8. Phosphorus halides

Phosphorus Halides

Phsophorus forms two types of halides , PX₃ [X = F , Cl , Br, I] and PX₅ [X = F , Cl , Br]

(a) Phosphorus Trichloride :

Preparation :

(i)  .It is obtained by passing dry chlorine over heated white phosphorus

 P₄ + 6 Cl₂ ®  4 PCl₃  

(ii) It is also obtained by the action of thionyl chloride with white phosphorus.

 P₄ + 8 SOCl₂  ®  4 PCl₃ + 4 SO₂  + 2 S₂Cl₂

Properties :

(i) It is a colourless oily liquid and hydrolyses in the presence of moisture.

 PCl₃ + 3 H₂O  ® H₃PO₃ + 3 HCl

(ii) It reacts with organic compounds containing – OH group such as CH₃COOH , C₂H₅OH.

  3 CH₃COOH + PCl₃  ® 3 CH₃COCl + H₃PO₃ 

  3 C₂H₅OH + PCl₃  ® 3 C₂H₅Cl +H₃PO₃ 

(b) Phosphorus pentachloride :

Preparation :

Phosphorus pentachloride is prepared by the reaction of white phosphorus with excess of dry chlorine.

P₄ + 10 Cl₂  ® 4 PCl₅ 

It can also be prepared by the action of SO₂Cl₂ on phosphorus.

P₄ + 10 SO₂Cl₂  ® 4 PCl₅ + 10 SO₂

 Properties :

(i) PCl₅ is a yellowish white powder and in moist air , it hydrolyses to POCl₃ and finally gets converted to phosphoric acid.

 PCl₅ + H₂O ®  POCl₃ + 2 HCl

 POCl₃ + 3 H₂O ® H₃PO₄ + 3 HCl

(ii) When heated it sublimes but decomposes on stronger heating.

PCl₅    PCl₃ + Cl₂  

(iii) It reacts with organic compounds containing – OH group converting them to chloro derivatives.

C₂H₅OH + PC_l5  ® C₂H₅Cl + POCl₃ + HCl

CH₃COOH + PCl₅  ® CH₃COCl + POCl₃ + HCl

(iv) PCl₅ on heating with finely divided metals give corresponding chlorides.

2 Ag + PCl₅  ® 2 AgCl + PCl₃

Sn + 2 PCl₅   ®  SnCl₄ + 2 PCl₃

It is used in the synthesis of some organic compounds , e.g., C₂H₅Cl , CH₃COCl.