Chemistry:Organophosphate

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Short description: Organic compounds with the structure O=P(OR)3


General chemical structure of the organophosphate functional group

In organic chemistry, organophosphates (also known as phosphate esters, or OPEs) are a class of organophosphorus compounds with the general structure O=P(OR)
3, a central phosphate molecule with alkyl or aromatic substituents.[1] They can be considered as esters of phosphoric acid.

Like most functional groups, organophosphates occur in a diverse range of forms,[2] 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.[3] 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.[4] Due to this, OPEs leak into the environment more readily through volatilization, leaching, and abrasion.[5] OPEs have been detected in diverse environmental compartments such as air, dust, water, sediment, soil and biota samples at higher frequency and concentration.[1][5]

Forms

Organophosphates are a class of compounds encompassing a number of distinct but closely related function groups. These are primarily the esters of phosphoric acid and can be mono‑esters, di‑esters or tri‑esters depending on the number of attached organic groups (abbreviated as 'R' in the image below). In general man‑made organophosphates are most often triesters, while biological organophosphates are usually mono- or di-esters. The hydolysis of triesters can form diesters and monoesters.[6]

Phosphorsäureester.png

In the context of pesticides, derivatives of organophosphates such as organothiophosphates (P=S) or phosphorodiamidates (P-N) are included as being organophosphates. The reason is that these compound are converted into organophosphates biologically.

Thiophosphorsäureester Dithiophosphorsäureester.svg

In biology the esters of diphosphoric acid and triphosphoric acid are generally included as organophosphates. The reason is again a practical one, as many cellular processes involve the mono- di and tri- phosphates of the same compound. For instance, the phosphates of adenosine (AMP, ADP, ATP) play a key role in many metabolic processes.

Di u Triphosphorsäureester.png

Synthesis

Alcoholysis of POCl
3

Phosphorus oxychloride reacts readily with alcohols to give organophosphates. This is the dominate industrial route and is responsible for almost all organophosphate production.

O=PCl
3 + 3 ROH → O=P(OR)
3 + 3 HCl

When aliphatic alcohols are used the HCl by-product can react with the phosphate esters to give organochlorides and a lower ester.

O=P(OR)
3 + HCl → O=P(OR)
2OH + RCl

This reaction is usually undesirable and is exacerbated by high reaction temperatures. It can be inhibited by the use of a base or the removal of HCl through sparging.

Esterification of phosphoric acid and P2O5

Esterifications of phosphoric acid with alcohols proceed less readily than the more common carboxylic acid esterifications, with the reactions rarely proceeding much further than the phosphate mono-ester. The reaction requires high temperatures, under which the phosphoric acid can dehydrate to form poly-phosphoric acids. These are exceedingly viscous and their linear polymeric structure renders them less reactive than phosphoric acid.[7] Despite these limitations the reaction does see industrial use for the formation of monoalkyl phosphates, which are used as surfactants.[8] A major appeal of this route is the low cost of phosphoric acid compared to phosphorus oxychloride.

OP(OH)
3 + ROH → OP(OH)
2(OR) + H
2O

P2O5 is the anhydride of phosphoric acid and acts similarly. The reaction yields equimolar amounts of di- and monoesters with no phosphoric acid. The process is mostly limited to primary alcohols, as secondary alcohols are prone to undesirable side reactions such as dehydration.[9]

Oxidation of phosphite esters

Organophosphites can be easily oxidised to give organophosphates. This might ordinarily be considered a specialised method, however large quantities of organophosphites are produced as antioxidant stabilisers for plastics, with their gradual oxidation forming organophosphates in the human environment.[10][11]

P(OR)
3 + [O] → OP(OR)
3
Phosphorylation
Main page: Biology:Phosphorylation

The formation of organophosphates is an important part of biochemistry and living systems achieve this using a variety of enzymes. Phosphorylation is essential to the processes of both anaerobic and aerobic respiration, which involve the production of adenosine triphosphate (ATP), the "high-energy" exchange medium in the cell.

