preparation and properties of phosphine

Phosphine or phosphane (PH3) is a hydride of phosphorous. It is also known as phosphorus trihydride. It was first prepared/discovered by a French chemist named Philippe Genembre in 1783. It is colourless but has a foul odour. It is highly toxic and can cause adverse effects on the human respiratory system when inhaled at low concentrations. It has a single phosphorous atom covalently bonded to three hydrogen atoms. The shape of a phosphine molecule is trigonal pyramidal. From semiconductors to flame retardants, phosphine gas has various uses.

Methods of Preparation

Industrial methods

• The action of caustic potash (potassium hydroxide) or sodium hydroxide on white phosphorus (P4) – The reactions form phosphine and potassium or sodium hypophosphite:

3 KOH + P4 + 3 H2O → 3 KH2PO2 + PH3

P4  + 3NaOH + 3 H2O → 3 NaH2PO2 + PH3

• Disproportionation of white phosphorous. Acids or alkalis are used as catalysts. The final products are phosphine and phosphoric acid.

• From metal phosphides – Aluminium phosphide and calcium phosphide yield phosphine in the presence of moisture or dilute hydrochloric acid (HCl):

Ca3P2 + 6 H2O → 3 Ca(OH)2 + 2PH3

Ca3P2  + 6 HCl → 3 CaCl2 + 2PH3

Pure phosphine, which does not contain P2H4, can be obtained by the reaction of potassium hydroxide and phosphonium iodide (PH4I).

 P4  + KOH → KI + H2O + PH3

Laboratory method

From phosphorous acid (H3PO3) – Phosphorous acid is disproportionated at 200 °C to give phosphine and orthophosphoric acid:

4 H3PO3 → PH3 + 3 H3PO4

Phosphine can also be prepared by aluminium phosphide, calcium phosphide, and tris (trimethylsilyl) phosphine.

Structure 

There is no hybridisation in phosphine (PH3), and only pure p-orbitals make bonds. It forms three bond pairs and a lone pair (s-orbital electrons). The cloud of electrons is present in the central atom. The lewis structure of phosphine has eight valence electrons. The shape of a phosphine molecule is trigonal bipyramidal. The P-H bond length is 142 pm, and the bond angles between H-P-H are 93.5°.

Properties 

Physical properties

• Its molecular weight is 33.997 g/mol.
• Phosphine is a colourless gas.
• It dissolves readily in non-polar solvents.
• It possesses a pungent smell like rotten fish or garlic.
• It is non-flammable in the pure state but turns inflammable in the presence of P2H4 or P4 molecules.
• It has a melting point of −132.8 °C.
• Its boiling point is −87.7 °C.
• It does not dissolve in hot water, but it is slightly soluble in cold water.
• It is heavier than air.
• It is soluble in alcohols and ethers.
• It is highly toxic and poisonous.

Chemical properties/reactions

• When oxidised by HNO3, Cl2, Br2, it explodes.
• When a mixture of phosphine and water is exposed to light, it forms red phosphorous and H2.
• It is weakly basic as it donates a lone pair of electrons. It forms phosphonium compounds when reacted with acids:

PH3 + HBr → PH4I

• It forms the following phosphides when added to copper sulphate or mercuric chloride:

3 CuSO4 + PH3 →  Cu3P2 + 3 H2SO4

3 HgCl2  + PH3 →  Hg3P2  + 6 HCl

• It does not ignite under normal conditions. When it is heated, phosphine catches fire and forms phosphoric acid:

 PH3 + 2O2 → H3PO4

• Tertiary phosphines are important ligands because it is possible to change their electronic and steric properties by shifting the R groups.
• They can stabilise a wide array of metal complexes.
• They are not performer ligands.

Uses of Phosphine Gas

• Phosphine is used to prepare many organophosphorus compounds.
• It is used as a dopant for n-type semiconductors in the semiconductor industry.
• It is used as an insecticide.
• Phosphine is an intermediate in the synthesis of flame retardants.
• It is used as a catalyst in condensation.
• It is also used to produce batteries.
• It plays a vital role in Holme’s signal as it can rapidly ignite. For mariners, they serve as signals.
• It is also used in smoke screens.
• It is used to make phosphine fumigants to keep away bugs and rodents.

Effects on Humans

It is a super toxic gas that can be lethal if inhaled. Short-term exposure to phosphine causes headaches, dizziness, fatigue, burning substernal pain, nausea, drowsiness,  vomiting, cough, laboured breathing, pulmonary irritation, pulmonary oedema, chest tightness, and tremors in humans. Long term exposure (chronic) exposure can cause inflammation of the nasal cavity and throat, weakness, nausea, dizziness, gastrointestinal, cardiorespiratory, and central nervous system symptomology, jaundice, liver effects, and increased bone density.

Diisopropyl Phosphine

Its molecular formula is C6H15P, and the IUPAC name is di(propan-2-yl) phosphane. The molecular weight of di isopropyl phosphine is 118.16 g/mol. It can catch fire rapidly if exposed to air, i.e., it is pyrophoric. It is toxic and lethal if swallowed. Exposure can cause severe skin burns and eye damage.

Chlorodiphenylphosphine

It is another organophosphorus compound with the formula (C6H5)2PCl. It is used as a reagent to introduce the Ph2P group into molecules, including various ligands.

Methods of preparation

On a commercial scale, it is produced by benzene (C6H6) and phosphorous trichloride (PCl3) at 600 °C. The products formed are dichlorophenylphosphine (PhPCl2) and HCl. At high temperatures, it gives chlorodiphenylphosphine.

2 PhPCl2 → Ph2PCl + PCl3

Properties

• Its molecular weight is 220.63 g/mol.
• It is colourless and oily.
• It possesses a pungent garlic-like odour.

Uses

• It is used to synthesise various phosphines using a Grignard reagent:

Ph2PCl + MgRX → Ph2PR + MgClX

• It is crucial in making some pesticides.
• It acts as an intermediate in the formation of flame retardants and catalysts.

Conclusion 

Phosphine is an important hydride of phosphorus. It is a colourless and highly toxic gas with various uses in the industry, like the processing of semiconductors and insecticides. Diisopropyl phosphine readily catches fire and is highly lethal to humans. Chlorodiphenylphosphine is colourless. It is an essential intermediate in the industry and serves commercial purposes.