Phosphine: Phosphorus is the second member of the Nitrogen family and belongs to the \({15^{{\rm{th}}}}\) group of the periodic table, located in the \({\rm{‘P’}}\) block. With the electronic configuration of: \(1{{\rm{s}}^2}2{{\rm{s}}^2}2{{\rm{p}}^6}3{{\rm{s}}^2}3{{\rm{p}}^3}\), phosphorus occurs widely distributed in nature. However, it occurs in a combined state with other elements.

Phosphorus has many allotropic forms; important ones are white, red, and black phosphorus. Phosphine is one of the hydrides of phosphorus and is an industrially significant compound of phosphorus.

What is Phosphine?

Phosphine is the hydride of phosphorus, with a formula of \({\rm{P}}{{\rm{H}}_3}\). Phosphine is inflammable gas and is toxic in nature. Because of its significant industrial applications, it is widely manufactured in industries and in laboratories.

In nature, phosphine occurs in human blood, urine and saliva.

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Structure and Formula of Phosphine

The structure of phosphine is trigonal pyramidal. The change in symmetry of the molecule is due to the lone pair of electrons present on phosphorus. Phosphine exhibits \({\rm{s}}{{\rm{p}}^3}\) hybridization. There are three bonded pairs and one lone pair of electrons. The \({\rm{H – P – H}}\) bond angle is \({93.5^{\rm{o}}}\), and the \({\rm{P – H}}\) bond length is \(1.42\,{\rm{pm}}\).

Preparation of Phosphine

1. Hydrolysis of metal phosphides: with calcium phosphide
   Phosphine is obtained when metal phosphides react with water or \({\rm{HCl}}\).
   \({\rm{C}}{{\rm{a}}_3}{{\rm{P}}_2} + 6{{\rm{H}}_2}{\rm{O}} \to 2{\rm{P}}{{\rm{H}}_3} + 3{\rm{Ca}}{({\rm{OH}})_2}\)
   \({\rm{C}}{{\rm{a}}_3}{{\rm{P}}_2} + 6{\rm{HCl}} \to 2{\rm{P}}{{\rm{H}}_3} + 3{\rm{CaC}}{{\rm{l}}_2}\)
2. From phosphorus acid
   On the decomposition of \({{\rm{H}}_3}{\rm{P}}{{\rm{O}}_3}\)_, pure phosphine is obtained.
   \(4{{\rm{H}}_3}{\rm{P}}{{\rm{O}}_3} \to 3{{\rm{H}}_3}{\rm{P}}{{\rm{O}}_4} + {\rm{P}}{{\rm{H}}_3}\)

Preparation of Phosphine from Phosphorus

White phosphorus on heating with concentrated \({\rm{NaOH}}\) solution in an inert atmosphere of \({\rm{C}}{{\rm{O}}_2}\) gives \({\rm{P}}{{\rm{H}}_3}\).

\({{\rm{P}}_4} + 3{\rm{NaOH}} + 3{{\rm{H}}_2}{\rm{O}}\mathop {{\rm{P}}{{\rm{H}}_3}}\limits_{{\rm{ Phosphine }}}  + \mathop {3{\rm{Na}}{{\rm{H}}_2}{\rm{P}}{{\rm{O}}_2}}\limits_{{\rm{ Sodium\, Hypophosphite }}} \)

Purification of Phosphine

Pure phosphine is non-inflammable. It becomes inflammable due to the presence of \({{\rm{P}}_2}{{\rm{H}}_4}\) or \({{\rm{P}}_4}\) vapours. Phosphine is purified from impurities by absorbing it in \({\rm{HI}}\) to form \({\rm{P}}{{\rm{H}}_4}{\rm{l}}\). This, on treatment with potassium hydroxide, gives phosphine.

\({\rm{P}}{{\rm{H}}_4}{\rm{I}} + {\rm{KOH}} \to {\rm{P}}{{\rm{H}}_3} + {\rm{KI}} + {{\rm{H}}_2}{\rm{O}}\)

Physical Properties of Phosphine

1. It is a colourless and highly poisonous gas.
2. Phosphine is non-inflammable in pure form, as mentioned. However, the presence of \({{\rm{P}}_4}\) vapour makes it flammable. Phosphine can catch fire when it comes in contact with oxidizing agents such as \({\rm{HN}}{{\rm{O}}_3}\), chlorine and bromine vapours. Even a small amount or traces of these oxidizing agents can cause it to catch fire.
3. It has a characteristic smell of rotten fish.
4. It is sparingly soluble in water.
5. Its boiling point is \( – 87.7^\circ {\rm{C}}\).

