How Does Phosphorus Trichloride React?
Phosphorus forms two types of halides:
1) Phosphorus trihalide (PX3). The covalency of phosphorus in trihalides is 3, and
2) Phosphorus pentahalide (PX5). The covalency of phosphorus in pentahalide is 5.
Phosphorus trichloride is one of the most common phosphorus halides, which is an inorganic compound made up of one phosphorus and three chlorine atoms. It exists in a liquid state. It is poisonous and volatile in nature. Phosphorus trichloride is a very reactive compound. It shows explosivity when it reacts with water.
The IUPAC name of phosphorus trichloride is Trichlorophosphane. The other names by which phosphorus trichloride is known as Phosphorus(III) Chloride and Phosphorus chloride.
Phosphorus trichloride cannot be obtained from nature in its natural form. It is obtained from the synthesis of organic substances. Phosphorus trichloride is very toxic and corrosive in nature, hence, it should not come in direct contact with eyes and skin. It also should not be directly inhaled or ingested. Phosphorus trichloride is also a very important and necessary industrial chemical and hence is used in manufacture of other important chemicals.
Phosphorus Trichloride Formula
Phosphorus trichloride chemical formula is PCl3. The hybridization of phosphorus in PCl3 is sp3. PCl3 has a trigonal bipyramidal shape due to its sp3 hybridization. The bond angles of this structure are less than 109 degrees.
Phosphorus Trichloride Structure
Chemical formula for phosphorus trichloride is PCl3. In Phosphorus trichloride structure PCl3, three sp3 hybrid orbitals of phosphorus overlap with p-orbitals of chlorine to form three P-Cl sigma bonds while the fourth sp3 hybrid orbital contains lone pair of electrons. PCl3 has a pyramidal structure as shown below.
Phosphorus Trichloride Preparation
It can be prepared by passing dry chlorine gas over overheated white phosphorus. The vapours of phosphorus trichloride distil over and are collected in receivers cooled by water.
P4 + 6Cl2 → 4PCl2
It can also be obtained by the reaction of thionyl chloride with the white phosphorus.
P4 + 8SOCl2 → 4PCl3 +4SO2 + S2Cl2
Phosphorus Trichloride Properties
Physical Properties:
It is a colourless oily liquid.
It might be yellowish in colour sometimes.
It is constantly fuming liquid in the moist air.
Its boiling point is 347 K.
Its melting point is 161 K.
It has a high pungent odour, kind of similar to the hydrochloric acid.
It has a molar mass of 137.33g/mol.
It has a density of 1.574 g/cm3
It has a vapour pressure of 13.3kPa.
It has a refractive index of 1.5122.
It’s dipole moment is 0.97D.
Chemical Properties:
It reacts with water violently and forms phosphorus acid.
PCl3 + 3H2O → H3PO3 + 3HCl
It combines with oxygen to form phosphorus oxychloride.
2PCl3 + O2 → 2POCl3
It reacts with sulphur trioxide to form phosphorus oxychloride.
PCl3 + SO3 → POCl3 +SO3
It reacts with halogens and sulphur monochloride to form phosphorus pentachloride.
PCl3 + Cl2 → PCl5 + SO2
PCl3 + S2Cl2 → PCl5 + 2PSCl3
It reacts with organic compounds that contain OH groups and substitute this group with the chlorine.
3CH3COOH + PCl3 → 3CH3COCl + H3PO3
It readily oxidises to the phosphorus derivatives.
It undergoes substitution reactions both in organic and inorganic reactions.
Uses of Phosphorus Trichloride PCl3
It is widely used in organic chemistry as an important regent to replace the hydroxyl group by chlorine atom.
It is used for making phosphorus oxychloride by oxidising it with oxygen.
It is used for production of phosphate ester insecticides.
It is used in the production of chlorinated compounds like phosphorous penta chloride, phosphoryl chloride, thiophosphoryl chloride and pseudohalogens.
Did you know?
Phosphorus trichloride can act as a nucleophile. Due to the presence of one lone pair it can donte this pair to the electron deficient compound.
PCl3 can also act as an electrophile. Due to the presence of an empty d orbital it can accept electrons from electron rich compounds and expands its valency to 5.
Reactions including PCl3 generally undergo redox reactions.
PCl3 is highly toxic in nature.
PCl3 violently reacts with water and produces large amounts of heat.
PCl3 causes irritation to eyes, skin and respiratory system.
FAQs on Phosphorus Trichloride: Properties, Preparation, and Uses
1. What is Phosphorus Trichloride (PCl₃) and what does its chemical formula represent?
Phosphorus Trichloride, with the chemical formula PCl₃, is an inorganic compound and one of the most important phosphorus halides. The formula indicates that each molecule is composed of one central phosphorus (P) atom covalently bonded to three chlorine (Cl) atoms. It is a colourless (or pale yellow) fuming liquid that serves as a crucial starting material in various chemical syntheses. For more details, you can refer to the explanation of Phosphorus Trichloride.
