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. They can be considered as esters of phosphoric acid. Organophosphates are best known for their use as pesticides. 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 low cost of production and compatibility with diverse polymers allowed 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. The popularity of OPEs as flame retardants came as a substitution for the highly regulated brominated flame retardants.
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 hydrolysis of triesters can form diesters and monoesters.
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.
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.
Synthesis
Alcoholysis of POCl3 Phosphorus oxychloride reacts readily with alcohols to give organophosphates. This is the dominant industrial route and is responsible for almost all organophosphate production. Aluminum trichloride or magnesium chloride are often employed as catalysts.
O=PCl3 + 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. Despite these limitations the reaction does see industrial use for the formation of monoalkyl phosphates, which are used as surfactants. 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) + H2O 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.
Oxidation of phosphite esters Organophosphites can be easily oxidised to give organophosphates. This is not a common industrial route, however large quantities of organophosphites are manufactured as antioxidant stabilisers for plastics. The gradual oxidation of these generates organophosphates in the human environment.
P(OR)3 + [O] → OP(OR)3
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. Protein phosphorylation is the most abundant post-translational modification in eukaryotes. Many enzymes and receptors are switched "on" or "off" by phosphorylation and dephosphorylation.
Laboratory methods Various specialised methods have been developed on the laboratory-scale for scientific investigations. These are rarely employed in bulk manufacturing. Examples include the Atherton-Todd reaction, which converts a dialkyl phosphite to a phosphoryl chloride. This can then react with an alcohol to give an organophosphate and HCl.
Properties
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