Pesticides , one of the first organophosphate insecticides. It remains important as a Vector control agent. Organophosphates are best known for their use as pesticides. The vast majority are
insecticides and are used either to protect crops, or as
vector control agents to reduce the transmission of diseases spread by insects, such as mosquitoes. Health concerns have seen their use significantly decrease since the turn of the century.
Glyphosate is sometimes called an organophosphate, but is in-fact a
phosphonate. Its chemistry, mechanism of toxicity and end-use as a herbicide are different from the organophosphate insecticides. The development of organophosphate insecticides dates back to the 1930s and is generally credited to
Gerhard Schrader. At the time, pesticides were largely limited to arsenic salts (
calcium arsenate,
lead arsenate and
Paris green) or
pyrethrin plant extracts, all of which had major problems. Schrader was seeking more effective agents, however while some organophosphates were found to be far more dangerous to insects than higher animals, the potential effectiveness of others as
chemical weapons did not go unnoticed. The development of organophosphate insecticides and the earliest
nerve agents was conjoined, with Schrader also developing the nerve agents
tabun and
sarin. Organophosphate pesticides were not commercialised until after WWII.
Parathion was among the first marketed, followed by
malathion and
azinphosmethyl. Although organophosphates were used in considerable quantities, they were originally less important than
organochlorine insecticides such as
DDT,
dieldrin, and
heptachlor. When many of the organochlorines were banned in the 1970s, following the publishing of
Silent Spring, organophosphates became the most important class of insecticides globally. Nearly 100 were commercialised, with the following being a varied selection: •
Acephate •
Azinphos-methyl •
Bensulide •
Chlorethoxyfos •
Coumaphos •
Diazinon •
Dichlorvos •
Dicrotophos •
Dimethoate •
Disulfoton •
Ethion •
Ethoprop •
Ethyl parathion •
Fenamiphos •
Fenitrothion •
Fonofos •
Isoxathion •
Malathion •
Methamidophos •
Methidathion •
Mevinphos •
Naled •
Phosmet •
Profenofos •
Propetamphos •
Quinalphos •
Sulfotep •
Tebupirimfos •
Temephos •
Terbufos •
Tetrachlorvinphos •
Triazofos Organophosphate insecticides are
acetylcholinesterase inhibitors, which disrupt the transmission of nerve signals in exposed organisms, with fatal results. The risk of human death through
organophosphate poisoning was obvious from the start and led to efforts to lower toxicity against mammals while not reducing efficacy against insects. The majority of organophosphate insecticides are
organothiophosphates (P=S) or
phosphorodiamidates (P-N), both of which are significantly weaker acetylcholinesterase inhibitors than the corresponding phosphates (P=O). They are 'activated' biologically by the exposed organism, via oxidative conversion of P=S to P=O, hydroxylation, or other related process which see them transformed into organophosphates. In mammals, these transformations occur almost exclusively in the liver, while in insects, they take place in the gut and
fat body. As the transformations are handled by different
enzymes in different classes of organism, it is possible to find compounds which activate more rapidly and completely in insects, and thus display more targeted lethal action. This selectivity is far from perfect and organophosphate insecticides remain
acutely toxic to humans, with many thousands estimated to be killed each year due to intentional (suicide) or unintentional poisoning. Beyond their acute toxicity, long-term exposure to organophosphates is associated with a number of heath risks, including
organophosphate-induced delayed neuropathy (muscle weakness) and developmental
neurotoxicity. There is limited evidence that certain compounds cause cancer, including
malathion and
diazinon. Children and farmworkers are considered to be at greater risk.
Pesticide regulation in the United States and the
regulation of pesticides in the European Union have both been increasing restrictions on organophosphate pesticides since the 1990s, particularly when used for crop protection. The use of organophosphates has decreased considerably since that time, having been replaced by
pyrethroids and
neonicotinoids, which are effective at much lower levels. Regulation in the global south can be less extensive. In 2015, only 3 of the 50 most common crop-specific pesticides used in the US were organophosphates (
Chlorpyrifos,
Bensulide,
Acephate). No new organophosphate pesticides have been commercialised in the 21st century. The situation in
vector control is fairly similar, despite different risk trade-offs, with the global use of organophosphate insecticides falling by nearly half between 2010 and 2019.
