Phosphine, halides PCl3, PCl5 and oxoacids

p-Block Elements: Group 15

Phosphine, halides PCl₃, PCl₅ and oxoacids

Phosphine is produced by the reacting calcium phosphide with water or dilutes HCl.

Ca₃P₂ + 6H₂O3Ca(OH)₂ + 2PH₃

Ca₃P₂ + 6HCl 3CaCl₂ + 2PH₃

At laboratory scale, it is made by heating white phosphorus with concentrated NaOH solution in an inert atmosphere of carbon dioxide and produces sodium hypophosphite.

P₄ +3NaOH + 3H₂O PH₃ + 3NaH₂ PO₂

In the pure state, phosphine is non-inflammable but becomes inflammable in the presence of P₂H₄ or P₄ vapours. To purify it from the impurities, it is absorbed in HI to form phosphoniumiodide (PH4I) which on treating with KOH gives off phosphine.

PH₄ I + KOH KI + H₂O + PH₃

Properties:-

1. 3CuSO₄ + 2PH₃ Cu₃P₂ + 3H₂SO₄

2. 3HgCl₂ + 2PH₃ Hg₃P₂ + 6HCl

3. Phosphine is weakly basic and like ammonia, gives phosphonium compounds with acids

e.g., PH₃ + HBr PH₄Br

Halides

The general formulas are PX₃ and PX₅

• PF₃ is found in gaseous state
• PCl₃ is found in liquid state( bp = 74^oC)
• PBr₃ is found in liquid state (bp = 175^oC)
• PI₃ is an unstable red solid (mp = 61^oC)

PX₃ + 3 H₂O_(l) H₃PO_{3 (aq)} + 3 HX_(aq)

• PCl₅ is an ionic saltof : PCl₆^– and PCl₄⁺ ions.
• PBr₅ is an ionic salt of PBr₄⁺ and Br ^– ions.
• The PX₅ compounds react with water to form Phosphoric acid.

PX₅ + 4 H₂O_(l) H₃PO_{4 (aq)} + 5 HX_(aq)

PCl₃ or P₄O₆ are hydrolysed in water to form ox0acids of phosphorous.

Phosphorous oxoacids

H₃PO₃ has only two acidic H atoms; the third is bonded to the central P and does not dissociate.

Orthophosphoric acid, H₃PO₄ has three acidic H atoms. It is a weak acid, but in the strong base, all three H⁺ is lost to give the phosphate anion.

Is the oldest known phosphorus compounds. It is a syrupy liquid made by direct reaction of ground phosphate rock with sulfuric acid. The pure acid is a colourless crystalline solid. Stable and has no oxidising properties below 350-400 °C. It will attack quartz. Hydrogen bonding persists in the concentrated solution and is responsible for the syrupy behaviour.

The acids in +3 oxidation state of phosphorus tend to disproportionate to higher and lower oxidation states. For example, ortho phosphorous acid (or phosphorous acid) on heating disproportionate to give orthophosphoric acid (or phosphoric acid) and phosphine.

4H₃PO₃ 3H₃PO₄ + PH₃

The acids which contain P–H bond has strong reducing properties. Thus, hypophosphorous acid is a good reducing agent. It contains two P–H bonds which can reduce AgNO3 to metallic silver.

4 AgNO₃ + 2H2O + H₃PO₂4Ag + 4HNO₃ + H₃PO₄

Fig 1: Oxoacids of phosphorus with their oxidation states.