Organophosphate
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.
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.
[4] OPEs have been detected in diverse environmental compartments such as air, dust, water, sediment, soil and biota samples at higher frequency and concentration.
For instance, the phosphates of adenosine (AMP, ADP, ATP) play a key role in many metabolic processes.
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.
[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 an oxygen 2p orbital.
[21] This is of great practical importance, as it makes these compounds far more resistant to degradation by hydrolysis or other forms of nucleophilic attack, due to electrostatic repulsion between negative charges.
Tri-esters such as flame retardants and plasticisers have positive log Kow values ranging between 1.44 and 9.49, signifying hydrophobicity.
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.
Nearly 100 were commercialised, with the following being a varied selection: 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[32] was obvious from the start and led to efforts to lower toxicity against mammals while not reducing efficacy against insects.
[33][34] 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).
[39][40][41] 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)[42] 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.
[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.
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.
[56][57] When some prominent brominated flame retardant were banned in the early 2000s phosphorus-based agents were promoted as safer replacements.
[63] 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.
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.
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.
[68] Plasticisers are added to polymers and plastics to improve their flexibility and processability, giving a softer more easily deformable material.
[76][77][78] Organophosphates have long been used in the field of extractive metallurgy to liberate valuable rare earth metals from their ores.
[7] 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.
Although the first phosphorus compounds observed to act as cholinesterase inhibitors were organophosphates,[85] the vast majority of nerve agents are instead phosphonates containing a P-C bond.
[89] Waters from rivers in Germany, Austria, and Spain have been consistently recorded for TBOEP and TCIPP at highest concentrations.