Flame retardants , a common organophosphate flame retardant Flame retardants are added to materials to prevent combustion and to delay the spread of fire after ignition. Organophosphate flame retardants are part of a wider family of phosphorus-based agents which include organic
phosphonate and
phosphinate esters, in addition to inorganic salts. When some prominent
brominated flame retardant were banned in the early 2000s, phosphorus-based agents were promoted as safer replacements. This has led to a large increase in their use, with an estimated 1 million tonnes of organophosphate flame retardants produced in 2018. Safety concerns have subsequently been raised about some of these reagents, with several under regulatory scrutiny. Organophosphate flame retardants were first developed in the first half of the twentieth century in the form of
triphenyl phosphate,
tricresyl phosphate and
tributyl phosphate for use in plastics like
cellulose nitrate and
cellulose acetate. Use in cellulose products is still significant, but the largest area of application is now in plasticized vinyl polymers, primarily
PVC. The more modern organophosphate flame retardants come in 2 major types;
chlorinated aliphatic compounds or aromatic diphosphates. which are commonly used to make casing for electrical items like TVs, computers and home appliances. Organophosphates act multifunctionally to retard fire in both the gas phase and condensed (solid) phase. Halogenated organophosphates are more active overall as their degradation products interfere with combustion directly in the gas phase. All organophosphates have activity in the condensed phase, by forming phosphorus acids which promote
char formation, insulating the surface from heat and air. Organophosphates were originally thought to be a safe replacements for brominated flame retardants, however many are now coming under regulatory pressure due to their apparent health risks. Bisphenol-A bis(diphenyl phosphate) can hydrolyse to form
Bisphenol-A which is under significant scrutiny as potential
endocrine-disrupting chemical. Although their names imply that they are a single chemical, some (but not all) are produced as complex mixtures. For instance, commercial grade TCPP can contain 7 different
isomers, while
tricresyl phosphate can contain up to 10. This makes their safety profiles harder to ascertain, as material from different producers can have different compositions.
Plasticisers an alkyl diaryl organophosphate used as both a plasticizer and flame retardant in PVC Plasticisers are added to polymers and plastics to improve their flexibility and processability, leading to a softer and more easily deformable material. In this way brittle polymers can be made more durable. The most frequently plasticised polymers are the vinyls (
PVC,
PVB,
PVA and
PVCA), as well as cellulose plastics (
cellulose acetate,
nitrocellulose and
cellulose acetate butyrate). PVC dominates the market, consuming 80-90% of global plasticiser production. PVC can accept large amounts of plasticiser; it is common for products to be 0-50% plasticiser by mass, but loadings can be as high as 70-80% in the case of
plastisols. Pure PVC is more than 60% chlorine by mass and is difficult to burn, but its flammability increases the more it is plasticised. Organophosphates find use because they are multifunctional; primarily plasticising but also imparting flame resistance. Compounds are typically triaryl or alkyl diaryl phosphates, with
cresyl diphenyl phosphate and
2-ethylhexyl diphenyl phosphate being important examples respectively. These are both liquids with high boiling points. Organophosphates are more expensive than traditional plasticisers and so tend be used in combination with other plasticisers and flame retardants.
Hydraulic fluids and lubricant additives Similar to their use as plastisiers, organophosphates are well suited to use as
hydraulic fluids due to their low freezing points and high boiling points, fire-resistance, non-corrosiveness, excellent boundary lubrication properties and good general chemical stability. The triaryl phosphates are the most important group, with tricresyl phosphate being the first to be commercialised in the 1940s, with
trixylyl phosphate following shortly after. Butylphenyl diphenyl phosphate and propylphenyl diphenyl phosphate became available after 1960. In addition to their use as hydraulic base-stock, organophosphates (tricresyl phosphate) and metal organothiophosphates (
zinc dithiophosphate) are used as both an
antiwear additives and
extreme pressure additives in
lubricants, where they remain effective even at high temperatures.
Metal extractants Organophosphates have long been used in the field of
extractive metallurgy to liberate valuable
rare earth metals from their
ores.
Di(2-ethylhexyl)phosphoric acid and
tributyl phosphate are used for the
liquid–liquid extraction of these elements from the acidic mixtures form by the
leaching of mineral deposits. The same compounds are also used in
nuclear reprocessing, as part of the
PUREX process.
Surfactants Mono- and di- phosphate esters of alcohols (or alcohol
ethoxylates) act as
surfactants (detergents). Although they are very common in biology as
phospholipids, their industrial use is largely limited to certain niche areas. Compared to the more common sulfur-based anionic surfactants (such as
LAS or
SLES), phosphate ester surfactants are more expensive and generate less foam. Low-levels of phosphate mono-esters, such as
potassium cetyl phosphate, find use in cosmetic creams and lotions. These in oil-in-water formulations are primarily based on non-ionic surfactants, with the anionic phosphate acting as emulsion-stabilisers. Phosphate tri-esters such as
tributyl phosphate are used as
anti-foaming agents in paints and concrete.
Nerve agents Although the first phosphorus compounds observed to act as cholinesterase inhibitors were organophosphates, the vast majority of nerve agents are instead
phosphonates containing a P-C bond. Only a handful of organophosphate nerve agents were developed between the 1930s and 1960s, including
diisopropylfluorophosphate,
VG and
NPF. Between 1971 and 1993 the
Soviet Union developed many new potential nerve agents, commonly known as the
Novichok agents. Some of these can be considered organophosphates (in a broad sense), being derivatives of
fluorophosphoric acid. Examples include
A-232,
A-234,
A-262,
C01-A035 and
C01-A039. The most notable of these is A-234, which was claimed to be responsible for the
poisoning of Sergei and Yulia Skripal in Salisbury (UK) 2018. ==In nature==