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.
• 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.\(P ~+~ 8SOCl2 ~→~ 4PCl3~ +~ 4SO2 ~+~ 2S_2Cl_2\)
• 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.
• 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.\(P_4 ~+~10SO_2Cl_2 \rightarrow 4PCl_5 + 10SO_2\)
• 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.