What Is Phosphorus Pentachloride? Properties, Preparation & Applications
Phosphorus pentachloride is a greenish-yellow crystalline solid, It is decomposed by water to form hydrochloric acid and phosphoric acid with the release of heat energy. It can also be prepared by the action of dry chlorine on phosphorus trichloride. The chemical formula for phosphorus pentachloride is PCl5. In a solid state, it exists as PCl4PCl4+PCl6PCl6-.
Phosphorus pentachloride vaporizes without dissociation in an atmosphere of phosphorus trichloride or chlorine gas. The dissociation equilibrium is shifted to the left side by the presence of the product.
Structure of Phosphorus Pentachloride
The hybridization in phosphorus pentachloride is sp3d and it has a trigonal bipyramidal geometry in gaseous and liquid states. It has two axial P−Cl bonds and three equatorial P−Cl bonds.
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Properties of Phosphorus Pentachloride
Uses of Phosphorus Pentachloride (PCl5)
It is used as a chlorinating agent.
It is used for the manufacture of penicillin and cephalosporin in the pharmaceutical industry.
Used to produce acid chlorides.
Used as a catalyst in the manufacture of acetyl cellulose and also a catalyst for condensation reactions and cyclization.
Methods of Preparation of Phosphorus Pentachloride (PCl5)
P4 + 10SO2Cl2 → 4PCl5 + 10SO2
P4 + 10Cl2 → 4PCl5
PCl3 + Cl2 → PCl5
Chemical Properties of Phosphorus Pentachloride (PCl5)
Phosphorus pentachloride dissociates as: PCl5 ⇌ PCl3 + Cl2
Reaction with water: PCl5 + 4H2O → H3PO4 + 5HCl
Reaction with metal: Zn + PCl5 → ZnCl2 + PCl3
Reactions with phosphorus pentoxide: 6PCl5 + P4O10 → 10POCl3
Reactions with sulfur dioxide: SO2 + PCl5 → POCl3 + SOCl2
Harmful Effects of Phosphorus Pentachloride
Phosphorus Pentachloride is a Reactive Chemical. Direct exposure to Phosphorus Pentachloride can cause weakness, nausea, headache, dizziness, and vomiting. It may damage the liver and kidneys.
As a precaution, one must be able to identify Phosphorus Pentachloride. It’s a crystalline solid whose color can range from white to pale yellow with a pungent odor. Phosphorus Pentachloride is a manufacturing agent for other chemicals, aluminum metallurgy, and the pharmaceutical industry.
Some of the acute health hazards that can be caused by Phosphorus Pentachloride are mentioned below:
If an individual comes in contact with Phosphorus Pentachloride, one may experience severe irritation and burning of skin and eyes with possible chronic eye damage
Breathing in Phosphorus Pentachloride can lead to irritation in the nasal cavity and throat.
If inhaled deeply, it can cause intense irritation and damage to the lungs resulting in coughing and/or shortness of breath. A medical emergency can also be caused because of higher exposure to Phosphorus Pentachloride. This can lead to fluid build-up in the lungs called pulmonary oedema with severe shortness of breath.
Exposure to Phosphorus Pentachloride can cause the development of cough, phlegm in bronchitis.
FAQs on Phosphorus Pentachloride Explained: Structure, Uses & Safety
1. What is Phosphorus Pentachloride (PCl₅), and what is its chemical formula?
Phosphorus Pentachloride is an inorganic compound and one of the most important phosphorus chlorides. It is a yellowish-white, crystalline solid that fumes in moist air due to hydrolysis. Its chemical formula is PCl₅, and it serves as a powerful chlorinating agent in chemical reactions.
2. What is the structure and hybridisation of a PCl₅ molecule in its gaseous state?
In the gaseous and liquid states, Phosphorus Pentachloride (PCl₅) has a trigonal bipyramidal structure. The central phosphorus atom undergoes sp³d hybridisation. This geometry features two types of P-Cl bonds: three equatorial bonds arranged in a plane at 120° to each other, and two axial bonds positioned at 90° to this plane, one above and one below.