Properties

Bonding

The bonding in organophosphates has been a matter of prolonged debate; the phosphorus atom is classically hypervalent, as it possesses more bonds than the octet rule should allow.[12] The focus of debate is usually on the nature of the phosphoryl P=O bond, which displays (in spite of the common depiction) non-classical bonding, with a bond order somewhere between 1 and 2. Early papers explained the hypervalence in terms of d-orbital hybridisation, with the energy penalty of promoting electrons into the higher energy orbitals being off-set by the stabilisation of additional bonding.[13] Later advances in computational chemistry showed that d-orbitals played little significant role in bonding.[14][15] Current models rely on either negative hyperconjugation,[16] or a more complex arraignment with a dative-type bond from P to O, combined with back-donation from a 2p orbital on the oxygen.[15][17] These models agree with the experimental observations of the phosphoryl as being shorter than P-OR bonds[18] and much more polarised. It has been argued that a more accurate depiction is dipolar (i.e. (RO)3P+-O-),[19] which is similar to the depiction phosphorous ylides such as methylenetriphenylphosphorane. However in contrast to ylides, the phosphoryl group is unreactive and organophosphates are poor nucleophiles, despite the high concentration of charge on the phosphoryl oxygen. The polarisation accounts in part for the higher melting points of phosphates when compared to their corresponding phosphites. The bonding in penta-coordinate phosphoranes (i.e. P(OR)5) is entirely different and involves three-center four-electron bonds.

Acidity

Phosphate esters bearing OH groups are acidic. The pKa of the first OH group is typically between 1-2, with the second OH deprotonating at a pKa of 6-7.[20] This is great practical importance as it means that phosphate mono- and di-esters are negatively charged at physiological pH. This includes biomolecules such as DNA and RNA. The presence of this negative charge make these compound much more water soluble and more resistant to degradation by hydrolysis or other forms of nucleophilic attack.

Water solubility

The water solubility of organophosphates is an important factor in biological, industrial and environmental settings. The wide variety of substitutes used in organophosphate esters results in great variations in physical properties. OPEs exhibit a wide range of octanol/water partition coefficients where log Kow values range from -0.98 up to 10.6.[3] Mono- and di- esters are usually water soluble, particularity biomolecules. Tri-esters such as flame retardants and plasticisers have positive log Kow values ranging between 1.44 and 9.49, signifying hydrophobicity.[3][21][5][22] Hydrophobic OPEs are more likely to be bioaccumulated and biomagnified in aquatic ecosystems.[4] Halogenated organophosphates tend to be denser than water and sink, causing them to accumulate in sediments.[23]

Industrial materials

Pesticides

Malathion, 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.[24][25] 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.[26] At the time pesticides were largely limited to arsenic salts (calcium arsenate, lead arsenate and Paris green)[27] or pyrethrin plant extracts, all of which had major problems.[28] Schrader was seeking more effective agents, however while some organophosphates were found to be far more dangerous to insects than higher animals,[29] 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 qualities 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:



Organophosphate insecticides are acetylcholinesterase inhibitors, and when introduced to an organism they act to fatally disrupt the transmission of nerve signals. The risk of human death through organophosphate poisoning[30] was obvious from the start and let to efforts to lower toxicity again mammals while not reducing efficacy again insects.[31][32]

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,[33] hydroxylation,[34][35] or other related process which see them transformed into organophosphates. In mammals these transformations occur almost exclusively in the liver,[36] while in insects they take place in the gut and fat body.[37][38][39] 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[40] or unintentional poisoning. Beyond their acute toxicity, exposure to organophosphates is associated with a number of heath risks, including organophosphate-induced delayed neuropathy (muscle weakness) and developmental neurotoxicity.[26][41][42] There is limited evidence that certain compounds cause cancer, including malathion and diazinon.[43] Children[44] and farmworkers[45] 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 a much lower levels.[46] Reported cases of organophosphate poisoning in the US have reduced during this period.[47][48] Regulation in the global south can be less extensive.[49][50]

