Phosphine
Phosphine is the common name for phosphorus hydride , also known by the
IUPAC name phosphane and, occasionally, phosphamine. It is a colorless, flammable gas with a boiling point of −88 C at standard pressure. Pure phosphine is odorless, but "technical grade" phosphine has a highly unpleasant odor like
garlic or rotting fish, due to the presence of substituted phosphine and diphosphine . Phosphines are also a group of substituted phosphines, with the structure R3P, where other functional groups replace hydrogens. They are important in catalysts where they complex to various metal ions; a chiral metal phosphine complex can catalyze a reaction to give chiral products.
Encyclopedia
| Phosphine |
|---|
| |
| General |
|---|
| Systematic name | Phosphane |
| Other names | Phosphine
Phosphamine
Phosphorus hydride
Phosphorated hydrogen |
| Molecular formula | PH3 |
| Molar mass | 34.00 g/mol |
| Appearance | colorless gas |
| CAS number | [7803-51-2] |
| Properties |
|---|
| Density and phase | 1.379 g/l, gas |
| Solubility in water | 31.2 mg/100 ml |
| Melting point | −134 °C |
| Boiling point | −87.8 °C |
| Structure |
|---|
| Molecular shape | Trigonal pyramidal |
| Dipole moment | 0.58 D |
| Hazards |
|---|
| MSDS | External MSDS |
| EU classification | Highly flammable
Very toxic
Dangerous for
the environment |
| NFPA 704 | |
| R-phrases | , , , , |
| S-phrases | , , , ,
, |
| Flash point | flammable gas |
| Autoignition temperature | 38 °C |
| Explosive limits | 1.8–? % |
| Supplementary data page |
|---|
Structure and
properties | n, er, etc. |
Thermodynamic
data | Phase behaviour
Solid, liquid, gas |
| Spectral data | UV, IR, NMR, MS |
| Related compounds |
|---|
| Other cations | Ammonia
Arsine
Stibine
Bismuthine |
| Related compounds | Trimethylphosphine
Triphenylphosphine |
Except where noted otherwise, data are given for
materials in their standard state
|
|
Phosphine is the common name for
phosphorus hydride , also known by the
IUPAC name
phosphane and, occasionally,
phosphamine. It is a colorless, flammable gas with a boiling point of −88 °C at standard pressure. Pure phosphine is odorless, but "technical grade" phosphine has a highly unpleasant odor like
garlic or rotting fish, due to the presence of substituted phosphine and diphosphine .
Phosphines are also a group of substituted phosphines, with the structure R
3P, where other functional groups replace hydrogens. They are important in catalysts where they complex to various metal ions; a chiral metal phosphine complex can catalyze a reaction to give chiral products.
Phosphine is highly
toxic; it can easily kill in relatively low concentrations. Because of this, the gas is used for
pest control by fumigation. For
farm use, it is often sold in the form of
aluminium phosphide, calcium phosphide, or
zinc phosphide pellets, which yield phosphine on contact with atmospheric water or rodents' stomach acid. These pellets also contain other chemicals which evolve
ammonia which helps to reduce the potential for spontaneous ignition or
explosion of the phosphine gas. They may also contain other agents, such as
methanethiol, to give the gas a detectable garlic smell to help warn against its presence in the atmosphere.
Phosphine is also used as a dopant in the
semiconductor industry.
Structure and properties
PH
3 is a trigonal pyramidal molecule with C
3v symmetry. The
length of the P-H bond 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: CH
3PH
2, 1.10 D;
2PH, 1.23 D;
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 PH
3 the P-H bonds are almost entirely ps – ss and the lone pair contributes only a little to the
molecular orbitals. The high positive chemical shift of the P atom in
31P NMR spectrum accords with the conclusion that the lone pair electrons occupy the 3s orbital and so are close to the P atom . This electronic structure leads to a lack of
nucleophilicity and an inability to form
hydrogen bonds.
