Phosphine

Phosphine
IUPAC name Phosphane
Other names Phosphamine Phosphorus trihydride Phosphorated hydrogen
Identifiers
CAS number	7803-51-2 Y
PubChem	24404
ChemSpider	22814 Y
EC number	232-260-8
UN number	2199
ChEBI	CHEBI:30278 Y
RTECS number	SY7525000
Jmol-3D images	Image 1
SMILES P
InChI InChI=1S/H3P/h1H3 Y Key: XYFCBTPGUUZFHI-UHFFFAOYSA-N Y InChI=1/H3P/h1H3 Key: XYFCBTPGUUZFHI-UHFFFAOYAP
Properties
Molecular formula	PH3
Molar mass	33.99758 g/mol
Appearance	colorless gas
Density	1.379 g/l, gas (25 °C)
Melting point	−132.8 °C; −207.0 °F; 140.3 K
Boiling point	−87.7 °C; −125.9 °F; 185.5 K
Solubility in water	31.2 mg/100 ml (17 °C)
Viscosity	1.1 x 10−5 Pa s
Structure
Molecular shape	Trigonal pyramidal
Dipole moment	0.58 D
Thermochemistry
Std enthalpy of formation ΔfHo298	5 kJ·mol−1[1]
Standard molar entropy So298	210 J·mol−1·K−1[1]
Hazards
MSDS	ICSC 0694
EU Index	015-181-00-1
EU classification	Highly flammable (F+) Very toxic (T+) Corrosive (C) Dangerous for the environment (N)
R-phrases	R12, R17, R26, R34, R50
S-phrases	(S1/2), S28, S36/37, S45, S61, S63
NFPA 704	4 4 2
Flash point	flammable gas
Autoignition temperature	38 °C; 100 °F; 311 K
Explosive limits	1.8% – ?
Related compounds
Other cations	Ammonia Arsine Stibine Bismuthine
Related compounds	Trimethylphosphine Triphenylphosphine
Y (verify) (what is: Y/N?) Except where noted otherwise, data are given for materials in their standard state (at 25 °C (77 °F), 100 kPa)
Infobox references

Phosphine (IUPAC name: phosphane) is the compound with the chemical formula PH₃. It is a colorless, flammable, toxic gas. 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, 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

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 “hydruyet of phosphorus, or phosphuret of hydrogen”.^{[citation needed]}

Thénard (1845) 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. 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.

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 and an ability to form only weak hydrogen bonds.^[2]

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 acts as neither an acid nor a base in water. 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_z = 41.6 × 10⁻²⁹.

Preparation and occurrence

Phosphine may be prepared in a variety of ways.^[3] Industrially it can be made by the reaction of white phosphorus with sodium hydroxide, producing sodium hypophosphite and sodium phosphite as a by-product. Alternatively the acid-catalyzed disproportioning of white phosphorus may be used, which 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).

Phosphine is probably a constituent of the atmosphere at very low and highly variable concentrations and hence may contribute to the global phosphorus biochemical cycle.^[4] The origin(s) of atmospheric phosphine is not certain. Possible sources include bacterial reduction of phosphate in decaying organic matter and the corrosion of phosphorus-containing metals.^[5]

Phosphines

Related to a PH₃ is the class of organophosphorus compounds commonly called phosphines. These alkyl and aryl derivatives of phosphine are analogous to organic amines. Common examples include triphenylphosphine ((C₆H₅)₃P) and BINAP, both used as ligands in homogeneous catalysis or triisopropylphosphine. Phosphines are easily oxidized to phosphine oxides as exemplified by the directed synthesis of a phospha-crown, the phosphorus analogue of an aza crown^[6] where it is not possible to isolate the phosphine itself.

In step 1 diphosphinoethane coordinates to a ferrocene containing additional carbon monoxide ligands and an acetonitrile ligand. The next step is a hydrophosphination with trivinylphosphine followed by alkylation with ethyl bromide and hydrogenation with hydrogen over palladium on carbon. In the final step the iron template is removed by bromine but oxidation of the phosphine groups is unavoidable.

When 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.^[7]

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.^[8]

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.

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 developing 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 may not have been as closely monitored.

Safety

Phosphine gas is more dense than air and hence may collect in low-lying areas. It can form explosive mixtures with air and also self-ignite. When phosphine burns it produces a dense white cloud of phosphorus pentoxide – a severe respiratory irritant.^[9]

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. 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; pulmonary edema.^[10][11] 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.^[12]

On January 2014, a whole family in Jerusalem was poisoned from phosphine after a section of their apartment was fumigated for pests, and some of the young children died from the poisoning. ^[13]

See also

• Diphosphane, H₂PPH₂
  • 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

• Fluck, E. (1973). "The Chemistry of Phosphine". Topics in Current Chemistry. Fortschritte der Chemischen Forschung 35: 1–64. doi:10.1007/BFb0051358. ISBN 3-540-06080-4. 
• WHO (World Health Organisation) (1988). Phosphine and Selected Metal Phosphides. Environmental Health Criteria 73. Geneva: Joint sponsorship of UNEP, ILO and WHO. 

External links

• International Chemical Safety Card 0694
• CDC - Phosphine - NIOSH Workplace Safety and Health Topic