Phosphorus Halides

You probably know that chlorine finds its use in your swimming pools. However, are you aware that a phosphorus halide is also used as a cleansing and bleaching agent? First of all, do you know what a phosphorus halide is? Well, in this chapter, let us look at the halides of phosphorus and their properties. In the end, we will also look at its uses.

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What is a Phosphorus Halide?

A phosphorus halide is a compound that a phosphorus forms with a halogen. A phosphorus halide is of two types. They are PX₃ and PX₅. Here, we refer X to a halogen. It could be anything from fluorine, chlorine, bromine or iodine. However, the most common phosphorus halide is that of chloride. These chlorides are usually covalent in their nature.

1) Phosphorus Trichloride

This is an oily and sleek fluid. It is very lethal in nature. The shape of this compound is that of a triangular pyramid. The atom of phosphorus exhibits sp³ hybridization.

As we can see in the above diagram, phosphorus has its SP³ orbitals. It has only one electron and it gives that electron to a p orbital electron from 3 chlorine atoms. The fourth sp³ orbital is full. It is a solitary lone pair. Thus, it cannot form a bond. However, it repels alternate bonds. This creates a state of the shape of trigonal pyramidal.

Preparation

• We obtain phosphorus trichloride by passing dry chlorine overheated white phosphorus. The reaction that takes place is as follows:

P₄   +   6Cl₂   →   4PCl₃

• We can also obtain this compound by the reaction of thionyl chloride with white phosphorus. Below is the reaction to it.

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

Chemical Properties

• Phosphorus trichloride hydrolyses when we dampen it.

PCl₃   +   3H₂O   →   H₃PO₃   +   3HCl

• It reacts with natural compounds having a  –OH group and gives their ‘chloro’ subsidiaries as products.

3C₂H₅OH   +   PCl₃   →   3C₂H₅Cl   +   H₃PO₃

Structure of PCl₃

The phosphorus particle in the centre of PCl₃ exhibits sp³ hybridisation. It has 3 bond sets and 1 lone pair of electrons. Due to this reason, it has a pyramidal shape. It acts as a Lewis base because it has the capacity to donate its lone pair of electrons to other electron-lacking particles or atoms.

2) Phosphorus Pentachloride

It is yellowish-white in colour. Phosphorus Pentachloride is a very water delicate solid. It dissolves in organic solvents like carbon tetrachloride, benzene, carbon disulphide, diethyl ether.

Its structure is that of a trigonal bi-pyramid. We find this structure primarily in vaporous and fluid stages. In the solid state, we can find it as an ionic solid, [PCl₄]⁺[PCl₆]^–. Here, the cation, [PCl₄]⁺ is tetrahedral and the anion, [PCl₆]^– is octahedral.

We must know that the molecule has three tropical P-Cl bonds and two pivotal P-Cl bonds. Due to the more prominent repulsion at hub positions as compared to the central positions, we see that the two axial bonds are longer than tropical bonds.

Preparation

• We can produce pentachloride by the reaction with an excess of dry chlorine.

P₄     +    10Cl₂     →    4PCl₅

• We can also produce it by the reaction of SO₂Cl₂ and phosphorus.

P₄     +    10SO₂Cl₂   →    4PCl₅    +    10SO₂

Chemical Properties

• In the presence of damp air, phosphorus pentachloride hydrolyses to POCl_3.  This compound changes over to phosphoric acid over a period of time.

PCl₅   +   H₂O   →   POCl₃   +   2HCl

 POCl₃  +    3H₂O   →   H₃PO₄      +     3HCl

• When we heat it, it sublimes and further disintegrates to phosphorus trichloride.

PCl₅    →   PCl₃   +    Cl₂

• It reacts with finely partitioned metals under the influence of heat to create metal chlorides.

2Ag    +    PCl₅    →    2AgCl    +    PCl₃

• It reacts with natural compounds containing –OH group and produces their ‘chloro’ subordinates.

C₂H₅OH    +    PCl₅    →    C₂H₅Cl   +    POCl₃   +    HCl

Structure of PCl₅

As we discussed earlier, the central phosphorus atom in phosphorus pentachloride experiences SP³d hybridisation. All the five electrons combine in these hybrid orbitals as bond sets. The molecular shape of the particle is trigonal bipyramidal.

After the five electrons get hybridised, we get five electrons of equivalent size and shape. Three of them frame a triangle (120° partition) in the centre. One bond is above and one is underneath those three.

However, you must remember that we notice trigonal bipyramidal geometry in phosphorus pentachloride only in its fluid and vaporous state. In its solid state, it exists as a salt.

Solved Example for You

Q: Mention some of the common uses of phosphorus halides.

Ans: The various uses of Phosphorus Halides are as follows:

• We use phosphorus halides as a chlorinating agent. Specifically, it is used in cleaning water bodies.
• Also, we utilise them for making water treatment agents. Too often, we use them to make organophosphorus pesticides.
• They are also an important component in the lube oil and paint added substances. They act as an intermediate in these substances.
• We use phosphorus halides in the preparation of phosphorus acid. We also use them to prepare chloroanhydrides and phosphoric acid subsidiaries as an intermediate.