1. History

2. Structure and properties

3. Preparation and occurrence
    Laboratory routes  Occurrence 

4. Phosphines
    Primary phosphines  Secondary phosphines  Tertiary phosphines  Cyclic phosphines 

5. Applications
    Organophosphorus chemistry  Microelectronics  Fumigant 

6. Safety

7. Toxicity

8. See also

9. References

10. Further reading

11. External links

Phosphine (IUPAC name: phosphane) is the compound with the chemical formula PH₃. It is a colorless, flammable, toxic gas and pnictogen hydride. Pure phosphine is odorless, but technical grade samples have a highly unpleasant odor like garlic or rotting fish, due to the presence of substituted phosphine and diphosphane (P₂H₄). With traces of P₂H₄ present, PH₃ is spontaneously flammable in air (pyrophoric), burning with a luminous flame. Phosphines are also a group of organophosphorus compounds with the formula R₃P (R = organic derivative). Organophosphines are important in catalysts where they complex to various metal ions; complexes derived from a chiral phosphine can catalyze reactions to give chiral, enantioenriched products.

History

Philippe Gengembre (1764-1838), a student of Lavoisier, first obtained phosphine in 1783 by heating phosphorus in an aqueous solution of potash (potassium carbonate).^[4][5]

Perhaps because of its strong association with elemental phosphorus, phosphine was once regarded as a gaseous form of the element, but Lavoisier (1789) recognised it as a combination of phosphorus with hydrogen and described it as phosphure d'hydrogène (phosphide of hydrogen).^[6]

In 1844, Paul Thénard, son of the French chemist Louis Jacques Thénard, used a cold trap to separate diphosphine from phosphine that had been generated from calcium phosphide, thereby demonstrating that P₂H₄ is responsible for spontaneous flammability associated with PH₃, and also for the characteristic orange/brown color that can form on surfaces, which is a polymerisation product.^[7] He considered diphosphine’s formula to be PH₂, and thus an intermediate between elemental phosphorus, the higher polymers, and phosphine. Calcium phosphide (nominally Ca₃P₂) produces more P₂H₄ than other phosphides because of the preponderance of P-P bonds in the starting material.

The name "phosphine" first appeared in combined form in 1857.^[8] The gas PH₃ was named "phosphine" by 1865 (or earlier).^[9]

Structure and properties

PH₃ is a trigonal pyramidal molecule with C_3v molecular symmetry. The length of the P-H bond is 1.42 Å, the H-P-H bond angles are 93.5°. The dipole moment is 0.58 D, which increases with substitution of methyl groups in the series: CH₃PH₂, 1.10 D; (CH₃)₂PH, 1.23 D; (CH₃)₃P, 1.19 D. In contrast, the dipole moments of amines decrease with substitution, starting with ammonia, which has a dipole moment of 1.47 D. The low dipole moment and almost orthogonal bond angles lead to the conclusion that in PH₃ the P-H bonds are almost entirely pσ(P) – sσ(H) and phosphorus 3s orbital contributes little to the bonding between phosphorus and hydrogen in this molecule. For this reason, the lone pair on phosphorus may be regarded as predominantly formed by the 3s orbital of phosphorus. The upfield chemical shift of the phosphorus atom in the ³¹P NMR spectrum accords with the conclusion that the lone pair electrons occupy the 3s orbital (Fluck, 1973). This electronic structure leads to a lack of nucleophilicity in general and lack of basicity in particular (pK_aH = –14),^[10] as well as an ability to form only weak hydrogen bonds.^[11]

The aqueous solubility of PH₃ is slight; 0.22 mL of gas dissolve in 1 mL of water. Phosphine dissolves more readily in non-polar solvents than in water because of the non-polar P-H bonds. It is technically amphoteric in water, but acid and base activity is poor. Proton exchange proceeds via a phosphonium (PH₄⁺) ion in acidic solutions and via PH₂⁻ at high pH, with equilibrium constants K_b = 4 × 10⁻²⁸ and K_a = 41.6 × 10⁻²⁹.

Phosphine burns producing a dense white cloud of phosphoric acid:

PH₃ + 2 O₂ → H₃PO₄

Preparation and occurrence

Phosphine may be prepared in a variety of ways.^[12] Industrially it can be made by the reaction of white phosphorus with sodium or potassium hydroxide, producing potassium or sodium hypophosphite as a by-product.

3 KOH + P₄ + 3 H₂O → 3 KH₂PO₂ + PH₃

Alternatively the acid-catalyzed disproportioning of white phosphorus yields phosphoric acid and phosphine. Both routes have industrial significance; the acid route is preferred method if further reaction of the phosphine to substituted phosphines is needed. The acid route requires purification and pressurizing. It can also be made (as described above) by the hydrolysis of a metal phosphide, such as aluminium phosphide or calcium phosphide. Pure samples of phosphine, free from P₂H₄, may be prepared using the action of potassium hydroxide on phosphonium iodide (PH₄I).