Chemical Properties of Phosphine

1. Phosphine acts as a Lewis base like ammonia. It is weakly basic and gives phosphonium compounds on reaction with \({\rm{HI, HBr}}\) and \({\rm{HCl}}\).
   i. \({\rm{P}}{{\rm{H}}_{\rm{3}}} + {\rm{Hl}} \to \mathop {{\rm{P}}{{\rm{H}}_4}{\rm{l}}}\limits_{{\rm{i}}.{\mkern 1mu} {\rm{Phosphonium}}{\mkern 1mu} {\rm{iodide}}} \)
   ii. \({\rm{P}}{{\rm{H}}_{\rm{3}}} + {\rm{HBr}} \to \mathop {{\rm{P}}{{\rm{H}}_4}{\rm{Br}}}\limits_{{\rm{i}}.{\mkern 1mu} {\rm{Phosphonium}}{\mkern 1mu} \,{\rm{bromide}}} \)
   iii. \({\rm{P}}{{\rm{H}}_{\rm{3}}} + {\rm{HCl}} \to \mathop {{\rm{P}}{{\rm{H}}_4}{\rm{Cl}}}\limits_{{\rm{i}}.{\mkern 1mu} {\rm{Phosphonium}}\,\,{\mkern 1mu} {\rm{chloride}}} \)
2. Combustibility
   Phosphine on combustion gives phosphorus pentoxide and water
   \(2{\rm{P}}{{\rm{H}}_3} + 4{{\rm{O}}_2} \to {{\rm{P}}_2}{{\rm{O}}_5} + 3{{\rm{H}}_2}{\rm{O}}\)
3. Reaction with chlorine
   \({\rm{P}}{{\rm{H}}_3} + 4{\rm{C}}{{\rm{l}}_2} \to {\rm{PC}}{{\rm{l}}_5} + 3{\rm{HCl}}\)
4. It is non-ignitable under normal conditions. On heating, it bursts with flame forming phosphoric acid.
5. When exposed to oxidizing agents like \({\rm{HN}}{{\rm{O}}_3},\,{\rm{C}}{{\rm{l}}_2}\) and \({\rm{B}}{{\rm{r}}_2}\), it explodes violently.
6. In the presence of light, \({\rm{P}}{{\rm{H}}_3}\) in water decomposes to give red \({\rm{P}}\) and \({{\rm{H}}_2}{\rm{O}}\).
   i. \({\rm{P}}{{\rm{H}}_{3({\rm{H}}2{\rm{O}})}} \to {\rm{P}}({\rm{red}}) + {{\rm{H}}_2}{\rm{O}}\)
7. When phosphine is absorbed in copper sulphate or mercuric chloride, copper phosphide and mercuric phosphide will be formed.
   \(3{\rm{CuS}}{{\rm{O}}_4} + 2{\rm{P}}{{\rm{H}}_3} \to {\rm{C}}{{\rm{u}}_3}{{\rm{P}}_2} + 3{{\rm{H}}_2}{\rm{S}}{{\rm{O}}_4}\)
   \(3{\rm{HgC}}{{\rm{l}}_2} + 2{\rm{P}}{{\rm{H}}_3} \to {\rm{H}}{{\rm{g}}_3}{{\rm{P}}_2} + 6{\rm{HCl}}\)

Uses of Phosphine

1. Phosphine is used for preparing metallic phosphides.
2. Phosphine is used in Holme’s signal and Smoke screens.

Holme’s signal:

The property of phosphine wherein it combusts spontaneously makes it a probable component for use in Holme’s signal. When ships need help, containers with calcium carbide and calcium phosphide, after piercing, are thrown into the sea. Due to the reaction with water, acetylene and phosphine gases are formed. These gases burn in the air and serve as a signal.

Smoke screen:

A pack of calcium phosphide, when soaked in water, produces phosphine in large quantity leading to a smoke screen. This was used during the war to hide the army from the enemies.

1. Phosphine fumigants are used in rodent and insect control formulations.

Summary

Phosphine is a colourless gas with rotten fish odour and is sparingly soluble in water. Phosphine can be prepared from white phosphorus in the laboratory, from metal phosphides and from phosphorus acid. It has a trigonal pyramidal structure with a bond angle of \(93.5\). It is a weak base. Phosphine explodes when exposed to oxidizing agents. Phosphine is used in a smoke screen, Holme’s signal, and as a fumigant in insecticides and rodenticides.

FAQs on Phosphine

Q.1. What are the properties of phosphine?
Ans: a. Phosphine is a colourless and poisonous gas.
b. It has a characteristic smell of rotten fish.
c. It is sparingly soluble in water.

Q.2. What are the uses of phosphine?
Ans: Phosphine is used in the smoke screen, Holme’s signal and as a fumigant in insecticides and rodenticides.

Q.3. What is the structure of phosphine?
Ans: The structure is trigonal bipyramidal with a lone pair of electrons on P. There are three bonded pairs and one lone pair of electrons. The bond angle is \({\rm{ 93}}{\rm{.5}}\). The \({\rm{P – H}}\) bond length is \(1.42\,Å\).

Q.4. How is phosphine gas prepared in the laboratory?
Ans: In the laboratory, phosphine is obtained when white phosphorus is heated with concentrated \({\rm{NaOH}}\) solution in an inert atmosphere of \({\rm{C}}{{\rm{O}}_2}\).
\({{\rm{P}}_4} + 3{\rm{NaOH}} + 3{{\rm{H}}_2}{\rm{O}} \to {\rm{P}}{{\rm{H}}_3} + 3{\rm{Na}}{{\rm{H}}_2}{\rm{P}}{{\rm{O}}_2}\)

Q.5. Is phosphine acidic or basic?
Ans: Phosphine is weakly basic. It is confirmed from the reaction with hydrogen halides.
\({\rm{P}}{{\rm{H}}_3} + {\rm{Hl}} \to {\rm{P}}{{\rm{H}}_4}{\rm{l}}\)

Q.6. How do you purify phosphine?
Ans: Phosphine is purified from impurities by absorbing it in \({\rm{HI}}\) forming \({\rm{P}}{{\rm{H}}_4}{\rm{l}}\), which on treatment with potassium hydroxide gives phosphine.
\({\rm{P}}{{\rm{H}}_4}{\rm{l}} + {\rm{KOH}} \to {\rm{KI}} + {{\rm{H}}_2}{\rm{O}} + {\rm{P}}{{\rm{H}}_3}\)

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