S-[(5-methoxy-2-oxo-1,3,4-thiadiazol-3(2H)-yl)methyl] O,O-dimethyl ester, CAS 950-37-8

Uses of Methidathion: Methidathion has been used in agriculture as a broad-spectrum, non-systemic acaricide and insecticide with both contact and stomach-poison properties.
Methidathion's efficacy is particularly pronounced against scale insects, including Psylla sylvestris and various armoured scale species; whiteflies, aphids, spider mites, and leafrollers are also among the susceptible species.

Methidathion has been applied against San Jose scale, mealybugs, and leafrollers on fruit trees such as citrus, apple, cherry, peach, walnut, and almond, and against overwintering pest populations on strawberry and grapevine.
Pests with chewing and sucking mouthparts on sunflower, artichoke, olive, alfalfa, safflower, cotton, and ornamental plants also fall within the target range of methidathion.

Methidathion's advantage in pest management stems largely from the selective efficacy of methidathion against scale insects, remaining effective against both larval and egg stages including overwintering populations.
However, the high mammalian toxicity of methidathion (rat oral LD50 approximately 25–54 mg/kg) and extreme toxicity to aquatic organisms (96-hour LC50 in bluegill sunfish approximately 2.2 μg/L) underline that the compound must be handled exclusively by authorised, trained personnel under strict precautionary measures.

Methidathion has been used in many countries to control caterpillars of Indarbela quadrinotata — a bark-eating caterpillar of the family Cossidae — on various crops.
Methidathion is also applied in residue analysis of serum and urine samples by HPLC and GC, and in routine monitoring of food and environmental matrices for organophosphate contamination.

Benefits and Advantages of Methidathion: Methidathion combines both insecticidal and acaricidal activity in a single compound, enabling multi-target control with a single application in conditions where secondary mite damage and scale insect infestation occur simultaneously.
This dual activity reduces the number of separate treatments required and simplifies pest management programmes in complex growing environments.

Methidathion's efficacy against overwintering larvae and eggs opens a winter-season application window that many alternative products cannot cover, especially for armoured scale management on deciduous fruit trees.
The rapid knockdown effect allows immediate intervention when pest populations reach critical density, which is particularly valuable during outbreaks on orchard crops.

Features of Methidathion: Methidathion is a colourless crystalline solid or powder at room temperature with the characteristic odour typical of organophosphorus compounds.
Specific gravity at 20°C is approximately 1.51 g/cm³ and melting point is 39–40°C; vapour pressure at 20°C is 2.5×10⁻⁴ Pa (0.186 mPa), indicating negligible volatility under ambient conditions.

Water solubility at 20°C is approximately 187–240 mg/L (0.024 g/100 mL), while solubility is high in organic solvents such as octanol, ethanol, xylene, acetone, and cyclohexane.
The LogPow of approximately 2.20 (experimental) indicates low-to-moderate bioaccumulation potential; Henry's Law constant is 1.4×10³ mol/(m³·Pa) at 25°C; pKa is -4.17 (predicted); aerobic soil half-life is 3–18 days, and hydrolysis half-life at pH 13 (25°C) is approximately 30 minutes.

Chemical Properties of Methidathion: Methidathion is a phosphorodithioate ester compound built around a 1,3,4-thiadiazole ring whose preferred IUPAC name is S-[(5-methoxy-2-oxo-1,3,4-thiadiazol-3(2H)-yl)methyl] O,O-dimethyl phosphorodithioate.
The structure contains three distinct sulfur atoms — one in the thiadiazole ring, one in the thioether bridge, and one in the thiophosphate group — together with one phosphorus atom and two methoxy groups; ChEBI defines methidathion as an organic thiophosphate and organothiophosphate insecticide functionally related to 5-methoxy-1,3,4-thiadiazol-2(3H)-one.

Methidathion is relatively stable in neutral or weakly acid solution but hydrolyses rapidly under strongly alkaline conditions; in the presence of strong reducing agents such as hydrides, highly toxic and flammable phosphine gas may form.
Partial oxidation by oxidising agents may release toxic phosphorus oxides; when heated to decomposition, methidathion emits very toxic fumes of nitrogen oxides (NOₓ), phosphorus oxides (POₓ), and sulfur oxides (SOₓ); the compound is classified as air and moisture sensitive.

