Halides of phosphorus preparation and properties, Group 15 P Block Elements

Halides of phosphorus preparation and properties, Group 15 P Block Elements

Halides of Phosphorus :

Phosphorus forms two types of halides :

• Phosphorus trihalide.
• Phosphorus pentahalide.

Phosphorus reacts with chlorine and forms two types of chlorides, which are discussed below:

Phosphorus Trichloride (PCl₃)

Preparation :

→ It can be prepared when dry chlorine gas is passed over heated white phosphorus. The vapours of PCI, formed which are then distilled over and condensed in a receiver by cold water.

P₄ + 6Cl₂ → 4PCl₃

→ It can also be prepared by the reaction of thionyl chloride with white phosphorus.

Structure :

→ Phosphorus atom in phosphorus trichloride is sp³-hybridised. Its shape is pyramidal. In PCl₃, three sp³-hybrid orbitals show overlapping with porbitals of chlorine atom. Thus, three P – Cl sigma bonds are formed with P – Cl bond length 204 pm. The fourth sp³-hybrid orbital contains one lone pair of electrons. Therefore, its shape is pyramidal. In PCl₃ the Cl – P – Cl bond angle is 100.4°.

→ Properties : Its physical and chemical properties are as follows:

→ Physical Properties

• It is colourless oily liquid.
• Its specific gravity is 1.6.
• Its boiling point is 347 K while melting point is 161 K.
• It has a pungent smell.

Chemical Properties : Its chemical properties are as follows:

It reacts with water and fumes in moist air. The fumes are due to the formation of HCl. With HCl it also forms phosphorus acid.

Reaction with oxygen : PCl₃ slowly reacts with dry oxygen and phosphorus oxychloride is formed.

Reaction with chlorine : When PCl₃ reacts with chlorine, phosphorus pentachloride is formed.

PCl₃ + Cl₂ → PCl₅

Reaction with sulphur : When PCl₃ reacts with sulphur, thiophosphorus oxychloride is formed.
PCl₃ + S → PSCl₃

Reaction with sulphur mono chloride : When PCl₃ reacts with sulphur mono chloride, PCl₃ is formed.
PCl₃ + S₂Cl₂ → PCl₅ + 2PSCl₃

Reaction with sulphur trioxide : When PCl₃ reacts with SO₃, phosphorus oxychloride is formed.

PCl₃ acts as a reducing agent : PCl₃ behaves as reducing agent and reduces various compounds.

Reaction with Grignard reagent : PCl₃ reacts with Grignard Reagents and form substituted phosphine.

Reaction with organic compounds : It reacts with those organic compounds which has-OH
group as shown below:

With alcohols, it forms alkyl chloride.

3C₂H₅OH + PCl₃ → 3C₂H₅Cl + H₃PO₃

With carboxylic acid, it forms acetyl chloride.

With benzenesulphonic acid, it forms benzene sulphonyl chloride.

3C₆H₅SO₂OH + PCl₃ → 3C₆H₅ SO₂Cl + H₃PO₃

Reaction with finely divided metals : On heating with finely divided metals it reacts to form metal chlorides.

Reaction with sulphuric acid : It reacts with sulphuric acid and converts it into chloro sulphonic acid.

Uses :

It is used as reducing agent.
It is used to replace hydroxyl group (-OH) by chlorine atom (-Cl) in organic compound.

Phosphorus Pentachloride (PCl₅)

Preparation :

It can be prepared by the reaction of white phosphorus with excess of chlorine.

When PCl₃ reacts with chlorine then PCl₅ is formed.

PCl₃ + Cl₂ → PCl₅

When phosphorus reacts with SO₂Cl₂ then phosphorus pentachloride is formed.
P₄ + 10SO₂Cl₂ → 4PCl₅ + 10SO₂

Structure Phosphorus atom in PCl₅ is sp³ hybridised. Its shape is triangular bipyramidal.

→ In PCl₅ there are two types of bond i.c., equatorial P – Cl bonds and axial P – Cl bonds. Axial bonds are longer than equatorial bonds due to the fact that the axial bond pairs suffer more repulsion as compared to equatorial bond pairs. The bond length are as follows:

Properties Its physical and chemical properties are as follows:

Physical Properties :

• It is a yellowish white powder.
• It has pungent smell.
• Its sublimation temperature is 433 K and melting point is 318 K on heating under pressure.
• It fumes in moist air due to the formation of HCl.
• It has strong affinity for water.

Chemical Properties : Its chemical properties are as follows:

Stability : PCl₅ is less stable. It easily decomposes into PCl₃ and Cl₂.

Reaction with water PCl₅ has great affinity for water. It fumes in moist air due to the formation of HCl. The related reactions are as follows:

Reaction with metals : Phosphorus pentachloride reacts with finely divided metals forming chlorides.

2Ag + PCl₅ → 2AgCl + PCl₃
Sn + 2PCl₅ → SnCl₄ + 2PCl₃

Reaction with phosphorus pentasulphide : It reacts with phosphorus pentasulphide, and phosphorus thiochloride is formed.

Reduction of PCl₅ : PCl₅ can be reduced by hydrogen into PCl₃.

PCl₅ + 2H → PCl₃ + 2HCl

Reaction with sulphur dioxide : When phosphorus pentachloride reacts with SO₂ then POCl₃ and SOCl₂ are formed.

Reaction with phosphorus pentoxide : When PCl₅ reacts with P₂O₅, POCl₃ is formed.

6PCl₅ + P₄O₁₀ → 10POCl₃

Reaction with chloride ion aceptors : Phosphorus pentachloride forms addition products with chloride ion aceptor such as BCl₃, AlCl₃ etc.

PCl₅ + BCl₃ → [PCl₄]⁺ [BCl₄]^–

Reaction with potassium fluoride : PCl₅ reacts with KF then K⁺[PF₆]^– complex is formed.

Reaction with organic compounds : PCl₅ is used to replace -OH group from any organic compound with-Cl.

With carboxylic acid

With ethyl alcohol

Uses It is used as a reagent to replace -OH group from organic compound with -Cl.

Chemistry Notes