In 2015, only 3 of the 50 most common crop-specific pesticides used in the US were organophosphates (Chlorpyrifos, Bensulide, Acephate),[51] of these Chlorpyrifos was banned in 2021.[52] No new organophosphate pesticides have been commercialised in the 21st century.[53] The situation in vector control is fairly similar, despite different risk trade-offs,[54] with the global use of organophosphate insecticides falling by nearly half between 2010 and 2019.[25] Pirimiphos-methyl, Malathion and Temefos are still important, primarily for the control of malaria in the Asia-Pacific region.[25] The continued use of these agents is being challenged by the emergence of insecticide resistance.[55]

Flame retardants

Bisphenol A diphenyl phosphate, 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 phosphorous-based agents which include organic phosphonate and phosphinate esters, in addition to inorganic salts.[56][57] When some prominent brominated flame retardant were banned in the early 2000s phosphorous-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.[58] Safety concerns have subsequently been raised about some of these reagents,[59][60] with several under regulatory scrutiny.[61][62]

Organophosphate flame retardants were first developed in the first half of the twentieth century in the from of triphenyl phosphate, tricresyl phosphate and tributyl phosphate for use in plastics like cellulose nitrate and cellulose acetate.[63] Use in cellulose products is still significant, but the largest area of application is now in plasticized vinyl polymers, principally PVC. The more modern organophosphate flame retardants come in 2 major types; chlorinated aliphatic compounds or aromatic diphosphates.[56] The chlorinated compounds TDCPP, TCPP and TCEP are all involatile liquids, of which TCPP is perhaps the most important. They are used in polyurethane (soft furnishings), PVC (wire and cable) phenolic resins and epoxy resinss (varnishes, coatings and adhesives). The most important of the diphosphates is bisphenol-A bis(diphenyl phosphate), with related analogues based around resorcinol and hydroquinone. These are used in polymer blends of engineering plastics, such as PPO/HIPS and PC/ABS,[64] 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.[62][65][66] The chlorinated organophosphates may be carcinogenic, while others such as tricresyl phosphate have necrotoxic properties.[67] 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, many are actually produced as complex mixtures. For instance, commercial grade TCPP can contain 7 different isomers,[68] while tricresyl phosphate can contain up to 10.[69] This makes their safety profiles harder to ascertain, as material from different producers can have different compositions.[70]

Plasticisers

Main page: Chemistry:Plasticizer
2-Ethylhexyl diphenyl phosphate 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, giving a softer more easily deformable material. In this way brittle polymers can be made more durable. Organophosphates find use because they are multifunctional; primarily plasticising but also imparting flame resistance. 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).[71] PVC dominates the market, consuming 80-90% of global plasticiser production.[71][72] PVC can accept large amounts of plasticiser; a PVC item may be 70-80% plasticiser by mass in extreme cases, but loadings of between 0-50% are more common.[73] The main applications of these products are in wire and cable insulation, flexible pipe, automotive interiors, plastic sheeting, vinyl flooring, and toys.

Pure PVC is more than 60% chlorine by mass and difficult to burn, but its flammability increases the more it is plasticised.[74] Organophosphates can act as both plasticisers and flame retarders. Compounds used are typically triaryl or alkyl diaryl phosphates, with cresyl diphenyl phosphate and 2-ethylhexyl diphenyl phosphate being important respective example.[75] 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.[76]

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.[77]

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.[78][79][80]

Metal extractants

Organophosphates have long been used in the field of extractive metallurgy to liberate valuable rare earths from their ores.[81] 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.[82] These compounds are also used for the PUREX (plutonium uranium reduction extraction) process, which is used for nuclear reprocessing.[83]