2. How is Phosphorus Trichloride typically prepared?
The most common method for preparing Phosphorus Trichloride is by reacting white phosphorus (P₄) with an excess of dry chlorine (Cl₂) gas. The reaction is exothermic and carefully controlled to prevent the formation of Phosphorus Pentachloride (PCl₅). The chemical equation for this preparation is:
P₄ (s) + 6Cl₂ (g) → 4PCl₃ (l)
The resulting PCl₃ is then purified by fractional distillation.
3. What are the key physical properties of Phosphorus Trichloride?
Phosphorus Trichloride exhibits several distinct physical properties as per the NCERT syllabus for the 2025-26 session:
Appearance: It is a colourless or slightly yellow, oily liquid.
Odour: It has a sharp, pungent odour similar to hydrochloric acid.
State: It is volatile and fumes in the presence of moist air due to hydrolysis.
Boiling Point: Its boiling point is approximately 76.1 °C (349.2 K).
Solubility: It is soluble in organic solvents like ether, benzene, and carbon disulphide but reacts violently with water and alcohol.
4. What happens when Phosphorus Trichloride reacts with water?
Phosphorus Trichloride undergoes a vigorous and rapid hydrolysis reaction when it comes into contact with water. This reaction produces phosphorous acid (H₃PO₃) and fumes of hydrochloric acid (HCl) gas. This reaction is why PCl₃ fumes in moist air. The balanced chemical equation is:
PCl₃ + 3H₂O → H₃PO₃ + 3HCl
This reaction is a key characteristic of many non-metal halides.
5. What are the major industrial uses of Phosphorus Trichloride?
Phosphorus Trichloride is a vital chemical intermediate with several important industrial applications:
It is used to manufacture other phosphorus compounds like Phosphorus Oxychloride (POCl₃) and Phosphorus Pentachloride (PCl₅).
In organic chemistry, it acts as a chlorinating agent to convert carboxylic acids into acyl chlorides and alcohols into alkyl chlorides.
It is a precursor in the production of pesticides, insecticides, plasticizers, and flame retardants.
It serves as a catalyst in certain organic reactions, such as the Hell-Volhard-Zelinsky (HVZ) reaction.
6. Why does the PCl₃ molecule have a trigonal pyramidal shape?
The shape of the PCl₃ molecule is determined by the VSEPR (Valence Shell Electron Pair Repulsion) theory. The central phosphorus atom undergoes sp³ hybridization. It forms three sigma (σ) bonds with three chlorine atoms, and one hybrid orbital is occupied by a lone pair of electrons. This lone pair exerts a stronger repulsive force than the bonding pairs, pushing the P-Cl bonds closer together and resulting in a trigonal pyramidal geometry, not a flat trigonal planar shape. The bond angle is approximately 100°, slightly less than the ideal tetrahedral angle of 109.5° due to this lone pair repulsion.
7. Is Phosphorus Trichloride a polar or non-polar molecule, and why?
Phosphorus Trichloride (PCl₃) is a polar molecule. Although the individual P-Cl bonds are polar due to the difference in electronegativity between phosphorus and chlorine, the molecule's overall polarity depends on its shape. Because PCl₃ has a trigonal pyramidal geometry, the bond dipoles do not cancel each other out. The asymmetrical distribution of charge, along with the presence of the lone pair on the phosphorus atom, results in a net dipole moment, making the entire molecule polar.
8. How does Phosphorus Trichloride (PCl₃) differ from Phosphorus Pentachloride (PCl₅)?
PCl₃ and PCl₅ are both important halides of phosphorus but differ significantly:
Oxidation State: The phosphorus atom is in the +3 oxidation state in PCl₃, while it is in the +5 oxidation state in Phosphorus Pentachloride (PCl₅).
Structure: PCl₃ has a trigonal pyramidal shape due to sp³ hybridization. In contrast, gaseous PCl₅ has a trigonal bipyramidal shape (sp³d hybridization).
Hydrolysis Products: PCl₃ hydrolyzes to form phosphorous acid (H₃PO₃) and HCl, whereas PCl₅ hydrolyzes to form phosphoric acid (H₃PO₄) and HCl.
Physical State: PCl₃ is a liquid at room temperature, while PCl₅ is a greenish-yellow crystalline solid.
9. Why is Phosphorus Trichloride considered a covalent compound and not an ionic one?
Phosphorus Trichloride is a covalent compound because the bonds between the phosphorus and chlorine atoms are formed by the sharing of electrons, not by the complete transfer of electrons. Both phosphorus and chlorine are non-metals with relatively high and similar electronegativity values. The difference in their electronegativity (approx. 0.97) is not large enough to facilitate the formation of an ionic bond. Instead, they share electrons to achieve a stable electron configuration, resulting in polar covalent P-Cl bonds.
