Halides of Phosphorus: Phosphorus TrichlorideII. Phosphorus pentachloride - Preparation, properties, Structure

Phosphorus  -

Atomic Number :  15  Electronic Configuration  :  [Ne] 3s2 3p3  Group Number : 15   Periodic Number  : 3

 Halides of Phosphorus (Compounds of Phosphorus)

Phosphorus combines with allthehalogens forming phosphorus halides which are all covalent compounds. Phosphorus chlorides are more important. Tri and pentachlorides of phosphorus are most common.

I.     Phosphorus Trichloride, PCl3

Preparation: PCl3 is prepared by heating white phosphorus in a current of dry chlorine.

P4 + 6Cl2 ® 4PCl3

Dry white phosphorus is placed in the retort and gently heated on a water bath. A current of pure, dry chlorine is led over the phosphorus. The phosphorus trichloride formed being volatile distils over and is collected in a water cooled receiver.

The phosphorus trichloride obtained as above contains some PCl5 as impurity. This is removed by distilling the PCl3 over white phosphorus.

Physical properties

1.         Colourless low boiling liquid

2.         It fumes in moist air

3.         It has pungent odour.

Chemical Properties

1. It is violently hydrolysed by water giving phosphorus acid and hydrochloric

acid gas.

PCl3 + 3 H2O ® 3HCl + H3PO3

In a similar manner it reacts with organic compounds containing hydroxyl (OH)

group, such as acids and alcohols.

PCl3 + 3CH3COOH   ® 3CH3COCl + H3PO3

Acetic Acid      Acetyl Chloride

PCl3 + 3C2H5OH ® 3 C2H5Cl + H3PO3

Ethyl alcohol    Ethyl Chloride

2.. It reacts with chlorine or sulphuryl chloride forming phosphorus pentachloride.

PCl3 + Cl2 ®         PCl5

PCl3 + SO2Cl2 ® PCl5 + SO2

3. It readily combines with oxygen forming phosphorus oxychloride

2PCl3 + O2 ®2POCl3

4. It reacts with SO3 to form phosphorus oxychloride and SO2

SO3 + PCl3 ® POCl3 + SO2

Structure: PCl3 molecule has a pyramidal shape, which arises from sp3

hybridisation of phosphorus atom. One of the tetrahedral positions is occupied by a lone pair of electrons.

II. Phosphorus pentachloride, PCl5

Preparation: Phosphorus pentachloride is usually prepared by the action of an

excess of chlorine on phosphorus trichloride.

PCl3 + Cl2 ® PCl 5

Physical properties

1.           Phosphorus pentachloride is a yellowish white crystalline solid.

2.     It sublimes on heating at 473 K and melts at 318 K under pressure.

Chemical properties

Phosphorus pentachloride dissociates on heating into phosphorus trichloride and chlorine.

PCl5  -- > < --- PCl3 + Cl2

It is violently hydrolysed by water giving phosphorus oxychloride or phosphoric acid depending upon the quantity of water.

PCl5 + H2O  -- ^{insufficient water}  --- > POCl3 + 2HCl

PCl5 + 4H2O  -- ^{Excess of water}  -- > H3PO4 + 5HCl

Structure

PCl5 molecule has trigonal bipyramidal shape in vapour state which arises from sp3d hybridisation of phosphorus atom.