Laboratory routes

It is prepared in the laboratory by disproportionation of phosphorous acid^[13]

4 H₃PO₃ → PH₃ + 3 H₃PO₄

Phosphine evolution occurs around 200 °C. Alternative methods involve the hydrolysis of aluminium phosphide, calcium phosphide, and tris(trimethylsilyl)phosphine.

Occurrence

Phosphine is a constituent of the atmosphere at very low and highly variable concentrations.^[14] It may contribute significantly to the global phosphorus biochemical cycle. The most likely source is reduction of phosphate in decaying organic matter, possibly via partial reductions and disproportionations, since environmental systems do not have known reducing agents of sufficient strength to directly convert phosphate to phosphine.^[15]

Phosphines

Organophosphines are compounds with the formula PR_nH_3−n. These compounds are often classified according to the value of n: primary phosphines (n = 1), secondary phosphines (n = 2), tertiary phosphines (n = 3). All adopt pyramidal structures. Their reactivity is also similar - they can be oxidized to the phosphorus(V) level, they can be protonated and alkylated at phosphorus to give phosphonium salts, and, for primary and secondary derivatives, they can be deprotonated by strong bases to give organophosphide derivatives.

Primary phosphines

Primary phosphines are typically prepared by alkylation of phosphine. Simple alkyl derivatives such as methylphosphine (CH₃PH₂) are prepared by alkylation of alkali metal derivatives MPH₂ (M = Li, Na, K). Another synthetic route involves treatment of the corresponding chlorophosphines with hydride reagents. For example, reduction of dichlorophenylphosphine with lithium aluminium hydride affords phenylphosphine (PhPH₂).

Secondary phosphines

Secondary phosphines are prepared analogously to the primary phosphines. They are also obtained by alkali-metal reductive cleavage of triarylphosphines followed by hydrolysis of the resulting phosphide salt. The latter route is employed to prepare diphenylphosphine (Ph₂PH). Diorganophosphinic acids, R₂P(O)OH, can also be reduced with diisobutylaluminium hydride. Secondary phosphines are typically protic in character.

When however modified with suitable substituents as in certain (rare) diazaphospholenes (scheme 3) the polarity of the P-H bond can be inverted (see: umpolung) and the resulting phosphine hydride can reduce a carbonyl group as in the example of benzophenone in yet another way.^[16]

Tertiary phosphines

Tertiary phosphines are generally obtained by treatment of phosphorus trichloride or triphenylphosphite with organolithium reagents or Grignard reagents. They are commonly used as ligands in coordination chemistry. Tertiary phosphines of the type PRR'R" are "P-chiral" and optically stable.

Cyclic phosphines

Secondary and tertiary phosphines occur in cyclic forms. Three-membered rings are phosphiranes (unsaturated: phosphirenes), five-membered rings are phospholanes (unsaturated: phosphole), and six-membered rings are phosphinanes.

Applications

Organophosphorus chemistry

Phosphine is mainly consumed as an intermediate in organophosphorus chemistry. In an illustrative reaction, formaldehyde adds in the presence of hydrogen chloride to give tetrakis(hydroxymethyl)phosphonium chloride, which is used in textiles.

Microelectronics

Small amounts of phosphine are used as a dopant in the semiconductor industry, and a precursor for the deposition of compound semiconductors.^[17]

Fumigant

For farm use, pellets of aluminium phosphide, calcium phosphide, or zinc phosphide release phosphine upon contact with atmospheric water or rodents' stomach acid. These pellets also contain agents to reduce the potential for ignition or explosion of the released phosphine. A more recent alternative is the use of phosphine gas itself which requires dilution with either CO₂ or N₂ or even air to bring it below the flammability point. Use of the gas avoids the issues related with the solid residues left by metal phosphide and results in faster, more efficient control of the target pests.

Because the previously popular fumigant methyl bromide has been phased out in some countries under the Montreal Protocol, phosphine is the only widely used, cost-effective, rapidly acting fumigant that does not leave residues on the stored product. Pests with high levels of resistance toward phosphine have become common in Asia, Australia and Brazil. High level resistance is also likely to occur in other regions, but has not been as closely monitored. Genetic variants that contribute to high level resistance to phosphine have been identified in the dihydrolipoamide dehydrogenase gene.^[18] Identification of this gene now allows rapid molecular identification of resistant insects.

Safety

Phosphine gas is denser than air and hence may collect in low-lying areas. It can form explosive mixtures with air and also self-ignite.