Metabolism and Environmental Fate of Methidathion: The main route of methidathion metabolism in both animals and plants is hydrolysis or oxidation to yield the 3-thiomethyl-5-methoxy-1,3,4-thiadiazole derivative, which may either lose methanethiol to yield the methoxy-1,3,4-thiadiazolinone or be conjugated as the cysteine derivative in plants.
An additional route in plants and mammals involves S-adenosylmethionine-mediated methylation followed by oxidation to the sulfoxide and sulfone, which are excreted in urine; demethylation of the 5-methoxy group is another detoxification route across insects, plants, and mammals.

Photolytic degradation in aqueous buffer (25°C, pH 7.0, UV >290 nm, 24 hours) decomposes 17% of methidathion to 5-methoxy-3H-1,3,4-thiadiazol-2-one; at 50°C, 56% was degraded within 24 hours via hydrolysis of the thiol bond of the phosphorodithioic ester.
Methidathion oxon, the active acetylcholinesterase inhibitor, is detected in metabolism studies but only at very low concentration due to its rapid hydrolysis; methidathion oxon has also been found in fogwater collected in agricultural areas.

Production of Methidathion: The industrial synthesis of methidathion begins with the cyclisation of appropriate hydrazine and carboxylic acid derivatives, followed by methoxylation at the 5-position to yield the 1,3,4-thiadiazole intermediate.
This intermediate is then reacted with O,O-dimethyl phosphorodithioic chloride in organic solvents such as toluene or xylene, with temperature and pH carefully controlled, to form the phosphorodithioate ester linkage.

Key raw materials include thionyl chloride, potassium hydroxide, hydrazinium hydroxide, carbon disulfide, paraformaldehyde, phosphorus pentasulfide, and dimethylphosphorodithioate.
Methidathion is available as a technical-grade solid (approximately 90–95% purity) or in high-purity certified form for analytical purposes (≥99%, GC); Certificate of Analysis (CoA) documentation is provided for analytical-grade material.

Methidathion Material Safety Data Sheet (MSDS):

Handling of Methidathion: As an extremely toxic organophosphorus compound, methidathion must be used only by specially trained personnel in full compliance with applicable regulations.
Work must be carried out in closed or well-ventilated systems with local exhaust ventilation; inhalation of dust and contact with skin and eyes must be strictly prevented, and eating, drinking, and smoking during use are not permitted.

Mandatory personal protective equipment includes chemically resistant impermeable gloves, full-face chemical safety goggles and face shield, and chemical-resistant protective clothing and boots.
Where dust or vapour cannot be adequately controlled by ventilation, an approved filter or pressure-fed air-supply respirator must be used; emergency antidotes (atropine and an appropriate oxime) and organophosphate-poisoning instructions must be kept readily available in the work area at all times.

Methidathion SDS:

Stability and Reactivity of Methidathion:

Chemical stability: Methidathion is stable under neutral and mildly acidic conditions.
Hydrolysis accelerates markedly in strongly alkaline media (pH >9); half-life at pH 13 and 25°C is approximately 30 minutes.

Reactivity: Contact with strong reducing agents (hydrides) may produce highly toxic and flammable phosphine gas.
Oxidising agents may cause the release of toxic phosphorus oxides; excessive heating causes decomposition with generation of very toxic NOₓ, POₓ, and SOₓ fumes.

Conditions to avoid: Excessive heat, open flames, and ignition sources.
Moisture and alkaline conditions (rapid hydrolysis).
Contact with strong reducing agents and oxidants.
Storage together with food, feed, and medicines.

Incompatible materials: Strong reducing agents (including hydrides).
Strong oxidants.
Strong bases and alkalis.

Hazardous decomposition products: Nitrogen oxides (NOₓ).
Phosphorus oxides (POₓ).
Sulfur oxides (SOₓ).
Carbon monoxide (CO) and carbon dioxide (CO₂).
Phosphine gas (upon contact with strong reducing agents).

Handling and Storage of Methidathion:

Handling: Use only in closed, well-ventilated systems or under a fume hood.
Strictly avoid inhalation of dust and contact with skin and eyes.
Use appropriate PPE (gloves, goggles, face shield, full protective clothing).
Do not eat, drink, or smoke in the work area.
Wash hands and exposed skin thoroughly after use.
Keep an organophosphate-poisoning antidote kit (atropine, oxime) readily available for emergencies.

