Phosphorus Halides: Phosphorus Trichloride & Phosphorus Pentachloride

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

Phosphorus Trichloride (PCl₃)

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

Table of Contents

• Preparation
• Chemical Properties
• Phosphorus Pentachloride(PCl5)
• Uses of Phosphorus Halides
• Frequently Asked Questions

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 (PCl₅)

Phosphorus pentachloride is a yellowish-white, water sensitive solid. It is soluble in carbon tetrachloride, carbon disulfide, benzene, and diethyl ether. It has a trigonal bipyramidal structure in gaseous and liquid phases. In a 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 an 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, chloro-anhydrides 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 – The Learning App.