Phosphorus Halides: Phosphorus Trichloride and Phosphorus Pentachloride

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Phosphorus Halide

When phosphorus reacts with halide, it forms phosphorus halides. Halides of phosphorus are of two types: PX3 and PX5. X here is denoted for halogen and it can be fluorine, chlorine, bromine, and iodine.


The most common type of phosphorus halide is phosphorus trichloride (PCl3) and phosphorus pentachloride (formula of phosphorus pentachloride - PCl5).


Phosphorus Trichloride PCl3:

  • Phosphorus trichloride is highly poisonous and is colorless in nature. 

  • The phosphorus trichloride formula (chemical) is PCl3.

  • This compound exhibits sp3 hybridization and is triangular pyramidal in shape.

Phosphorus Trichloride Structure:

  • It consists of three bonded chlorine atoms to phosphorus and one lone pair.

  • Phosphorus in the structure of PCl3 shows sp3 hybridization. These three sp3 hybridized orbits of phosphorus overlap with the p orbital of each chlorine atom thus forming a three-sigma P-Cl bond.

  • The fourth sp3 hybridized orbital of phosphorus contains a lone pair.

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Preparation of Phosphorus Trichloride:

  1. Phosphorus trichloride is formed when dry chlorine is passed over warmed white phosphorus.

P4   +   6Cl2   →   4PCl3

  1. Phosphorus trichloride is produced when thionyl chloride reacts with white phosphorus.

P    +     8SOCl2     →    4PCl3  +    4SO2   +    2S2Cl2


Chemical Properties 

  1. Hydrolysis of phosphorus trichloride in the presence of moisture.

PCl3 +   3H2O   →   H3PO3  +   3HCl

  1. To produce its chloro-derivatives, it reacts with organic compounds containing the OH group.

3C2H5OH   +   PCl3   →   3C2H5Cl   +   H3PO3


Phosphorus Pentachloride(PCl5):

  • It is a yellowish-white powder in colour with a pungent smell.

  • It is very sensitive to water.

  • It is soluble in carbon tetrachloride, benzene, diethyl ether, and carbon disulfide.

  • The phosphorus pentachloride formula is PCl5

Structure of Phosphorus Pentachloride:

  • Phosphorus pentachloride shape is trigonal bipyramidal (in liquid and gaseous form).

  • There are two axial bonds and three equatorial bonds in the structure of phosphorus pentachloride PCl5. There is more repulsion in axial bonds as compared to equatorial bonds. So, axial bonds are longer than the equatorial bonds in the structure.

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Note: Phosphorus pentachloride exists in ionic form in solid-state. Its ionic form consists of [PCl4]+[PCl6]-. The cation [PCl4]+ is tetrahedral while [PCl6]- is octahedral in shape.


Preparation:

  1. Phosphorus pentachloride is formed when white phosphorus reacts with an excess of dry chlorine.

P4    +    10Cl2    →    4PCl5

  1. By the reaction of SO2Cl2 and phosphorus, we can also produce it.

P4 + 10SO2Cl→      4PCl5 + 10SO2


Chemical Properties:

  1. Phosphorus pentachloride hydrolyzes into POCl3 in the presence of moist air. Over a period of time, this compound converts to phosphoric acid.

PCl5   +   H2O   →   POCl3   +   2HCl

 POCl3  +    3H2O   →   H3PO4    +     3HCl

  1. It sublimates and further disintegrates into phosphorus trichloride as we heat it.

PCl5    →   PCl3  +    Cl2

  1. Under the influence of heat, it reacts with finely partitioned metals to produce metal chlorides.

2Ag    +    PCl5    →    2AgCl    +    PCl3

  1. It reacts and produces its 'chloro' subordinates with natural compounds containing-OH group.

C2H5OH    +    PCl5   →    C2H5Cl   +    POCl3   +    HCl


Uses of Phosphorus Halides:

  • It is used as an agent for chlorinating.

  • It is used as an intermediate in the manufacturing of phosphorus acid, chloro-anhydrides, and phosphoric acid derivatives.

  • It is used for the manufacturing of organophosphate pesticides.

  • It is used in water treatments and paint additives.

FAQ (Frequently Asked Questions)

Question: Phosphorus Trichloride is an Acid or Base?

Answer: According to Lewis’s acid-base theory, compounds that accept lone pairs of electrons are lewis acids, and the compound that donates lone pairs of electrons is Lewis base. Phosphorus trichloride structure contains three sp³ hybridized orbitals bonded with three chlorine atoms to form a sigma bond while one sp³ hybridized orbital contains a lone pair. So it can readily donate the lone pair of electrons and hence acts as a Lewis base.

Question: How has PCl₃ Been Formed?

Answer: Through the reaction of chlorine with a reflux solution of white phosphorus in phosphorus trichloride, with continuous removal of PCl₃ as it is formed, Phosphorus trichloride is industrially prepared. Using less toxic red phosphorus could be more convenient in the laboratory.

Question: Is PCl₅  a Liquid. If Yes, then Explain.

Answer: Yes, it is a liquid since between the molecules there are only van der Waals dispersion forces and dipole-dipole attractions.

Note: There is a permanent dipole in the phosphorus(III) chloride molecule, so dipole-dipole attractions are feasible.