Organophosphorus chemistry is the scientific study of the synthesis and properties oforganophosphorus compounds, which areorganic compounds containingphosphorus.[1] They are used primarily inpest control as an alternative tochlorinated hydrocarbons that persist in the environment. Some organophosphorus compounds are highly effectiveinsecticides, although some are extremely toxic to humans, includingsarin andVX nerve agents.[2]
Phosphorus, likenitrogen, is ingroup 15 of the periodic table, and thus phosphorus compounds and nitrogen compounds have many similar properties.[3][4][5] The definition of organophosphorus compounds is variable, which can lead to confusion. In industrial and environmental chemistry, an organophosphorus compound need contain only an organicsubstituent, but need not have a direct phosphorus-carbon (P-C) bond.[citation needed] Thus a large proportion of pesticides (e.g.,malathion), are often included in this class of compounds.
Phosphorus can adopt a variety ofoxidation states, 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 theircoordination numberσ and theirvalencyλ. In this system, a phosphine is a σ3λ3 compound.
Phosphate esters have the general structure P(=O)(OR)3 feature P(V). Such species are of technological importance asflame retardant agents, andplasticizers. Lacking a P−C bond, these compounds are in the technical sense not organophosphorus compounds but esters of phosphoric acid. Many derivatives are found in nature, such asphosphatidylcholine. Phosphate ester are synthesized byalcoholysis of phosphorus oxychloride. A variety of mixed amido-alkoxo derivatives are known, one medically significant example being the anti-cancer drugcyclophosphamide. Also derivatives containing the thiophosphoryl group (P=S) include the pesticidemalathion. The organophosphates prepared on the largest scale are thezinc dithiophosphates, as additives for motor oil. Several million kilograms of thiscoordination complex are produced annually by the reaction of phosphorus pentasulfide with alcohols.[6]
Phosphoryl thioates are thermodynamically much stabler than thiophosphates, which can rearrange at high temperature or with a catalytic alkylant to the former:[7]: 73–76
In the environment, all these phosphorus(V) compounds break down viahydrolysis to eventually affordphosphate and the organic alcohol or amine from which they are derived.
Phosphonates are esters of phosphonic acid and have the general formula RP(=O)(OR')2. Phosphonates have many technical applications, a well-known member beingglyphosate, better known as Roundup. With the formula (HO)2P(O)CH2NHCH2CO2H, this derivative ofglycine is one of the most widely used herbicides.Bisphosphonates are a class of drugs to treatosteoporosis. The nerve gas agentsarin, containing both C–P and F–P bonds, is a phosphonate.[citation needed]
Phosphinates featuretwo P–C bonds, with the general formula R2P(=O)(OR'). A commercially significant member is the herbicideglufosinate. Similar to glyphosate mentioned above, it has the structure CH3P(O)(OH)CH2CH2CH(NH2)CO2H.
TheMichaelis–Arbuzov reaction is the main method for the synthesis of these compounds. For example, dimethylmethylphosphonate (see figure above) arises from the rearrangement oftrimethylphosphite, which is catalyzed bymethyl iodide. In theHorner–Wadsworth–Emmons reaction and theSeyferth–Gilbert homologation, phosphonates are used in reactions withcarbonyl compounds. TheKabachnik–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.[citation needed]
Phosphine oxides (designation σ4λ5) have the general structure R3P=O with formal oxidation state +5. Phosphine oxides formhydrogen bonds and some are therefore soluble in water. The P=O bond is very polar with adipole moment of 4.51 D fortriphenylphosphine oxide.[citation needed]
Compounds related to phosphine oxides includephosphine imides (R3PNR') and relatedchalcogenides (R3PE, where E =S,Se,Te). 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:[7]: 73
Some phosphorus sulfides can undergo a reverseArbuzov rearrangement to a dialkylthiophosphinate ester.[7]: 55
Compounds with the formula [PR4+]X− comprise thephosphonium salts. These species are tetrahedral phosphorus(V) compounds. From the commercial perspective, the most important member istetrakis(hydroxymethyl)phosphonium chloride, [P(CH2OH)4]Cl, which is used as a fire retardant intextiles. Approximately 2M kg are produced annually of the chloride and the related sulfate.[6] They are generated by the reaction of phosphine withformaldehyde in the presence of the mineral acid:
A variety of phosphonium salts can be prepared byalkylation andarylation of organophosphines:
The methylation of triphenylphosphine is the first step in the preparation of the Wittig reagent.
The parentphosphorane (σ5λ5) is PH5, which is unknown.[citation needed] 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 fromP(C6H5)4+ by reaction withphenyllithium.[citation needed]
Phosphorusylides are unsaturated phosphoranes, known asWittig reagents, 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.[citation needed]
Phosphites, sometimes calledphosphite esters, 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 thePerkow reaction and theMichaelis–Arbuzov reaction. They also serve as ligands in organometallic chemistry.
Intermediate between phosphites and phosphines arephosphonites (P(OR)2R') andphosphinite (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 aphosphorus trichloride with apoor metal-alkyl complex, e.g.organomercury,organolead, or a mixedlithium-organoaluminum compound.[8]
The parent compound of the phosphines is PH3, calledphosphine in the US and British Commonwealth, but phosphane elsewhere.[9] Replacement of one or more hydrogen centers by an organic substituents (alkyl, aryl), gives PH3−xRx, an organophosphine, generally referred to as phosphines.[citation needed]
From the commercial perspective, the most important phosphine istriphenylphosphine, several million kilograms being produced annually. It is prepared from the reaction ofchlorobenzene,PCl3, and sodium.[6] Phosphines of a more specialized nature are usually prepared by other routes.[10] Phosphorus halides undergonucleophilic displacement by organometallic reagents such asGrignard reagents. Organophosphines are nucleophiles andligands. Two major applications are as reagents in theWittig reaction and as supportingphosphine ligands inhomogeneous catalysis.[citation needed]
Their nucleophilicity is evidenced by their reactions withalkyl halides to givephosphonium salts. Phosphines arenucleophilic catalysts inorganic synthesis, e.g. theRauhut–Currier reaction andBaylis-Hillman reaction. Phosphines arereducing agents, as illustrated in theStaudinger reduction for the conversion of organic azides to amines and in theMitsunobu 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.[11]
A few halophosphines are known, although phosphorus' strongnucleophilicity predisposes them to decomposition, anddimethylphosphinyl fluoride spontaneously disproportionates todimethylphosphine trifluoride andtetramethylbiphosphine.[12] One common synthesis adds halogens totetramethylbiphosphine disulfide.[13] Alternatively alkylation ofphosphorus trichloride gives a halophosphonium cation, which metals reduce to halophosphines.
Compounds with carbon phosphorus(III) multiple bonds are calledphosphaalkenes (R2C=PR) andphosphaalkynes (RC≡P). They are similar in structure, but not in reactivity, toimines (R2C=NR) andnitriles (RC≡N), respectively. In the compoundphosphorine, 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 by1,2-elimination of suitable precursors, initiated thermally or by base such asDBU,DABCO, ortriethylamine:
Thermolysis of Me2PH generates CH2=PMe, an unstable species in the condensed phase.
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 anN-heterocyclic carbene.[14] With the formulae (RP)n and (R2P)2, respectively,compounds of phosphorus(I) and (II) are generated by reduction of the related organophosphorus(III) chlorides:[citation needed]
Diphosphenes, 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 preventcatenation. Bulky substituents alsostabilize phosphorus radicals.
Manymixed-valence compounds are known, e.g. the cage P7(CH3)3.
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