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Organophosphorus chemistry is the scientific study of the synthesis and properties of organophosphorus compounds, which are containing phosphorus. They are used primarily in pest control as an alternative to chlorinated hydrocarbons that persist in the environment. Some organophosphorus compounds are highly effective , although some are extremely toxic to humans, including sarin and VX nerve agents.
Phosphorus, like nitrogen, is in pnictogen of the periodic table, and thus phosphorus compounds and nitrogen compounds have many similar properties.Dillon, K. B.; Mathey, F.; Nixon, J. F. (1997) Phosphorus. The Carbon Copy; John Wiley & Sons, Quin, L. D. (2000) A Guide to Organophosphorus Chemistry; John Wiley & Sons, Racke, K.D. (1992). "Degradation of organophosphorus insecticides in environmental matrices", pp. 47–73 in: Chambers, J.E., Levi, P.E. (eds.), Organophosphates: Chemistry, Fate, and Effects. Academic Press, San Diego, . The definition of organophosphorus compounds is variable, which can lead to confusion. In industrial and environmental chemistry, an organophosphorus compound need contain only an organic substituent, but need not have a direct phosphorus-carbon (P−C) bond. Thus a large proportion of pesticides (e.g., malathion), are often included in this class of compounds.
Phosphorus can adopt a variety of , and it is general to classify organophosphorus compounds based on their being derivatives of phosphorus(V) vs phosphorus(III), which are the predominant classes of compounds. In a descriptive but only intermittently used nomenclature, phosphorus compounds are identified by their coordination number σ and their valency lambda. In this system, a phosphine is a σ3λ3 compound.
Phosphoryl thioates are thermodynamically much stabler than thiophosphates, which can rearrange at high temperature or with a catalytic alkylant to the former:
In the environment, all these phosphorus(V) compounds break down via hydrolysis to eventually afford phosphate and the organic alcohol or amine from which they are derived.
Phosphinates feature two P–C bonds, with the general formula R2P(=O)(OR'). A commercially significant member is the herbicide glufosinate. Similar to glyphosate mentioned above, it has the structure CH3P(O)(OH)CH2CH2CH(NH2)CO2H.
The Michaelis–Arbuzov reaction is the main method for the synthesis of these compounds. For example, dimethylmethylphosphonate (see figure above) arises from the rearrangement of trimethylphosphite, which is catalyzed by methyl iodide. In the Horner–Wadsworth–Emmons reaction and the Seyferth–Gilbert homologation, phosphonates are used in reactions with carbonyl compounds. The Kabachnik–Fields reaction is a method for the preparation of aminophosphonates. These compounds contain a very inert bond between phosphorus and carbon. Consequently, they hydrolyze to give phosphonic and phosphinic acid derivatives, but not phosphate.
Compounds related to phosphine oxides include (R3PNR') and related (R3PE, where E = sulfur, selenium, tellurium). These compounds are some of the most thermally stable organophosphorus compounds. In general, they are less basic than the corresponding phosphine oxides, which can adduce to thiophosphoryl halides:
Some phosphorus sulfides can undergo a reverse Arbuzov rearrangement to a dialkylthiophosphinate ester.
The parent phosphorane (σ5λ5) is PH5, which is unknown. Related compounds containing both halide and organic substituents on phosphorus are fairly common. Those with five organic substituents are rare, although P(C6H5)5 is known, being derived from P(C6H5)4+ by reaction with phenyllithium.
Phosphorus are unsaturated phosphoranes, known as , e.g. CH2P(C6H5)3. These compounds feature tetrahedral phosphorus(V) and are considered relatives of phosphine oxides. They also are derived from phosphonium salts, but by deprotonation not alkylation.
Intermediate between phosphites and phosphines are (P(OR)2R') and phosphinite (P(OR)R'2). Such species arise via alcoholysis reactions of the corresponding phosphonous and phosphinous chlorides ((PCl2R') and (PClR'2) , respectively). The latter are produced by reaction of a phosphorus trichloride with a poor metal-alkyl complex, e.g. organomercury, organolead, or a mixed lithium-organoaluminum compound.
From the commercial perspective, the most important phosphine is triphenylphosphine, several million kilograms being produced annually. It is prepared from the reaction of chlorobenzene, PCl3, and sodium. Phosphines of a more specialized nature are usually prepared by other routes.