Surfactants

Mono- and di- phosphate esters of alcohols or alcohol ethoxylates are used as surfactants (detergents).[84] Compared to the more common sulfur-based anionic surfactants (such as LAS or SLES), phosphate ester surfactants are more expensive and generate less foam.[84] Benefits include stability over a broad pH range, low skin irritation and a high tolerance to dissolved salts.[85] In agricultural settings monoesters of fatty alcohol ethoxylates are used, which are able to disperse poorly miscible or insoluble pesticides into water. As they are low-foaming these mixtures can be sprayed effectively onto fields, while a high salt tolerance allows co-spraying of pesticides and inorganic fertilisers.[86] Low-levels of phosphate mono-esters, such as potassium cetyl phosphate, find use in cosmetic creams and lotions.[87] 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 agent in paints and concrete.

Nerve agents

Although the first phosphorous compounds observed to act as cholinesterase inhibitors were organophosphates,[88] 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.[89] 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.[90]

In nature

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.[22] OPEs were measured in high frequency in air and water and widely distributed in northern hemisphere.[91][92] 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.[92] Waters from rivers in Germany, Austria, and Spain have been consistently recorded for TBOEP and TCIPP at highest concentrations.[22] 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.

References

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  37. ↑ Spencer, E. Y.; O'Brien, R. D. (August 1953). "Schradan, Enhancement of Anticholinesterase Activity in Octamethylpyrophosphoramide by Chlorine". Journal of Agricultural and Food Chemistry 1 (11): 716–720. doi:10.1021/jf60011a003. 
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  48. ↑ Souza, Marília Cristina Oliveira; Cruz, Jonas Carneiro; Cesila, Cibele Aparecida; Gonzalez, Neus; Rocha, Bruno Alves; Adeyemi, Joseph A.; Nadal, Marti; Domingo, José L. et al. (July 2023). "Recent trends in pesticides in crops: A critical review of the duality of risks-benefits and the Brazilian legislation issue". Environmental Research 228: 115811. doi:10.1016/j.envres.2023.115811. PMID 37030406. 
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  50. ↑ Maggi, Federico; Tang, Fiona H. M.; la Cecilia, Daniele; McBratney, Alexander (12 September 2019). "PEST-CHEMGRIDS, global gridded maps of the top 20 crop-specific pesticide application rates from 2015 to 2025". Scientific Data 6 (1): 170. doi:10.1038/s41597-019-0169-4. PMID 31515508. 
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  52. ↑ Umetsu, Noriharu; Shirai, Yuichi (20 May 2020). "Development of novel pesticides in the 21st century". Journal of Pesticide Science 45 (2): 54–74. doi:10.1584/jpestics.D20-201. PMID 33132734. 
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  54. ↑ Siegfried, Blair D.; Scharf, Michael E. (2001). "Mechanisms of Organophosphate Resistance in Insects". Biochemical Sites of Insecticide Action and Resistance. pp. 269–291. doi:10.1007/978-3-642-59549-3_13. ISBN 978-3-540-67625-6. 
