Organophosphate Ester

In organic chemistry, organophosphates (also known as phosphate esters, or OPEs) are a class of organophosphorus compounds with the general structure , a central phosphate molecule with alkyl or aromatic substituents. They can be considered as esters of phosphoric acid.

Like most functional groups, organophosphates occur in a diverse range of forms, with important examples including key biomolecules such as DNA, RNA and ATP, as well as many insecticides, herbicides, nerve agents and flame retardants. OPEs have been widely used in various products as flame retardants, plasticizers, and performance additives to engine oil. The popularity of OPEs as flame retardants came as a substitution for the highly regulated brominated flame retardants. The low cost of production and compatibility to diverse polymers made OPEs to be widely used in industry including textile, furniture, electronics as plasticizers and flame retardants. These compounds are added to the final product physically rather than by chemical bond. Due to this, OPEs leak into the environment more readily through volatilization, leaching, and abrasion. OPEs have been detected in diverse environmental compartments such as air, dust, water, sediment, soil and biota samples at higher frequency and concentration.

Chemistry

Synthesis

Various routes exist for the synthesis of organophosphates:

;Esterification of phosphoric acid
:
:
:
Alcohols can be detached from phosphate esters by hydrolysis, which is the reverse of the above reactions. For this reason, phosphate esters are common carriers of organic groups in biosynthesis.

;Oxidation of phosphite esters
:
Organophosphites can be readily oxidised to give the above organophosphates.

;Alcoholysis of
:
Phosphorus oxychloride reacts readily with alcohols to give the above organophosphates.

Properties

The phosphate esters bearing OH groups are acidic and partially deprotonated in aqueous solution. For example, DNA and RNA are polymers of the type . Polyphosphates also form esters; an important example of an ester of a polyphosphate is ATP, which is the monoester of triphosphoric acid ().

OPEs have a central phosphate molecular group. In the case of organophosphate (OP) triesters, these are three ester bonds with alkyl or aromatic substituents. However, OP diesters are distinct from the triesters as one of the alkyl ester groups is replaced by a hydroxyl group, making OP diesters phosphoric acids. The wide variety of substitutes used in organophosphate esters results in great variations in physicochemical properties, varying from highly polar to very hydrolysis resistant characteristics. OPEs exhibit a wide range of octanol water partitioning coefficient where log Kow values range from -0.98 up to 10.6. The predominant OPEs used as flame retardants and plasticizers have a positive log Kow values ranging between 1.44 and 9.49 signifying hydrophobicity. Thus, owing to this hydrophobicity OPEs are presumptively bioaccumulated and biomagnified in aquatic ecosystems. Laboratory experiments had shown that the non-halogenated OPEs are prone to photolysis, while the chlorinated OPEs such as TCEP and TCPP, however, seemed to be resistant to degradation by sunlight.

In nature

Guanitoxin is a naturally occurring organophosphate produced by cyanobacteria.

The detection of OPEs in the air as far away as Antarctica at concentrations around 1 ng/m³ suggests their persistence in air, and their potential for long-range transport. OPEs were measured in high frequency in air and water and widely distributed in northern hemisphere. The chlorinated OPEs (TCEP, TCIPP, TDCIPP) in urban sampling sites and non-halogenated like TBOEP in rural areas respectively were frequently measured in the environment across multiple sites. In the Laurentian Great Lakes total OPEs concentrations were found to be 2–3 orders of magnitude higher than concentrations of brominated flame retardants measured in similar air. Waters from rivers in Germany, Austria, and Spain have been consistently recorded for TBOEP and TCIPP at highest concentrations. From these studies, it is clear that OPE concentrations in both air and water samples are often orders of magnitude higher than other flame retardants, and that concentrations are largely dependent on sampling location, with higher concentrations in more urban, polluted locations.

Pesticides

Today, organophosphates make up about 50% of the killing agents in chemical pesticides.

Organophosphate pesticides (OPPs), like some nerve agents, inhibit acetylcholinesterase (IRAC mode of action 1b), which is broadly essential for normal function in insects, but also in humans and many other animals. OPPs affect this enzyme in varied ways, a principal one being through irreversible covalent inhibition, and so create potentials for poisoning that vary in degree. The brain sends out neurotransmitters to the nerve endings in the body; organophosphates disrupt this process from occurring. This chemical, organophosphate works by disrupting the enzyme acetylcholinesterase. Acetylcholinesterase breaks down the acetylcholine neurotransmitter, which sends out signals to other nerve endings in the body.

For instance, parathion, one of the first OPPs commercialized, is many times more potent than malathion, an insecticide used in combating the Mediterranean fruit fly (Med-fly) and West Nile virus-transmitting mosquitoes. Human and animal exposure to them can be through ingestion of foods containing them, or via absorption through the skin or lungs.

The human and animal toxicity of OPPs make them a societal health and environmental concern; the EPA banned most residential uses of organophosphates in 2001, but their agricultural use, as pesticides on fruits and vegetables, is still permitted, as is their use in mosquito abatement in public spaces such as parks. For instance, the most commonly used OPP in the U.S., malathion, sees wide application in agriculture, residential landscaping, and pest control programs (including mosquito control in public recreation areas). As of 2010, forty such OPPs were registered for use in the U.S., with at least 73 million pounds used in one time period in agricultural and residential settings. Commonly used organophosphates have included:

* parathion
* malathion
* methyl parathion
* chlorpyrifos
* diazinon
* dichlorvos
* phosmet
* fenitrothion
* tetrachlorvinphos
* azamethiphos
* azinphos-methyl
* terbufos

Studies have shown that prolonged exposure to OPPs—e.g., in the case of farm workers—can lead to health problems, including increased risks for cardiovascular and respiratory disease, and cancer. In the case of pregnant women, exposure can result in premature births. In addition, permanent damage to the brain's chemical make-up, and changes in human behavior and emotion can occur to the fetus in pregnant women.

Organophosphate pesticides degrade rapidly by hydrolysis on exposure to sunlight, air, and soil, although small amounts can be detected in food and drinking water. Organophosphates contaminate drinking water by moving through the soil to the ground water. When the pesticide degrades, it is broken down into several chemicals. Organophosphates degrade faster than the organochlorides. The greater acute toxicity of OPPs results in the elevated risk associated with this class of compounds (see the Toxicity section below).

Nerve agents

History

Early pioneers in the field include Jean Louis Lassaigne (early 19th century) and Philippe de Clermont (1854). In 1932, German chemist Willy Lange and his graduate student, Gerde von Krueger, first described the cholinergic nervous system effects of organophosphates, noting a choking sensation and a dimming of vision after exposure on themselves, which they attributed to the esters themselves. This discovery later inspired German chemist Gerhard Schrader at company IG Farben in the 1930s to experiment with these compounds as insecticides. Their potential use as chemical warfare agents soon became apparent, and the Nazi government put Schrader in charge of developing organophosphate (in the broader sense of the word) nerve gases. Schrader's laboratory discovered the G series of weapons, which included Sarin

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