Their nucleophilicity is evidenced by their reactions with to give . Phosphines are organocatalysis in organic synthesis, e.g. the Rauhut–Currier reaction and Baylis-Hillman reaction. Phosphines are , as illustrated in the Staudinger reduction for the conversion of organic azides to amines and in the Mitsunobu reaction for converting alcohols into esters. In these processes, the phosphine is oxidized to phosphorus(V). Phosphines have also been found to reduce activated carbonyl groups, for instance the reduction of an α-keto ester to an α-hydroxy ester.
A few halophosphines are known, although phosphorus' strong predisposes them to decomposition, and dimethylphosphinyl fluoride spontaneously disproportionates to dimethylphosphine trifluoride and tetramethylbiphosphine. One common synthesis adds halogens to tetramethylbiphosphine disulfide. Adapted from Alternatively alkylation of phosphorus trichloride gives a halophosphonium cation, which metals reduce to halophosphines.
Many mixed-valence compounds are known, e.g. the cage P7(CH3)3.
Phosphonic and phosphinic acids and their esters
are esters of phosphonic acid and have the general formula RP(=O)(OR')2. Phosphonates have many technical applications, a well-known member being glyphosate, better known as Roundup. With the formula (HO)2P(O)CH2NHCH2CO2H, this derivative of glycine is one of the most widely used herbicides. are a class of drugs to treat osteoporosis. The nerve gas agent sarin, containing both C–P and F–P bonds, is a phosphonate.
Phosphine oxides, imides, and chalcogenides
Phosphine oxides (designation σ4λ5) have the general structure R3P=O with formal oxidation state +5. Phosphine oxides form and some are therefore soluble in water. The P=O bond is very polar with a dipole moment of 4.51 D for triphenylphosphine oxide.
Phosphonium salts and phosphoranes
Compounds with the formula PR4+X− comprise the phosphonium. These species are tetrahedral phosphorus(V) compounds. From the commercial perspective, the most important member is tetrakis(hydroxymethyl)phosphonium chloride, P(CH2OH)4Cl, which is used as a fire retardant in . Approximately 2M kg are produced annually of the chloride and the related sulfate. They are generated by the reaction of phosphine with formaldehyde in the presence of the mineral acid:
A variety of phosphonium salts can be prepared by alkylation and arylation of organophosphines:
The methylation of triphenylphosphine is the first step in the preparation of the Wittig reagent.
Organophosphorus(III) compounds, main categories
Phosphites, phosphonites, and phosphinites
Phosphites, sometimes called , have the general structure P(OR)3 with oxidation state +3. Such species arise from the alcoholysis of phosphorus trichloride:
The reaction is general, thus a vast number of such species are known. Phosphites are employed in the Perkow reaction and the Michaelis–Arbuzov reaction. They also serve as ligands in organometallic chemistry.
Phosphines
The parent compound of the phosphines is PH3, called phosphine in the US and British Commonwealth, but phosphane elsewhere. Replacement of one or more hydrogen centers by an organic substituents (alkyl, aryl), gives PH3−xRx, an organophosphine, generally referred to as phosphines.
(2025). 9780080437484 ISBN 9780080437484 Phosphorus halides undergo nucleophilic displacement by organometallic reagents such as . Organophosphines are nucleophiles and . Two major applications are as reagents in the Wittig reaction and as supporting in homogeneous catalysis.
Phosphaalkenes and phosphaalkynes
Compounds with carbon phosphorus(III) multiple bonds are called (R2C=PR) and (RC≡P). They are similar in structure, but not in reactivity, to (R2C=NR) and (RC≡N), respectively. In the compound phosphorine, one carbon atom in benzene is replaced by phosphorus. Species of this type are relatively rare but for that reason are of interest to researchers. A general method for the synthesis of phosphaalkenes is by 1,2-elimination of suitable precursors, initiated thermally or by base such as DBU, DABCO, or triethylamine:
Thermolysis of Me2PH generates CH2=PMe, an unstable species in the condensed phase.
Organophosphorus(0), (I), and (II) compounds
Compounds where phosphorus exists in a formal oxidation state of less than III are uncommon, but examples are known for each class. Organophosphorus(0) species are debatably illustrated by the carbene adducts, P(NHC)2, where NHC is an N-heterocyclic carbene. With the formulae (RP)n and (R2P)2, respectively, cyclophosphines and (II) are generated by reduction of the related organophosphorus(III) chlorides:
, with the formula R2P2, formally contain phosphorus-phosphorus double bonds. These phosphorus(I) species are rare but are stable provided that the organic substituents are large enough to prevent catenation. Bulky substituents also stabilize phosphorus radicals.
See also
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