Triphenylphosphate
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Triphenyl phosphate (TPhP) is the
chemical compound A chemical compound is a chemical substance composed of many identical molecules (or molecular entities) containing atoms from more than one chemical element held together by chemical bonds. A molecule consisting of atoms of only one element ...
with the formula OP(OC6H5)3. This colourless solid is the
ester In chemistry, an ester is a compound derived from an oxoacid (organic or inorganic) in which at least one hydroxyl group () is replaced by an alkoxy group (), as in the substitution reaction of a carboxylic acid and an alcohol. Glycerides a ...
(triester) of
phosphoric acid Phosphoric acid (orthophosphoric acid, monophosphoric acid or phosphoric(V) acid) is a colorless, odorless phosphorus-containing solid, and inorganic compound with the chemical formula . It is commonly encountered as an 85% aqueous solution, w ...
and
phenol Phenol (also called carbolic acid) is an aromatic organic compound with the molecular formula . It is a white crystalline solid that is volatile. The molecule consists of a phenyl group () bonded to a hydroxy group (). Mildly acidic, it ...
. It is used as a
plasticizer A plasticizer ( UK: plasticiser) is a substance that is added to a material to make it softer and more flexible, to increase its plasticity, to decrease its viscosity, and/or to decrease friction during its handling in manufacture. Plasticiz ...
and a
fire retardant A fire retardant is a substance that is used to slow down or stop the spread of fire or reduce its intensity. This is commonly accomplished by chemical reactions that reduce the flammability of fuels or delay their combustion. Fire retardants m ...
in a wide variety of settings and products.


Preparation

Triphenyl phosphate is prepared by the
SN2 reaction The SN2 reaction is a type of reaction mechanism that is common in organic chemistry. In this mechanism, one bond is broken and one bond is formed in a concerted way, i.e., in one step. The name SN2 refers to the Hughes-Ingold symbol of the m ...
of
phosphorus oxychloride Phosphoryl chloride (commonly called phosphorus oxychloride) is a colourless liquid with the formula . It hydrolyses in moist air releasing phosphoric acid and fumes of hydrogen chloride. It is manufactured industrially on a large scale from pho ...
and
phenol Phenol (also called carbolic acid) is an aromatic organic compound with the molecular formula . It is a white crystalline solid that is volatile. The molecule consists of a phenyl group () bonded to a hydroxy group (). Mildly acidic, it ...
.


Uses

Triphenyl phosphate has been used widely as a flame retardant and plasticizer. It has been used as a flame retardant for a variety of materials, including electronic equipment, PVC, hydraulic fluids, glues, in nail polishes, and casting resins. Its mechanism of action as a flame retardant is as follows: first, during
thermal decomposition Thermal decomposition, or thermolysis, is a chemical decomposition caused by heat. The decomposition temperature of a substance is the temperature at which the substance chemically decomposes. The reaction is usually endothermic as heat is re ...
,
phosphoric acid Phosphoric acid (orthophosphoric acid, monophosphoric acid or phosphoric(V) acid) is a colorless, odorless phosphorus-containing solid, and inorganic compound with the chemical formula . It is commonly encountered as an 85% aqueous solution, w ...
is formed. This reacts to form
pyrophosphoric acid Pyrophosphoric acid, also known as diphosphoric acid, is the inorganic compound with the formula H4P2O7 or, more descriptively, HO)2P(O)sub>2O. Colorless and odorless, it is soluble in water, diethyl ether, and ethyl alcohol. The anhydrous acid c ...
, which, when in its condensed phase, acts to block heat transfer. One of the most effective flame retardants for certain polymers, TPhP is only active as an additive flame retardant in its gas phase. Phase out of PBDEs may have increased the use of TPhP in recent years. TPhP is also used as a
plasticizer A plasticizer ( UK: plasticiser) is a substance that is added to a material to make it softer and more flexible, to increase its plasticity, to decrease its viscosity, and/or to decrease friction during its handling in manufacture. Plasticiz ...
in lacquers, varnishes, and hydraulic fluids. Nail polish has received particular interest as a source of exposure to TPhP.


