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.

Halides of phosphorus preparation and properties, Group 15 P Block Elements 1

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°.

Halides of phosphorus preparation and properties, Group 15 P Block Elements 2

→ 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.

Halides of phosphorus preparation and properties, Group 15 P Block Elements 3

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

Halides of phosphorus preparation and properties, Group 15 P Block Elements 4

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

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

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.

Halides of phosphorus preparation and properties, Group 15 P Block Elements 5

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

Halides of phosphorus preparation and properties, Group 15 P Block Elements 6

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

Halides of phosphorus preparation and properties, Group 15 P Block Elements 7

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.

Halides of phosphorus preparation and properties, Group 15 P Block Elements 8

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.

Halides of phosphorus preparation and properties, Group 15 P Block Elements 9

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

Halides of phosphorus preparation and properties, Group 15 P Block Elements 10

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.

Halides of phosphorus preparation and properties, Group 15 P Block Elements 11

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

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

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:

Halides of phosphorus preparation and properties, Group 15 P Block Elements 12

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.

Halides of phosphorus preparation and properties, Group 15 P Block Elements 13

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:

Halides of phosphorus preparation and properties, Group 15 P Block Elements 14

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.

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

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.

Halides of phosphorus preparation and properties, Group 15 P Block Elements 16

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]

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

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

Halides of phosphorus preparation and properties, Group 15 P Block Elements 17

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

With carboxylic acid

Halides of phosphorus preparation and properties, Group 15 P Block Elements 18

With ethyl alcohol

Halides of phosphorus preparation and properties, Group 15 P Block Elements 19

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

Chemistry Notes