  55. ↑ 56.0 56.1 van der Veen, Ike; de Boer, Jacob (August 2012). "Phosphorus flame retardants: Properties, production, environmental occurrence, toxicity and analysis". Chemosphere 88 (10): 1119–1153. doi:10.1016/j.chemosphere.2012.03.067. PMID 22537891. Bibcode: 2012Chmsp..88.1119V. 
  56. ↑ Schmitt, Elmar (May 2007). "Phosphorus-based flame retardants for thermoplastics". Plastics, Additives and Compounding 9 (3): 26–30. doi:10.1016/S1464-391X(07)70067-3. 
  57. ↑ He, Huan; Gao, Zhanqi; Zhu, Donglin; Guo, Jiehong; Yang, Shaogui; Li, Shiyin; Zhang, Limin; Sun, Cheng (December 2017). "Assessing bioaccessibility and bioavailability of chlorinated organophosphorus flame retardants in sediments". Chemosphere 189: 239–246. doi:10.1016/j.chemosphere.2017.09.017. PMID 28942249. 
  58. ↑ Blum, Arlene; Behl, Mamta; Birnbaum, Linda S.; Diamond, Miriam L.; Phillips, Allison; Singla, Veena; Sipes, Nisha S.; Stapleton, Heather M. et al. (12 November 2019). "Organophosphate Ester Flame Retardants: Are They a Regrettable Substitution for Polybrominated Diphenyl Ethers?". Environmental Science & Technology Letters 6 (11): 638–649. doi:10.1021/acs.estlett.9b00582. PMID 32494578. 
  59. ↑ Du, Jia; Li, Huanxuan; Xu, Shaodan; Zhou, Qingwei; Jin, Meiqing; Tang, Junhong (August 2019). "A review of organophosphorus flame retardants (OPFRs): occurrence, bioaccumulation, toxicity, and organism exposure". Environmental Science and Pollution Research 26 (22): 22126–22136. doi:10.1007/s11356-019-05669-y. PMID 31243659. 
  60. ↑ "ECHA identifies certain brominated flame retardants as candidates for restriction". European Chemicals Agency. https://echa.europa.eu/-/echa-identifies-certain-brominated-flame-retardants-as-candidates-for-restriction. 
  61. ↑ 62.0 62.1 "Regulatory strategy for flame retardant". European Chemicals Agency. https://echa.europa.eu/documents/10162/2082415/flame_retardants_strategy_en.pdf/. doi:10.2823/854233
  62. ↑ Weil, E.D.; Levchik, S.V. (26 January 2001). "Phosphorus Flame Retardants". Kirk-Othmer Encyclopedia of Chemical Technology: 1–34. doi:10.1002/0471238961.1608151923050912.a01.pub3. ISBN 978-0-471-48494-3. 
  63. ↑ Pawlowski, Kristin H; Schartel, Bernhard (November 2007). "Flame retardancy mechanisms of triphenyl phosphate, resorcinol bis(diphenyl phosphate) and bisphenol A bis(diphenyl phosphate) in polycarbonate/acrylonitrile–butadiene–styrene blends". Polymer International 56 (11): 1404–1414. doi:10.1002/pi.2290. 
  64. ↑ Blum, Arlene; Behl, Mamta; Birnbaum, Linda S.; Diamond, Miriam L.; Phillips, Allison; Singla, Veena; Sipes, Nisha S.; Stapleton, Heather M. et al. (12 November 2019). "Organophosphate Ester Flame Retardants: Are They a Regrettable Substitution for Polybrominated Diphenyl Ethers?". Environmental Science & Technology Letters 6 (11): 638–649. doi:10.1021/acs.estlett.9b00582. PMID 32494578. 
  65. ↑ Du, Jia; Li, Huanxuan; Xu, Shaodan; Zhou, Qingwei; Jin, Meiqing; Tang, Junhong (August 2019). "A review of organophosphorus flame retardants (OPFRs): occurrence, bioaccumulation, toxicity, and organism exposure". Environmental Science and Pollution Research 26 (22): 22126–22136. doi:10.1007/s11356-019-05669-y. PMID 31243659. 
