# Phosphorus Halides: Phosphorus Trichloride, Phosphorus Pentachloride

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Phosphorus is known to form two types of halides $$PX_3$$ and $$PX_5$$. Of all the phosphorus halides formed, most common are phosphorus pentachloride, $$PCl_5$$ and phosphorus trichloride, $$PCl_3$$.

Phosphorus trichloride: It is a colourless oily liquid and highly toxic compound. The compound has triangular pyramidal shape in which phosphorus is $$sp^3$$ hybridized.

Preparation:

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

$$P_4 ~+ ~6Cl_2 → 4PCl_3$$

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

$$P ~+~ 8SOCl2 ~→~ 4PCl3~ +~ 4SO2 ~+~ 2S_2Cl_2$$

Chemical properties:

• Phosphorus trichloride hydrolyses in presence of moisture.

$$PCl_3~ +~ 3H_2O ~→~ H_3PO_3 ~+ ~3HCl$$

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

$$3C_2H_5OH ~+~ PCl_3 → 3C_2H_5Cl ~+~ H_3PO_3$$

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,$$[PCl_4]^+ [PCl6]^–$$ in which the cation, $$[PCl_4]^+$$ is tetrahedral and the anion, $$[PCl_6]^–$$ 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.

$$P_4~+~10Cl_2 \rightarrow 4PCl_5$$

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

$$P_4 ~+~10SO_2Cl_2 \rightarrow 4PCl_5 + 10SO_2$$

Chemical properties:

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

$$PCl_5 ~+ ~H_2O \rightarrow POCl_3~ +~ 2HCl$$

$$POCl_3 ~+~ 3H_2O \rightarrow H_3PO_4 ~+~ 3HCl$$

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

$$PCl_5 \rightarrow PCl_3 ~+ ~Cl_2$$

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

$$2Ag~ +~ PCl_5 \rightarrow 2AgCl ~+ ~PCl_3$$

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

$$C_2H_5OH ~+ ~PCl_5 \rightarrow C_2H_5Cl~ +~ POCl_3 ~+~ HCl$$

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.

For detailed discussions on other phosphorus halides, please visit Byju’s.’

#### Practise This Question

In the gaseous state, PCl5 exists in the form of trigonal bipyramidal. In the structure, all the bonds are of equal length and can be treated equivalent. True or False?