Phosphorus Halides: Phosphorus Trichloride, Phosphorus Pentachloride

Phosphorus is known to form two types of halides PX3 and PX5. Of all the phosphorus halides formed, most common are phosphorus pentachloride, PCl5 and phosphorus trichloride, PCl3.

Phosphorus trichloride: It is a colourless oily liquid and highly toxic compound. The compound has triangular pyramidal shape in which phosphorus is sp3 hybridized.

Preparation:

• When dry chlorine is passed over heated white phosphorus, phosphorus trichloride is produced.

P4 + 6Cl2→4PCl3

• When thionyl chloride reacts with white phosphorus, phosphorus trichloride is produced.

P + 8SOCl2 → 4PCl3 + 4SO2 + 2S2Cl2

Chemical properties:

Phosphorus trichloride hydrolyses in presence of moisture.

PCl3 + 3H2O → H3PO3 + 3HCl

• It reacts with organic compounds containing –OH group to produce their chloro derivatives.

3C2H5OH + PCl3→3C2H5Cl + H3PO3

Phosphorus pentachloride: It is a yellowish white, water sensitive solid. It is soluble in carbon tetrachloride, carbon disulfide, benzene, diethyl ether. It has a trigonal bypyramidal structure in gaseous and liquid phases. In solid state, it exists as an ionic solid,[PCl4]+[PCl6]– in which the cation, [PCl4]+ is tetrahedral and the anion, [PCl6]– is octahedral.

From the figure, we acknowledge the formation of three equatorial P-Cl bonds and two axial P-Cl bonds. Due to greater repulsion at axial positions in comparison to equatorial positions the two axial bonds are longer than equatorial bonds.

Preparation:

• When white phosphorus reacts with excess of dry chlorine, phosphorus pentachloride is produced.

P4 + 10Cl2→4PCl5

• It can also be prepared by the reaction of SO2Cl2 and phosphorus.

P4 + 10SO2Cl2→4PCl5+10SO2

Chemical properties:

• In moist air, phosphorus pentachloride hydrolyses to POCl3 which finally converts to phosphoric acid.

PCl5 + H2O→POCl3 + 2HCl

POCl3 + 3H2O→H3PO4 + 3HCl

• It sublimes on heating and further decomposes to phosphorus trichloride in case of stronger heating.

PCl5→PCl3 + Cl2

• It reacts with finely divided metals under the influence of heat to produce metal chlorides.

2Ag + PCl5→2AgCl + PCl3

• It reacts with organic compounds containing –OH group to produce their chloro derivatives.

C2H5OH + PCl5→C2H5Cl + POCl3 + HCl

Uses of phosphorus halides:

• It is used as a chlorinating agent.

• It is used as an intermediate in the manufacturing of phosphorus acid, chloroanhydrides and phosphoric acid derivatives.

• It is used as an intermediate for making organophosphorus pesticides, water treatment agents, lube oil and paint additives etc.