The aqueous solubility of PH
3 is slight; 0.22 mL of gas dissolve in 1 mL of water. Phosphine dissolves more readily in non-polar solvents than in water. It acts as neither an acid nor a base in water. Proton exchange proceeds via a phosphonium ion in acidic solutions and via PH
2- at high pH, with equilibribium constants K
b = 4 x 10
-28 and K
z = 41.6 x 10
-29.
History
Perhaps because of its strong association with elemental
phosphorus, phosphine was once regarded as a gaseous form of the element but
Lavoisier recognised it as a combination of phosphorus with hydrogen by describing it as “hydruyet of phosphorus, or phosphuret of hydrogen”.
Ernst von Meyer described the early history of phosphine research thus:
"The discovery of phosphuretted hydrogen by Gengembre in 1783, and the examination of it by Pelletier , only became fruitful after
Humphry Davy’s investigations; and the last-named elucidated the composition of this gas, and pointed out its analogy to
ammonia, this being emphasised still more sharply by H. Rose later on."
Thénard used a cold trap to separate
diphosphine from phosphine that had been generated from calcium phosphide, thereby demonstrating that P
2H
4 is responsible for spontaneous flammability associated with PH
3, and also for the characteristic orange/brown colour that can form on surfaces, which is a polymerisation product. He considered diphosphine’s formula to be PH
2, and thus an intermediate between elemental phosphorus, the higher polymers, and phosphine. Calcium phosphide produces more P
2H
4 than other phosphides because of the preponderance of P-P bonds in the starting material.
Chemistry
Phosphine may be prepared in a variety of ways
[2]. 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, with the acid route as the preferred method if further reaction of the phosphine to substituted phosphines is needed. This latter step requires purification and pressurizing. It can also be made by the hydrolysis of a metal phosphide such as
aluminium phosphide or calcium phosphide. Pure samples of phosphine, free from P
2H
4, may be prepared using the action of
potassium hydroxide on phosphonium iodide .
Related to PH
3 is the class of compounds commonly called
phosphines. These are alkyl or aryl derivatives of phosphine, just as
amines can be regarded as derivatives of
ammonia. Common examples include
triphenylphosphine and
BINAP, both used as phosphine ligands in metal complexes such as
Wilkinson's catalyst. Metal phosphine complexes are
catalysts for reactions such as the Sonogashira coupling. Most of these phosphines, with the exception of
triphenyl phosphine, are made from pressurized, purified phosphine gas as described above.
A large industrial application of phosphine is found in the production of tetrakis phosphonium salts, made by passing phosphine gas through a solution of
formaldehyde and a mineral acid such as
hydrochloric acid. These find application as flame retardants for textile and as biocides.
Phosphine is often confused with
phosgene, which has a similar-sounding name but contains no phosphorus.
Use as a fumigant
Phosphine is highly toxic to organisms undergoing oxidative respiration, but is non toxic to organisms kept under low oxygen or that can anaerobically respire . Because of these characteristics, phosphine is widely used as a fumigant of metabolically dormant stored products such as
grain. The toxicity of phosphine kills insect pests that might infest the grain, but does not affect the viability of the dormant grain.
Because continued use of the previously widely used fumigant
methyl bromide has been banned under the
Montreal Protocol, phosphine is the only widely used, cost effective, rapidly acting fumigant that does not leave residues on the stored product. Given the heavy reliance on phosphine as a means of protecting grain from insect infestation, it is disturbing to note that high levels of resistance toward phosphine have become commonplace in many countries of Asia and in Australia as well. Active research in Australia into the mode of action of phosphine and the mechanisms whereby insects acquire resistance is being carried out by the
CSIRO in Canberra, QDPI&F in Queensland and the
University of Queensland.
References
- E. Fluck, The chemistry of phosphine, Topics in Current Chemistry Vol. 35, 64 pp, 1973.
- A.D.F. Toy, The Chemistry of Phosphorus, Pergamon Press, Oxford, UK, 1973.
- WHO , Phosphine and selected metal phosphides, Environmental Health Criteria. Published under the joint sponsorship of UNEP, ILO and WHO, Geneva, Vol. 73, 100 pp, 1988.
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