Parathione

Among the best known organophosphorus pesticides are malathion, parathion, schradan and dimefox. 

Noncatalytic Reactions Chemical kinetic methods are not as common for the quantitative analysis of analytes in noncatalytic reactions. Because they lack the enhancement of reaction rate obtained when using a catalyst, noncatalytic methods generally are not used for the determination of analytes at low concentrations. Noncatalytic methods for analyzing inorganic analytes are usually based on a com-plexation reaction. One example was outlined in Example 13.4, in which the concentration of aluminum in serum was determined by the initial rate of formation of its complex with 2-hydroxy-1-naphthaldehyde p-methoxybenzoyl-hydrazone. ° The greatest number of noncatalytic methods, however, are for the quantitative analysis of organic analytes. For example, the insecticide methyl parathion has been determined by measuring its rate of hydrolysis in alkaline solutions. 

Compounds with organophosphate moieties, such as Diazinon, Methyl Parathion, Coumaphos and Glyphosate are usually hydrolyzed at the phosphoms atom (40,58). Indeed several Flavobacterium isolates are able to grow using parathion and diazinon as sole sources of carbon. 

The rat LD qS are 13, 3.6 (oral) and 21, 6.8 (dermal) mg/kg. Parathion is resistant to aqueous hydrolysis, but is hydroly2ed by alkah to form the noninsecticidal diethjlphosphorothioic acid and -nitrophenol. The time required for 50% hydrolysis is 120 d ia a saturated aqueous solution, or 8 h ia a solution of lime water. At temperatures above 130°C, parathion slowly isomerizes to 0,%diethyl 0-(4-nitrophenyl) phosphorothioate [597-88-6] which is much less stable and less effective as an insecticide. Parathion is readily reduced, eg, by bacillus subtilis ia polluted water and ia the mammalian mmen to nontoxic 0,0-diethyl 0-(4-aminophenyl) phosphorothioate, and is oxidized with difficulty to the highly toxic paraoxon [511-45-5] diethyl 4-nitrophenyl phosphate d 1.268, soluble ia water to 2.4 mg/L), rat oral LD q 1.2 mg/kg. 

The hazards of human poisoning by the parathions have stimulated the development of safer analogues. Two chlorinated derivatives have gready reduced mammalian toxicides. Dicapthon [2463-84-5], 0,0-dimethyl 0-(2-chloro-4-nitrophenyl) phosphorothioate (63) (mp 53°C), has rat LD qS of 400, 330 (oral) and 790, 1250 (dermal) mg/kg. Chlorthion [500-20-8], 0,0-dimethyl 0-(3-chloro-4-nitrophenyl) phosphorothioate (64) (mp 21°C, <71.437), has rat LD qS of 890, 980 (oral) and 4500, 4100 (dermal) mg/kg. These compounds have been used as household insecticides. 

The reactivity of the individual O—P insecticides is determined by the magnitude of the electrophilic character of the phosphoms atom, the strength of the bond P—X, and the steric effects of the substituents. The electrophilic nature of the central P atom is determined by the relative positions of the shared electron pairs, between atoms bonded to phosphoms, and is a function of the relative electronegativities of the two atoms in each bond (P, 2.1 O, 3.5 S, 2.5 N, 3.0 and C, 2.5). Therefore, it is clear that in phosphate esters (P=0) the phosphoms is much more electrophilic and these are more reactive than phosphorothioate esters (P=S). The latter generally are so stable as to be relatively unreactive with AChE. They owe their biological activity to m vivo oxidation by a microsomal oxidase, a reaction that takes place in insect gut and fat body tissues and in the mammalian Hver. A typical example is the oxidation of parathion (61) to paraoxon [311-45-5] (110). 

The development of malathion in 1950 was an important milestone in the emergence of selective insecticides. Malathion is from one-half to one-twentieth as toxic to insects as parathion but is only about one two-hundredths as toxic to mammals. Its worldwide usage in quantities of thousands of metric tons in the home, garden, field, orchard, woodland, on animals, and in pubHc health programs has demonstrated substantial safety coupled with pest control effectiveness. The biochemical basis for the selectivity of malathion is its rapid detoxication in the mammalian Hver, but not in the insect, through the attack of carboxyesterase enzymes on the aUphatic ester moieties of the molecule. 

Slow release formulations incorporate nonpersistent compounds, eg, methyl parathion, insect growth regulators, and sex pheromones, in a variety of granular, laminated, microencapsulated, and hoUow-ftber preparations. 

Encapsulated fonofos, a soil insecticide, was developed to coat seeds before they were planted (72). Encapsulation reduces oral toxicity 100-fold and dermal toxicity 10-fold while extending activity of the fonofos. Other encapsulated pesticides available include permethrin and parathion (69). Significantly, all commercial encapsulated pesticides are prepared by interfacial polymeri2ation. 

Dialkyl and diaryl dithiophosphoric acids are the bases of many high pressure lubricants, oil additives (see Lubrication and lubricants), and ore flotation chemicals (see Mineral recovery and processing). Organophosphoms insecticides such as Parathion are made by chlorination of the appropriate diaLkyl dithiophosphate and subsequent reaction of the intermediate dialkyl thiophosphoric chloride with sodium -nitrophenolate according to the following (see... 

The use of alkali or alkaline-earth sulfides cataly2es the reaction so that it is complete in a few hours at 150—160°C use of aluminum chloride as the catalyst gives a comparable reaction rate at 115°C. When an excess of sulfur is used, the product can be distilled out of the reactor, and the residue of sulfur forms part of the charge in the following batch reaction. The reaction is carried out in a stainless steel autoclave, and the yield is better than 98% based on either reactant. Phosphoms sulfochloride is used primarily in the manufacture of insecticides (53—55), such as Parathion. 

Reduction of Nitro Substituents. These reactions are very common in anaerobic environments and result in amine-substituted pesticides anaerobic bacteria capable of reducing nitrate to ammonia appear to be primarily responsible. All nitro-substituted pesticides appear to be susceptible to this transformation, eg, methyl parathion (7) (eq. 9), triduralin, and pendimethalin. 

S-oxidation of sulfur-containing pesticides such as aldicarb, parathion, and malathion can be of importance in the absence of microbial activity (29). The products of chemical vs biological oxidation are generally identical (eq. 8). 

Uses. Sulfur dichloride is used as a chlorinating agent Hi the manufacture of parathion [56-38-2] Hisecticide Hitermediates (see Insect control technology) ... 

R. M. Gamson and co-workers. Detection of GB, UX and Parathion in Water, Edgewood Arsenal Report No. ED-TR-74015, June 1974. ... 

Chlorfenvinphos, cythion, diazinon, elsan, fenitrothion, fenthion, malathion, methyl parathion, mevinphos, monocrotophos, parathion, quinalphos, temephos, TEPA, tetrachlorvinphos... 

Dichloro-4,4 -methylene dianiline (MDOCA) Parathion-methyl... 

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