Phosphorus Pentachloride: A Versatile Chemical Compound
Phosphorus pentachloride, with the chemical formula PCl5, is an important industrial chemical that has a wide range of applications. In this article, we will explore the properties, structure, production, uses and safety precautions of phosphorus pentachloride.
Chemical and Physical Properties
Phosphorus pentachloride is a colourless crystalline solid at room temperature with a freezing point of -101°C. It has a pungent and suffocating odour. It has a molecular weight of 197.33 g/mol and a density of 1.63 g/cm3. When heated, it sublimes readily without melting. It is non-flammable but supports combustion. It reacts rapidly with water to produce hydrochloric acid. Due to its reactivity, phosphorus pentachloride needs to be handled and stored carefully.
Molecular Structure
At the molecular level, phosphorus pentachloride is made up of one phosphorus atom bonded to five chlorine atoms in a trigonal bipyramidal molecular geometry. The phosphorus atom occupies the central position and the five chlorine atoms surround it in the axial and equatorial positions. The P-Cl bonds have partially covalent and partially ionic character. The chlorine atoms are symmetrically arranged around the phosphorus atom with bond angles of around 90° between the axial chloride ligands and 120° between the equatorial chloride ligands.
Methods of Production
Industrially, phosphorus pentachloride is produced by the direct chlorination of white or red phosphorus. In the laboratory, it can be synthesized by the following reactions:
1) By the direct combination of phosphorus and chlorine gases at 400-450°C in a stream of chlorine gas:
P4 + 10Cl2 → 4PCl5
2) By the reaction of phosphorus trichloride with additional chlorine:
PCl3 + Cl2 → PCl5
3) By the dehydration of phosphorus oxychloride with thionyl chloride:
POCl3 + SOCl2 → PCl5 + SO2
These methods exploit the high reactivity of phosphorus with chlorine to produce phosphorus pentachloride on an industrial scale.
Uses and Applications
Due to its versatile reactive nature, phosphorus pentachloride finds extensive applications as a chlorinating and dehydrating agent in industrial processes. Some of its major uses are:
- Production of phosphorus trichloride, phosphorus oxychloride and other phosphorus chlorides.
- Production of polyvinyl chloride (PVC) by chlorination of ethylene or propylene to produce alkoxychlorides as intermediates.
- Production of phosphorus-containing flame retardants, plasticizers and pesticides.
- Catalyst in the chlorination of benzene to produce chlorobenzene.
- Catalyst in the production of polycarbonates through chlorination of bisphenol-A.
- Dehydrating agent for alcohols to produce alkyl chlorides in esterification reactions.
- Agent for nitration, sulfonation, oxidation and phosphorylation reactions.
Its wide applicability stems from its ability to readily donate chlorine to other organic and inorganic compounds through both electrophilic aromatic substitution and nucleophilic substitution pathways.
Safety Precautions
Given its reactivity with moisture and combustion supporting property, phosphorus pentachloride needs careful handling. The following safety measures must be adopted while using this compound:
- It should be stored in airtight containers in a cool, dry place away from sources of heat and flame.
- Gloves, eye protection and protective clothing should be worn while handling it.
- It should not come in contact with water as the reaction is highly exothermic producing toxic hydrogen chloride gas.
- Spills should be cleaned up immediately using sodium bicarbonate and the area well ventilated.
- Its vapours are heavier than air and might cause suffocation in confined spaces, so ventilation is important especially while heating or subliming it.
- It may cause burns to skin and eyes on contact. In case of exposure, the affected area should be flushed with water and medical assistance sought immediately.
In summary, phosphorus pentachloride is a very reactive and industrially important chlorinating agent. Its distinctive molecular structure and reactivity allow it to act as an intermediate in numerous organic syntheses. With proper safety precautions during handling and storage, it serves as a versatile building block in the large-scale production of many valuable pharmaceuticals, agrochemicals and polymers. Further research is ongoing to explore new applications of this versatile compound.
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