By : Sanekh Das

Phosphorus combines with all the halogens forming phosphorus halides which are all covalent compounds. This is known as phosphorous halides.

Phosphorous Chlorides are the most improtant.

1. Phosphorous Trichloride .

2. Phosphorous Pentachloride.

Phosphorous trichloride is a chemical compound of phosphorus and chlorine, having chemical formula PCl3. It has a trigonal pyramidal shape. It is the most important of the two phosphorus chlorides.

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

P4 + 6Cl2 → 4PCl3

By the action of thionyl Chloride with white phosphorous.

P4 + 8SOCl2 → 4PCl3 + 4SO2 + 2S2Cl2

Chemical Properties

Physical Properties

It is a colourless oily liquid and hydrolysis in the presence of mositure.

PCl3 + 3 H2O → 3HCl + H3PO3

On reaction with compounds containing hydroxyl (OH) groups, such as acids & alcohols.

PCl3 + 3CH3COOH → 3CH3COCl + H3PO3

PCl3 + 3C2H5OH → 3 C2H5Cl + H3PO3

It has a pyramidal shape.

Colourless low boiling liquid

It fumes in moist air.

It has pungent odour.

PCl3 is also used directly as a reagent in organic synthesis.

It is used to convert primary and secondary alcohols into alkyl chlorides, or carboxylic acids into acyl chlorides, although thionylchloride generally gives better yields than PCl3.

PCl3 molecule has a trigonal pyramidal shape, which arises from sp3 hybridisation of phosphorus atom.

One of the tetrahedral positions is occupiedby a lone pair of electrons.

Phosphorus pentachloride is the chemical compound with the formula PCl5.

It is mainly used as a chlorinating reagent.

It is usually prepared by the action of dry chlorine on phosphorus trichloride.

PCl3 + Cl2 → PCl5

It can also be prepared by the action of SO2Cl2on phosphorous.

P4+10SO2Cl2 → 4PCl5 + 10SO2

Chemical Properties.

Physical Properties.

Phosphorus pentachloride dissociates on heating into phosphorus trichloride and chlorine.

PCl5 → PCl3 + Cl2

Depending upon the quantity of water on hydrolysis it is converted into the following ;

o Insufficient water :

PCl5 + H2O → POCl3 + 2HCl

o Excess of water :

PCl5 + 3H2O → H3PO4 + 3HCl

It is a colourless, water and moisture sensitive solid.

It is a yellowish white crystalline solid.

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

Used in the synthesis of some organic compounds.

Eg : C2H5Cl, CH3COCl.

PCl5 molecule has trigonal bi-pyramidal shape in vapour state which arises from sp₃dhybridisation of phosphorus atom.