Phosphine can be absorbed into the body by inhalation. Direct contact with phosphine liquid – although unlikely to occur – may cause frostbite, like other cryogenic liquids. The main target organ of phosphine gas is the respiratory tract.^[19] According to the 2009 U.S. National Institute for Occupational Safety and Health (NIOSH) pocket guide, and U.S. Occupational Safety and Health Administration (OSHA) regulation, the 8 hour average respiratory exposure should not exceed 0.3 ppm. NIOSH recommends that the short term respiratory exposure to phosphine gas should not exceed 1 ppm. The Immediately Dangerous to Life or Health level is 50 ppm. Overexposure to phosphine gas causes nausea, vomiting, abdominal pain, diarrhea, thirst, chest tightness, dyspnea (breathing difficulty), muscle pain, chills, stupor or syncope, and pulmonary edema.^[20][21] Phosphine has been reported to have the odor of decaying fish or garlic at concentrations below 0.3 ppm. The smell is normally restricted to laboratory areas or phosphine processing since the smell comes from the way the phosphine is extracted from the environment. However, it may occur elsewhere, such as in industrial waste landfills. Exposure to higher concentrations may cause olfactory fatigue.^[22]

Toxicity

Deaths have resulted from accidental exposure to fumigation materials containing aluminum phosphide or phosphine. It can be absorbed either by inhalation or transdermally.^[30] As a respiratory poison, it affects the transport of oxygen or interferes with the utilization of oxygen by various cells in the body.^[31] Exposure results in pulmonary edema (the lungs fill with fluid).^[32] Phosphine gas is heavier than air so stays nearer the floor where children are likely to play.^[33]

• Two toddlers died in Jerusalem in January 2014, when a licensed exterminator used Phostoxin (a brand name for aluminum phosphide) as insecticide in one room of their apartment and the ad-hoc isolation protocol he used failed. According to the regulations then in place, the use of such a gas apparently required all residents remain outside the home for at least 10 days, and it was only approved for use in private homes that are isolated. The police recommended the indictment of the exterminator.^[23]
• In February 2014, three members of a Sevilla family, the father of which warehoused for recycling waste plastic jugs in their apartment bathroom, perished when phosphine was liberated because aluminum phosphide came into contact with water.^[24][25]
• The 2012 death in Thailand of a pair of women tourists from Quebec was blamed in March 2014 by The Fifth Estate on phosphine gas, which was employed by staff at the hotel where they stayed to control bedbugs. Their deaths were part of a series of tourist deaths that affected not only other tourists to the Laleena Guest House on Phi Phi Island, but also tourists to Chiang Mai. In all, more than a dozen deaths have been linked to the use of phosphine gas to control bedbugs.^[26]
• In a Fort McMurray apartment building, an eight month-old baby perished when a Pakistani home remedy for bedbugs failed in February 2015. The mother used the illegally imported tablets inadvertently causing the death of the child.^[27]
• In January 2017, four children were killed in Amarillo, Texas when their father sought to control an apparent rodent problem with an aluminum phosphide product designed to control weevils. The Washington Post at the time of the incident would not name the product.^[28] In November 2017, Weevil-Cide manufacturer United Phosporus was blamed in a wrongful death lawsuit. The father, who is English-illiterate, claimed damages because the product was not labelled in Spanish.^[29]
• In February 2019, five children with one adult woman died in a guest house in Karachi. The room was fumigated with Aluminum Phosphide tablets. The tablets were in excessive number and kept discharging Phosphine all night, which caused everyone's death.^[30]

See also

• Diphosphane, H₂PPH₂, simplified to H₄P₂
  • Diphosphines, R₂PPR₂, R₂P(CH₂)_nPR₂
• Diphosphene, HP=PH
  • Diphosphenes, R-P=P-R'
• Phosphine oxide, R₃P=O
• Phosphorane, PR₅, R₃P=CR₂
• Phosphinite, P(OR)R₂
• Phosphonite, P(OR)₂R
• Phosphite, P(OR)₃
• Phosphinate, R₂P(RO)O
• Phosphonate, RP(RO)₂O
• Phosphate, P(RO)₃O

References

Further reading

• {{ cite journal | author = Fluck, E. | title = The Chemistry of Phosphine | journal = Topics in Current Chemistry | year = 1973 | volume = 35 | pages = 1–64 | doi = 10.1007/BFb0051358 | series = Fortschritte der Chemischen Forschung | isbn = 3-540-06080-4 }}
• {{ cite book | author = WHO (World Health Organisation) | title = Phosphine and Selected Metal Phosphides | series = Environmental Health Criteria | publisher = Joint sponsorship of UNEP, ILO and WHO | location = Geneva | volume = 73 | year = 1988 | url = http://www.inchem.org/documents/ehc/ehc/ehc73.htm }}

External links

• International Chemical Safety Card 0694
• [https://www.cdc.gov/niosh/topics/phosphine/ CDC - Phosphine - NIOSH Workplace Safety and Health Topic]

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