PHOSPHONE (PH₃)

Phosphine is prepared by the reaction of calcium phosphide with water or dilute HCl.

Ca₃P₂ + 6H₂O → 3Ca(OH)₂ + 2PH₃

Ca₃P₂ + 6HCl → 3CaCl₂ + 2PH₃

In Laboratory, it is prepared by heating white phosphorous with conc. NaOH solution in an inert atmosphere of CO₂.

P₄ + 3NaOH + 3H₂O → PH₃ + 3NaH₂PO₂

                                                   (Sodium hypophosphide)

Pure PH₃ is not inflammable, but catches fire in air due to the presence of P₂H₄ (diphosphine) or P₄ vapours. To purify it form impurities, it is absorbed in HI to form phosphonium iodine (PH₄I) which on treating with KOH gives off Phosphine. PH₃ + HI → PH₄I

PH₄I + KOH → KI + H₂O + PH₃

Properties:-

(1) Physical Nature:- It is a colourless, unpleasant smelling (garlic or rotten fish), and highly poisonous gas.

(2) Solubility:- Phosphine gas is slightly soluble in water. The solution of phosphine in water decomposes in presence of air and light to give phosphorous and H₂.

(3) Basic Nature:- Phosphine is weakly basic in nature and gives phosphonium compounds with acids:

PH₃ + HBr → PH₄Br

(4) Reaction with CuSO₄ & Mercuric Chloride (HgCl₂):- When absorbed in these salts corresponding phosphides are formed:

3CuSO₄ + 2PH₃ → Cu₃P₂ + 3H₂SO₄

3HgCl₂ + 2PH₃ → Hg₃P₂ + 6HCl

Uses:-

(1) It is used as Holmes’s signals in deep seas and oceans for signaling danger points to steamers. Container containing a mixture of calcium carbide and calcium phosphide are pierced and put near danger point. In contact with water, a mixture of phosphine and acetylene is formed. Phosphine contains the traces of di-phosphine (P₂H₄) which is highly inflammable and catches fire. This ignites acetylene which burns with a luminous flame and thus serves as a signal to the approaching ship.

(2) For the production of smoke screens: Shells containing calcium phosphide are exsploded by war – ships, which burn in air to give clouds of P₄O₁₀ which acts as smoke screen.

                                           PHOSPHORUS HALIDES

Phosphorus forms two halides PX₃ (trihalides) and PX₅ (penta halides).

Phosphorus Trichloride (PCl₃):-

Phosphorus Trichloride is prepared by the action of thionyl chloride on white phosphorous:

P₄ + 8SOCl₂ → 4PCl₃ + 4SO₂ + 2S₂Cl₂

In laboratory, it is prepared by heating white phosphorus in a current of dry chlorine when it is distilled.

P₄ + 3Cl₂ → 4PCl₃

 

Structure:- In PCl₈, P atom is sp³ hybridized, having a lone pair of electrons over it. The bond angle is 100.5^° due to larger size of chlorine atom.

Properties:- It is colourless pungent smelling liquid with following chemical properties.

(1) Action with water:- It reacts violently with water to produce phosphorous acid and hydrochloric acid.

PCl₃ + 3H₂O → H₃PO₃ + 3HCl

Due to the formation of HCl, it fumes with water:-

(2) Reaction with hydroxyl and carboxylic group:- PCl₃ replaces these groups with chlorine e.g.

3CH₃CH₂OH + PCl₃ → 3CH₃CH₂Cl + H₃PO₃

3CH₃COOH + PCl₃ → 3CH₃COCl + H₃PO₃

Phosphorus Penta Chloride PCl₅:-

Phosphorous penta chloride is prepared by reaction of Cl₂ with P₄ and PCl₃.

P₄ + 10Cl₂ → 4PCl₅

PCl₃ + Cl₂ → PCl₅

It is also prepared by the action of sulphuryl chloride (SO₂Cl₂) on white phosphorous.

P₄ + 10SO₂Cl₂ → 4PCl₅ + 10SO₂

Structure:- In PCl₅ phosphorous undergoes sp³ d-hybridization and has trigonalbipyramidal geometry in gaseous and liquid states. It has three equatorial bonds and two axial bonds. Since three equatorial bonds are repelled by two axial bonds; and two axial bonds are repelled by three equatorial bonds. Hence axial bond lengths suffer more repulsion and are greater than equatorial bond lengths.

Properties:- Phosphorus pentachloride is a pale yellow crystalline solid with a characteristic pungent smell.

In the solid state it exists as an ionic solid, [PCl₄]⁺[PCl₆]^- in which the cation [PCl₄]⁺ is tetrahedral and the anion [PCl₆]^- is octahedral. The major chemical properties are:

(i) Dissociation:- On heating PCl₅ dissociates into PCl₃ and Cl₂

PCl₅ ⇌ PCl₃ + Cl₂

(ii) Action of water (Moisture):-

PCl₅ + H₂O → POCl₃ + 2HCl

PCl₅ + 4H₂O → H₃PO₄ + 5HCl

           (Excess)

Due to the formation of HCl it fumes in water (Moist air).

(iii) Reaction with metals:- Metals on heating with PCl₅ form corresponding chlorides.

Ag + PCl₅ → 2AgCl + PCl₃

Sn + 2PCl₅ → SnCl₄ + 2PCl₃

(iv) Reaction with compounds containing hydroxyl groups:- The compound is replaced by Cl – group

CH₃COOH + PCl₅ → CH₃COCl + POCl₃ + HCl

CH₃CH₂OH + PCl₅ → CH₃CH₂Cl + POCl₃ + HCl

Uses:- PCl₅ as well as PCl₃ is used as an important reagent in organic chemistry.