Storage: Store in original tightly closed containers in a cool (0–6°C), dry, well-ventilated, locked dedicated area — colour code Blue (Health Hazard/Poison).
Protect from direct sunlight, heat, and moisture; classified as air and moisture sensitive.
Store separately from food, beverages, feed, and medicines.
Ensure the storage area has no direct access to drains or sewers.
For small quantities, treat with strong alkali and bury; for large quantities, use high-temperature incineration with flue-gas scrubbing.

First Aid Measures for Methidathion:

Inhalation: Remove the affected person to fresh air immediately; if breathing has stopped, begin rescue breathing using universal precautions including a resuscitation mask, and apply CPR if cardiac action has also stopped.
Administer 100% oxygen, call for medical assistance immediately, and keep the person under observation as symptoms may be delayed; rescuers must use full respiratory protection.

Skin contact: Remove contaminated clothing and footwear immediately — speed is of extreme importance; wash the affected skin with soap and plenty of water, and shampoo the hair promptly if contaminated, as methidathion is absorbed through the skin.
Seek immediate medical attention and administer antidote if necessary.

Eye contact: Remove contact lenses if present and rinse immediately with clean running water for at least 15 minutes, occasionally lifting upper and lower eyelids.
Obtain immediate ophthalmological examination.

Ingestion: Rinse the mouth immediately; give large quantities of water to a conscious person and do not induce vomiting for an unconscious or convulsing person.
Call a poison centre or emergency services immediately, as effects may be delayed; keep the victim quiet and maintain normal body temperature.

Firefighting Measures for Methidathion:

Suitable extinguishing media: Water spray, dry chemical powder, carbon dioxide, or alcohol-resistant foam.

Specific hazards: Methidathion is a combustible solid; commercial liquid formulations may be flammable due to organic solvent content.
Combustion releases nitrogen oxides, phosphorus oxides, sulfur oxides, carbon monoxide, and highly toxic fumes; containers exposed to fire may rupture or explode, and runoff from fire control water may produce irritating or poisonous gases.

Protective equipment for firefighters: Positive-pressure self-contained breathing apparatus (SCBA) and full chemical-protective clothing must be used.

Firefighting procedures: Evacuate all non-essential personnel; approach the fire from a safe distance and remain upwind, cooling exposed containers with water spray.
Prevent contaminated firefighting water from entering drains, soil, or water sources; collect separately and dispose of as hazardous waste.

Accidental Release Measures for Methidathion:

Personal precautions: Evacuate non-essential personnel and position upwind; avoid inhalation of dust and contact with skin and eyes.
Wear full PPE and take precautions against electrostatic discharge.

Environmental precautions: Methidathion is extremely toxic to aquatic organisms; strictly prevent contamination of sewers, surface water, and groundwater.
Notify environmental authorities immediately if a significant release occurs.

Clean-up methods: Avoid generating dust; carefully sweep or vacuum using equipment appropriate for toxic materials, and collect the material with an inert, non-combustible absorbent in sealed, labelled containers.
Prevent collected washings from entering drains and send all waste to an authorised hazardous-waste facility.

Exposure Controls / Personal Protective Equipment for Methidathion:

Engineering controls: Use a closed system or effective local exhaust ventilation.
Emergency eyewash stations and safety showers must be positioned close to the work area.

Eye protection: Full-face chemical safety goggles and face shield — both must be used together.

Hand protection: Chemically resistant impermeable gloves (butyl rubber or neoprene); select according to formulation and duration of exposure, and inspect before use.

Skin and body protection: Chemical-resistant full-body protective clothing and boots; fully encapsulating protective suit for emergency spills.

Respiratory protection: Approved full-face mask with combined organic vapour/particulate filter cartridge for controlled environments; positive-pressure SCBA for emergencies, high concentrations, or unknown exposure levels.

Hygiene measures: Wash hands and exposed skin thoroughly after each use; keep contaminated clothing separate from personal clothing; do not eat, drink, or smoke in the work area.

