Phosphorus Trichloride

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What Is Phosphorus Trichloride?

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 a 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.

Phosphorus Trichloride Formula

Phosphorus trichloride chemical formula is PCl3. The hybridization of phosphorus in PCl3 is sp3

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 → 4PCl3

  • 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  colourless oily liquid.

  • 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.

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 oxidized 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 oxidizing 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.

FAQ (Frequently Asked Questions)

Question: Explain the Structure of Phosphorus Trichloride.

Answer: Phosphorus in PCl₃ undergoes sp³ hybridization. Three of the sp³ hybrid orbitals overlap with P-orbitals of chlorine to form three P-Cl sigma bonds while the fourth sp³ hybridised orbital contains lone pair of electrons. Therefore, PCl₃ has a pyramidal structure. The bond angle is 100.4 degrees in PCl₃. The structure of PH₃ and PCl₃ is similar, but the bond angle in both the compounds is different. In PCl₃ the bond angle is more than the PH₃ due to steric hindrance caused by the larger sized chlorine atom.

Question: Write Five Physical Properties of Phosphorus Trichloride.

Answer: Five physical properties of phosphorus trichloride are given below:

  1. It is  colourless oily liquid.

  2. It is constantly fuming liquid in the moist air.

  3. Its boiling point is 347 K.

  4. Its melting point is 161 K.

  5. It has a high pungent odour.