  66. ↑ Barth, Mary L.; Craig, Peter H. (October 1999). "Evaluation of the hazards of industrial exposure to tricresyl phosphate:a review and interpretation of the literature". Journal of Toxicology and Environmental Health, Part B 2 (4): 281–300. doi:10.1080/109374099281142. PMID 10596299. 
  67. ↑ Truong, Jimmy W.; Diamond, Miriam L.; Helm, Paul A.; Jantunen, Liisa M. (December 2017). "Isomers of tris(chloropropyl) phosphate (TCPP) in technical mixtures and environmental samples". Analytical and Bioanalytical Chemistry 409 (30): 6989–6997. doi:10.1007/s00216-017-0572-7. PMID 29147747. 
  68. ↑ Amiri, Roshanak; Bissram, Meera J.; Hashemihedeshi, Mahin; Dorman, Frank L.; Megson, David; Jobst, Karl J. (5 April 2023). "Differentiating Toxic and Nontoxic Tricresyl Phosphate Isomers Using Ion–Molecule Reactions with Oxygen". Journal of the American Society for Mass Spectrometry 34 (4): 640–648. doi:10.1021/jasms.2c00334. PMID 36942790. 
  69. ↑ Duarte, Daniel J.; Rutten, Joost M.M.; van den Berg, Martin; Westerink, Remco H.S. (March 2017). "In vitro neurotoxic hazard characterization of different tricresyl phosphate (TCP) isomers and mixtures". NeuroToxicology 59: 222–230. doi:10.1016/j.neuro.2016.02.001. PMID 26851706. 
  70. ↑ 71.0 71.1 "Plasticizers". Ullmann's Encyclopedia of Industrial Chemistry 27: 613–614. 15 June 2000. doi:10.1002/14356007.a20_439. ISBN 3527306730. 
  71. ↑ Rahman, M; Brazel, C (December 2004). "The plasticizer market: an assessment of traditional plasticizers and research trends to meet new challenges". Progress in Polymer Science 29 (12): 1223–1248. doi:10.1016/j.progpolymsci.2004.10.001. 
  72. ↑ "3.13 Plasticizers". Plastics additives handbook (6. ed.). Munich: Carl Hanser Verlag. 2009. pp. 485–511. ISBN 978-3-446-40801-2. 
  73. ↑ William Coaker, A. (September 2003). "Fire and flame retardants for PVC". Journal of Vinyl and Additive Technology 9 (3): 108–115. doi:10.1002/vnl.10072. 
  74. ↑ Grossman, Richard F (2008-05-02). Handbook of Vinyl Formulating. John Wiley & Sons. p. 289. ISBN 978-0-470-25354-0. 
  75. ↑ Levchik, Sergei V.; Weil, Edward D. (October 2005). "Overview of the recent literature on flame retardancy and smoke suppression in PVC". Polymers for Advanced Technologies 16 (10): 707–716. doi:10.1002/pat.645. 
  76. ↑ Rudnick, L.R. (2013). "Chapter 4: Neutral Phosphate Esters". Synthetics, Mineral Oils, and Bio-Based Lubricants: Chemistry and Technology, Second Edition. Chemical Industries. CRC Press. pp. 81–104. doi:10.1201/9781315158150-4. ISBN 978-1-4398-5538-6. https://books.google.com/books?id=9YXRBQAAQBAJ&pg=PA81. Retrieved 2024-01-07. 
  77. ↑ Guan, Bihan; Pochopien, Bernadeta A.; Wright, Dominic S. (August 2016). "The chemistry, mechanism and function of tricresyl phosphate (TCP) as an anti-wear lubricant additive". Lubrication Science 28 (5): 257–265. doi:10.1002/ls.1327. 
  78. ↑ Johnson, David; Hils, John (18 December 2013). "Phosphate Esters, Thiophosphate Esters and Metal Thiophosphates as Lubricant Additives". Lubricants 1 (4): 132–148. doi:10.3390/lubricants1040132. 
  79. ↑ Li, Haogang; Zhang, Yanbin; Li, Changhe; Zhou, Zongming; Nie, Xiaolin; Chen, Yun; Cao, Huajun; Liu, Bo et al. (May 2022). "Extreme pressure and antiwear additives for lubricant: academic insights and perspectives". The International Journal of Advanced Manufacturing Technology 120 (1–2): 1–27. doi:10.1007/s00170-021-08614-x. 