Methidathion Identifiers:
CAS Number: 950-37-8
EC Number: 213-449-4
EU Index Number: 015-069-00-2
UN Number: UN 2783
RIDADR: UN 2811
Transport Class: 6.1 (a)
Packing Group: II
WGK Germany: 3
Storage Class: 6.1A
IUPAC Name: S-[(5-Methoxy-2-oxo-1,3,4-thiadiazol-3(2H)-yl)methyl] O,O-dimethyl phosphorodithioate
Molecular Formula: C₆H₁₁N₂O₄PS₃
Molecular Weight: 302.33 g/mol
Monoisotopic Mass: 301.962 g/mol
InChIKey: MEBQXILRKZHVCX-UHFFFAOYSA-N
SMILES: COC1=NN(CSP(=S)(OC)OC)C(=O)S1
PubChem CID: 13709
ChemSpider: 13115
ChEBI: CHEBI:34837
ChEMBL: ChEMBL226651
ECHA InfoCard: 100.012.227
KEGG: C14431
CompTox (EPA): DTXSID5020819
UNII: Y2P145U7KK
BRN: 619915
RTECS: TE2100000
MeSH: C005828
GHS Signal Word: Danger
GHS Hazard Statements (ECHA CLP): H300, H312, H400, H410 (Acute Tox. 2 oral, Acute Tox. 4 dermal, Aquatic Acute 1, Aquatic Chronic 1)
Risk Statements (R-phrases): R21, R28, R41, R50/53
Safety Statements (S-phrases): S22, S26, S28, S36/37, S39, S45, S60, S61
Rat oral LD50: 25–54 mg/kg (males 31, females 32 mg/kg per Gaines & Linder)
Rat inhalation LC50: 3.6 mg/L (4 hours)
NOEL (2 yr, rat): 4 mg/kg diet (0.2 mg/kg/day)
ADI: 1 µg/kg body weight
Aquatic LC50 (bluegill sunfish, 96 hours): ~2.2 µg/L
Daphnia magna EC50 (48 hours): ~6.4 µg/L
EPA Classification: Restricted Use Pesticide (RUP), Toxicity Class I
EU Status: Use banned
USA Status: Use banned
TSCA: Listed

Properties of Methidathion:
Physical state: Solid
Appearance: Colourless crystalline powder
Odour: Characteristic organophosphorus odour
Molecular formula: C₆H₁₁N₂O₄PS₃
Molecular weight: 302.33 g/mol
Density: 1.51 g/cm³ (20°C)
Melting point: 39–40°C
Boiling point: 347.7 ± 52.0°C (predicted)
Flash point: 100°C (combustible solid)
Vapour pressure: 2.5×10⁻⁴ Pa (20°C); 3.37×10⁻⁶ mmHg (25°C, experimental)
Water solubility (20°C): ~187–240 mg/L (0.024 g/100 mL)
Solubility in acetone: 670 g/L (20°C)
Solubility in toluene: 720 g/L (20°C)
Solubility in ethanol: 150 g/L (20°C)
LogPow: 2.20 (experimental, expkow database)
pKa: -4.17 ± 0.40 (predicted)
Henry's Law constant: 1.4×10³ mol/(m³·Pa) at 25°C
Aerobic soil DT50: 3–18 days
Hydrolysis half-life at pH 13: ~30 minutes (25°C)
Stability: Air sensitive, moisture sensitive
GHS Classification: Danger — H300, H312, H400, H410
Storage temperature: 0–6°C (tightly closed, locked, cool, dry)

Methidathion Properties — Specifications:
Product name: Methidathion
CAS Number: 950-37-8
EC Number: 213-449-4
Molecular Formula: C₆H₁₁N₂O₄PS₃
Molecular Weight: 302.33 g/mol
Purity (GC): ≥99% (analytical grade)
Technical purity: ~90–95%
Appearance: Colourless crystalline solid
Melting point: 39–40°C
Density: ~1.51 g/cm³ (20°C)
Storage: 0–6°C; tightly closed, locked, cool and dry
Format: Solid (pure); formulated products also available
Packaging: According to customer requirements
Documents: CoA (Certificate of Analysis), MSDS/SDS available

Names of Methidathion: Methidathion DMTP Supracide Ultracide Suprathion Somonil Surpracide Somonic Cobracide Medathion Metidation Faat Sphat OMS 844 NC2964 GS-13005 GS 13005 Hionate Geigy 13005 Geigy GS 13005 Fisons NC 2964 S-[(5-Methoxy-2-oxo-1,3,4-thiadiazol-3(2H)-yl)methyl] O,O-dimethyl phosphorodithioate 3-(Dimethoxyphosphinothioylsulfanylmethyl)-5-methoxy-1,3,4-thiadiazol-2-one Phosphorodithioic acid, S-[(5-methoxy-2-oxo-1,3,4-thiadiazol-3(2H)-yl)methyl] O,O-dimethyl ester S-2,3-dihydro-5-methoxy-2-oxo-1,3,4-thiadiazol-3-ylmethyl O,O-dimethyl phosphorodithioate CAS 950-37-8 UN 2783