Toxicology

Limited information is available indicating significant
toxicological Toxicology is a scientific discipline, overlapping with biology, chemistry, pharmacology, and medicine, that involves the study of the adverse effects of chemical substances on living organisms and the practice of diagnosing and treating expo ...
effects of TPhP. Although it was initially expected to have an overall low impact, a growing body of evidence suggests that the effects may not be so harmless. Triphenyl phosphate exhibits low
acute toxicity Acute toxicity describes the adverse effects of a substance that result either from a single exposure or from multiple exposures in a short period of time (usually less than 24 hours). To be described as ''acute'' toxicity, the adverse effect ...
by dermal or oral contact. However, an increasing number of studies have linked exposure to TPhP with reproductive and
developmental toxicity Development of the human body is the process of growth to maturity. The process begins with fertilization, where an egg released from the ovary of a female is penetrated by a sperm cell from a male. The resulting zygote develops through m ...
, neurotoxicity, metabolic disruption, endocrine effects, and
genotoxicity Genotoxicity is the property of chemical agents that damage the genetic information within a cell causing mutations, which may lead to cancer. While genotoxicity is often confused with mutagenicity, all mutagens are genotoxic, but some genotoxic su ...
. TPhP has also been found to induce significant estrogenic activity. One study found that concentrations above the lowest observable effect level have been observed in a variety of other studies in coral grouper, yellow striped goat fish, and freshwater perch. This indicates that TPhP may be present in the environment at high enough concentrations to have harmful ecological effects. The European Chemicals agency considers TPhP to be "very toxic" to aquatic life, with potentially long-lasting effects. In contrast to many
persistent organic pollutants Persistent organic pollutants (POPs), sometimes known as "forever chemicals", are organic compounds that are resistant to environmental degradation through chemical, biological, and photolytic processes. They are toxic chemicals that adversel ...
, TPHP has limited affinity for
lipid Lipids are a broad group of naturally-occurring molecules which includes fats, waxes, sterols, fat-soluble vitamins (such as vitamins A, D, E and K), monoglycerides, diglycerides, phospholipids, and others. The functions of lipids includ ...
s. Still, bioaccumulation of the compound has been found to occur at varying levels in fish, with the strongest patterns showing up based on gender, feeding patterns, and metabolic efficiency. However, the mechanisms explaining why and how TPhP accumulates in this manner are not yet known.


Environmental transport and transformations

Triphenyl phosphate has been detected in the environment. Other triaryl phosphates have been known to enter aquatic environments through volatilization and leaching from plastics, through hydraulic fluid leakages, and, to a lesser degree, through manufacturing processes. TPhP, in particular, has been found to enter the environment through industrial use, as in the manufacturing process, and through indoor use, for example through paints and electronic equipment. As with many other phosphorus-containing flame retardants, TPhP has been found widely in sediment, soil, indoor dust, and air. Once in water, TPhP has been found to
biodegrade Biodegradation is the breakdown of organic matter by microorganisms, such as bacteria and fungi. It is generally assumed to be a natural process, which differentiates it from composting. Composting is a human-driven process in which biodegradati ...
relatively quickly under both aerobic and anaerobic conditions, and does not meet criteria for being categorized as persistent. However, although the compound biodegrades easily and does not bioaccumulate, it is readily detected because of the sheer volume that is utilized. In 2014, the
United States Environmental Protection Agency The Environmental Protection Agency (EPA) is an independent executive agency of the United States federal government tasked with environmental protection matters. President Richard Nixon proposed the establishment of EPA on July 9, 1970; it ...
added TPhP to its list of Toxic Substance Control Act Work Plan for Chemicals on the basis that the compound has exhibited "acute and chronic aquatic toxicity," "moderate bioaccumulation potential," and "moderate environmental persistence." Still, there is not yet enough information to fully assess the environmental impact of TPhP.


References

{{Reflist Aromatic compounds Organophosphates Phosphate esters