  80. ↑ Hidayah, Nur Nadiatul; Abidin, Sumaiya Zainal (June 2018). "The evolution of mineral processing in extraction of rare earth elements using liquid-liquid extraction: A review". Minerals Engineering 121: 146–157. doi:10.1016/j.mineng.2018.03.018. 
  81. ↑ Xie, Feng; Zhang, Ting An; Dreisinger, David; Doyle, Fiona (February 2014). "A critical review on solvent extraction of rare earths from aqueous solutions". Minerals Engineering 56: 10–28. doi:10.1016/j.mineng.2013.10.021. 
  82. ↑ Paiva, A. P.; Malik, P. (2004). "Recent advances on the chemistry of solvent extraction applied to the reprocessing of spent nuclear fuels and radioactive wastes". Journal of Radioanalytical and Nuclear Chemistry 261 (2): 485–496. doi:10.1023/B:JRNC.0000034890.23325.b5. 
  83. ↑ 84.0 84.1 Farn, R.J. (2008). Chemistry and Technology of Surfactants. Wiley. pp. 122–124. ISBN 978-1-4051-7179-3. https://books.google.com/books?id=CCn81pnPrgwC&pg=PP2. Retrieved 2023-05-27. 
  84. ↑ Arora, Pinklesh; Singh, Rakhi; Seshadri, Geetha; Tyagi, Ajay Kumar (16 July 2018). "Synthesis, Properties and Applications of Anionic Phosphate Ester Surfactants: A Review". Tenside Surfactants Detergents 55 (4): 266–272. doi:10.3139/113.110570. 
  85. ↑ Kaneko, T.M.; Spicer, L.D. (1985). Pesticide Formulations and Application Systems: Fourth Symposium : a Symposium Sponsored by ASTM Committee E-35 on Pesticides, New Orleans, La., 2-3 Nov. 1983. ASTM special technical publication. ASTM. pp. 5–14. ISBN 978-0-8031-0413-6. https://books.google.com/books?id=NSvmvgBsn8cC&pg=PA5. Retrieved 2023-05-27. 
  86. ↑ Miller, Dennis; Wiener, Eva-Maria; Turowski, Angelika; Thunig, Christine; Hoffmann, Heinz (July 1999). "O/W emulsions for cosmetics products stabilized by alkyl phosphates — rheology and storage tests". Colloids and Surfaces A: Physicochemical and Engineering Aspects 152 (1–2): 155–160. doi:10.1016/S0927-7757(98)00630-X. 
  87. ↑ Petroianu, G. A. (2010-10-01). "Toxicity of phosphor esters: Willy Lange (1900-1976) and Gerda von Krueger (1907-after 1970)". Die Pharmazie 65 (10): 776–780. ISSN 0031-7144. PMID 21105582. 
  88. ↑ Chai, Peter R.; Hayes, Bryan D.; Erickson, Timothy B.; Boyer, Edward W. (January 2018). "Novichok agents: a historical, current, and toxicological perspective". Toxicology Communications 2 (1): 45–48. doi:10.1080/24734306.2018.1475151. PMID 30003185. 
  89. ↑ Vale, J. Allister; Marrs, Timothy C.; Maynard, Robert L. (2 November 2018). "Novichok: a murderous nerve agent attack in the UK". Clinical Toxicology 56 (11): 1093–1097. doi:10.1080/15563650.2018.1469759. PMID 29757015. 
  90. ↑ Salamova, Amina; Ma, Yuning; Venier, Marta; Hites, Ronald A. (2014-01-14). "High Levels of Organophosphate Flame Retardants in the Great Lakes Atmosphere". Environmental Science & Technology Letters 1 (1): 8–14. doi:10.1021/ez400034n. https://doi.org/10.1021/ez400034n. 
  91. ↑ 92.0 92.1 Venier, Marta; Dove, Alice; Romanak, Kevin; Backus, Sean; Hites, Ronald (2014-08-19). "Flame Retardants and Legacy Chemicals in Great Lakes' Water" (in en). Environmental Science & Technology 48 (16): 9563–9572. doi:10.1021/es501509r. ISSN 0013-936X. PMID 25045802. Bibcode: 2014EnST...48.9563V. https://pubs.acs.org/doi/10.1021/es501509r. 



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