3. Why are the axial bonds in PCl₅ longer and weaker than its equatorial bonds?
The axial P-Cl bonds in PCl₅ are longer and weaker because they experience greater electronic repulsion from the three equatorial bond pairs. An axial bond pair is repelled by three bond pairs at 90°, whereas an equatorial bond pair is only repelled by two bond pairs at 90° and two at 120°. This stronger repulsion on the axial bonds causes them to elongate, making them weaker and more reactive than the shorter, stronger equatorial bonds.
4. Why is Phosphorus Pentachloride (PCl₅) considered a Lewis acid?
Phosphorus Pentachloride acts as a Lewis acid because the central phosphorus atom has vacant d-orbitals in its valence shell. These empty orbitals can accept a pair of electrons from a Lewis base (an electron-pair donor). For instance, PCl₅ can accept a chloride ion (Cl⁻) to form the hexachlorophosphate anion, [PCl₆]⁻.
5. What happens when PCl₅ is heated or when it reacts with water?
PCl₅ exhibits distinct behaviours under these conditions:
- On Heating: When heated, PCl₅ undergoes thermal decomposition, sublimating and breaking down into Phosphorus Trichloride (PCl₃) and Chlorine gas (Cl₂). The reaction is reversible: PCl₅ ⇌ PCl₃ + Cl₂.
- With Water: PCl₅ reacts violently with water in a process called hydrolysis. With excess water, it fully hydrolyses to form phosphoric acid (H₃PO₄) and hydrochloric acid (HCl). The reaction is: PCl₅ + 4H₂O → H₃PO₄ + 5HCl.
6. What are the most common industrial and laboratory uses of Phosphorus Pentachloride?
Phosphorus Pentachloride is primarily used for its chlorinating properties. Key applications include:
- As a chlorinating agent to convert organic compounds like alcohols and carboxylic acids into their corresponding chloro-derivatives (alkyl chlorides and acyl chlorides).
- As a catalyst in the production of acetyl cellulose, a type of plastic film.
- In the manufacturing of intermediates, dyes, and various organic chemicals.
- To produce phosphoryl chloride (POCl₃) by reacting it with boric acid or oxalic acid.
7. What are the key safety hazards and precautions for handling PCl₅?
PCl₅ is a hazardous substance that requires careful handling. Key safety precautions include:
- It is highly corrosive and reacts violently with water. It must be stored in a moisture-free, cool, and dry environment.
- Upon contact with moisture, it releases toxic and corrosive hydrogen chloride (HCl) gas, which is harmful if inhaled and can cause severe skin burns.
- Always use appropriate Personal Protective Equipment (PPE), including safety goggles, chemical-resistant gloves, and a lab coat.
- All work involving PCl₅ must be conducted inside a well-ventilated chemical fume hood.
8. Why does PCl₅ exist as an ionic solid but as a covalent molecule in the gas phase?
This dual nature is a classic example of how structure can change with physical state to achieve maximum stability. In the gaseous or non-polar solvent phase, PCl₅ exists as discrete, covalent trigonal bipyramidal molecules. However, in the solid state, it achieves greater lattice energy and stability by existing as an ionic solid with the formula [PCl₄]⁺[PCl₆]⁻. This ionic lattice is composed of a tetrahedral cation ([PCl₄]⁺) and an octahedral anion ([PCl₆]⁻).
9. How is Phosphorus Pentachloride used as a test for hydroxyl (-OH) groups in organic chemistry?
PCl₅ is a definitive reagent for converting hydroxyl (-OH) groups into chloro groups (-Cl). When an organic compound containing an -OH group (like an alcohol or carboxylic acid) is treated with PCl₅, the -OH group is substituted by a -Cl atom. The reaction produces steamy white fumes of HCl gas and liquid phosphoryl chloride (POCl₃) as by-products. The formation of HCl fumes serves as a positive test for the presence of a hydroxyl group.
